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17β-estradiol: See estradiol.
5α-reductase: An enzyme that converts testosterone into dihydrotestosterone.
5-HT: See serotonin.

A

ABI: See auditory brainstem implant.
absence attack: See petit mal seizure.
absolute refractory phase: See refractory phase (definition 1).
accommodation: The process of focusing by the ciliary muscles and the lens to form a sharp image on the retina.
acetylcholine (ACh): A neurotransmitter produced and released by parasympathetic postganglionic neurons, by motoneurons, and by neurons throughout the brain.
acetylcholinesterase (AChE): An enzyme that inactivates the transmitter acetylcholine both at synaptic sites and elsewhere in the nervous system.
ACh: See acetylcholine.
AChE: See acetylcholinesterase.
acid: See LSD.
acquired dyslexia: See dyslexia.
acquired prosopagnosia: See prosopagnosia.
act: Also called action pattern. Complex behavior, as distinct from a simple movement.
ACTH: See adrenocorticotropic hormone.
actin: A protein that, along with myosin, mediates the contraction of muscle fibers. See Figure 11.7.
action pattern: See act.
action potential: Also called nerve impulse. The propagated electrical message of a neuron that travels along the axon to the presynaptic axon terminals. See Figures 3.6, 3.7.
activational effect: A temporary change in behavior resulting from the administration of a hormone to an adult animal. Compare organizational effect.
acupuncture: The insertion of needles at designated points on the skin to alleviate pain or neurological malfunction.
adaptation: 1. In the context of evolution, a trait that increases the probability that an individual will leave offspring in subsequent generations. 2. In the context of sensory processing, the progressive loss of receptor sensitivity as stimulation is maintained. See Figure 8.7.
adaptation stage: The second stage in the stress response, including successful activation of the appropriate response systems and the reestablishment of homeostatic balance.
addiction: See dependence.
Aδ fiber: A moderately large, myelinated, and therefore fast-conducting axon, usually transmitting pain information. See Table 8.2. Compare C fiber.
adenohypophysis: See anterior pituitary.
adenosine: In the context of neural transmission, a neuromodulator that alters synaptic activity. Adenosine receptors are the site of action of caffeine.
adequate stimulus: The type of stimulus for which a given sensory organ is particularly adapted. Light energy, for example, is the adequate stimulus for photoreceptors.
ADH: See arginine vasopressin.
ADHD: See attention deficit hyperactivity disorder.
adipose tissue: Tissue made up of fat cells.
adrenal cortex: The outer rind of the adrenal gland. Each of the three cellular layers of the adrenal cortex produces different hormones. See Figures 5.1, 5.16; Table 5.2.
adrenal gland: An endocrine gland atop the kidney. See Figures 5.1, 5.16.
adrenal medulla: The inner core of the adrenal gland. The adrenal medulla secretes epinephrine and norepinephrine. See Figures 5.1, 5.16.
adrenal steroids: See adrenocorticoids.
adrenaline: See epinephrine.
adrenocorticoids: Also called adrenal steroids. A class of steroid hormones that are secreted by the adrenal cortex.
adrenocorticotropic hormone (ACTH): A tropic hormone secreted by the anterior pituitary gland that controls the production and release of hormones of the adrenal cortex. See Table 5.2; Figure 5.15.
adult neurogenesis: The creation of new neurons in the brain of an adult.
affective disorder: A disorder of mood, such as depression or bipolar disorder.
afferent: In reference to an axon, carrying nerve impulses from a sensory organ to the central nervous system, or from one region to another region of interest. See Box 2.2. Compare efferent.
affinity: See binding affinity.
afterpotential: The positive or negative change in membrane potential that may follow an action potential.
agnosia: The inability to recognize objects, despite being able to describe them in terms of form and color; may occur after localized brain damage.
agonist: 1. A molecule, usually a drug, that binds a receptor molecule and initiates a response like that of another molecule, usually a neurotransmitter. Compare antagonist (definition 1). 2. A muscle that moves a body part in the same general way as the muscle of interest; a synergistic muscle. Compare antagonist (definition 2). See also synergist.
agouti-related peptide (AgRP): A peptide that is a naturally occurring antagonist to α-melanocyte-stimulating hormone at melanocortin receptors.
agraphia: The inability to write. Compare alexia.
AgRP: See agouti-related peptide.
AIS: See androgen insensitivity syndrome.
alarm reaction: The initial response to stress.
aldosterone: A mineralocorticoid hormone, secreted by the adrenal cortex, that induces the kidneys to conserve sodium ions.
alexia: The inability to read. Compare agraphia.
all-or-none property: The fact that the amplitude of the action potential is independent of the magnitude of the stimulus. See Table 3.1. Compare postsynaptic potential.
allele: Any particular version of a gene.
allocortex: Formerly called archicortex or paleocortex. Brain tissue with three layers or unlayered organization. Compare neocortex.
allomone: A chemical signal that is released outside the body by one species and affects the behavior of other species. See Figures 5.3, 5.4. Compare pheromone.
allopregnanolone: A naturally occurring steroid that modulates GABA receptor activity in much the same way that benzodiazepine anxiolytics do.
α-fetoprotein: A protein found in the plasma of fetuses. In rodents, α-fetoprotein binds estrogens and prevents them from entering the brain.
α-melanocyte stimulating hormone (α-MSH): A peptide that binds the melanocortin recetor.
alpha motoneuron: A motoneuron that controls the main contractile fibers (extrafusal fibers) of a muscle. See Figure 11.9. Compare gamma motoneuron.
α-MSH: See α-melanocyte stimulating hormone.
alpha rhythm: A brain potential of 8–12 Hz that occurs during relaxed wakefulness. See Figure 14.11. Compare desynchronized EEG.
α-synuclein: A protein that has been implicated in Parkinson’s disease.
ALS: See amyotrophic lateral sclerosis.
altricial: Referring to animals that are born in an undeveloped state and depend on maternal care, as human infants do. Compare precocial.
Alzheimer’s disease: A form of dementia that may appear in middle age but is more frequent among the aged.
amacrine cells: Specialized retinal cells that contact both the bipolar cells and the ganglion cells, and are especially significant in inhibitory interactions within the retina.
amblyopia: Reduced visual acuity that is not caused by optical or retinal impairments.
AMH: See anti-müllerian hormone.
amine hormones: Also called monoamine hormones. A class of hormones, each composed of a single amino acid that has been modified into a related molecule, such as melatonin or epinephrine.
amine neurotransmitter: A neurotransmitter based on modifications of a single amino acid nucleus. Examples include acetylcholine, serotonin, ordopamine.
amino acid neurotransmitter: A neurotransmitter that is itself an amino acid. Examples include GABA, glycine, or glutamate.
amnesia: Severe impairment of memory.
AMPA receptor: A glutamate receptor that also binds the glutamate agonist AMPA. The AMPA receptor is responsible for most of the activity at glutamatergic synapses. See Figure 17.22.
amphetamine: A molecule that resembles the structure of the catecholamine transmitters and enhances their activity.
amphetamine psychosis: A delusional and psychotic state, closely resembling acute schizophrenia, that is brought on by repeated use of high doses of amphetamine.
amplitude: The maximum extent of a single oscillation in a periodic event, such as a sound wave, measured as the distance from peak to trough in a single cycle. In practical terms, amplitude corresponds to the “volume” of a sound. See Box 9.1.
ampulla (pl. ampullae): An enlarged region of each semicircular canal that contains the receptor cells (hair cells) of the vestibular system. See Figure 9.16.
amusia: A disorder characterized by the inability to discern tunes accurately.
amygdala: A group of nuclei in the medial anterior part of the temporal lobe. See Figure 2.15.
amyloid plaques: See senile plaques.
amyloid precursor protein (APP): A protein that, when cleaved by several enzymes, produces β-amyloid. Buildup of β-amyloid is thought to cause Alzheimer’s disease.
amyotrophic lateral sclerosis (ALS): Also called Lou Gehrig’s disease. A disease in which motoneurons and their target muscles waste away.
analgesia: Absence of or reduction in pain.
analgesic: Referring to painkilling properties.
analogy: Similarity of function, although the structures of interest may look different. The human hand and an elephant’s trunk are analogous features. Compare homology.
anandamide: An endogenous substance that binds the cannabinoid receptor molecule.
androgen insensitivity syndrome (AIS): A syndrome caused by a mutation of the androgen receptor gene that renders tissues insensitive to androgenic hormones like testosterone. Affected XY individuals are phenotypic females, but they have internal testes and regressed internal genital structures. See Figure 12.16.
androgens: A class of hormones that includes testosterone and other male hormones. See Figure 5.19; Table 5.2.
androstenedione: The chief sex hormone secreted by the human adrenal cortex. Androstenedione is responsible for the adult pattern of body hair in men and women.
angel dust: See phencyclidine.
angiography: A brain-imaging technique in which a specialized X-ray image of the head is taken shortly after the cerebral blood vessels have been filled with a radiopaque dye by means of a catheter. This technique allows visualization of the major blood vessels and is used to assess stroke risk and other conditions.
angiotensin II: A substance that is produced in the blood by the action of renin and that play a role in the control of thirst.
angular gyrus: A brain region in which strokes can lead to word blindness.
anion: A negatively charged ion, such as a protein or chloride ion. Compare cation.
annulospiral ending: See primary sensory ending.
anomia: The inability to name persons or objects readily.
anorexia nervosa: A syndrome in which individuals severely deprive themselves of food.
anosmia: The inability to smell.
anosognosia: Denial of illness.
ANP: See atrial natriuretic peptide.
antagonist: 1. A molecule, usually a drug, that interferes with or prevents the action of a transmitter. Compare agonist (definition 1). 2. A muscle that counteracts the effect of another muscle. Compare agonist (definition 2) and synergist.
anterior: Also called rostral. In anatomy, toward the head end of an organism. See Box 2.2. Compare posterior.
anterior cerebral arteries: Two large arteries, arising from the internal carotids, that provide blood to the anterior poles and medial surfaces of the cerebral hemispheres.
anterior pituitary: Also called adenohypophysis. The front division of the pituitary gland; secretes tropic hormones. See Figures 5.1, 5.14, 5.15; Table 5.2. Compare posterior pituitary.
anterograde amnesia: The inability to form new memories beginning with the onset of a disorder. Compare retrograde amnesia.
anterograde degeneration: Also called Wallerian degeneration. The loss of the distal portion of an axon resulting from injury to the axon. See Box 7.1. Compare retrograde degeneration.
anterograde transport: Movement of cellular substances away from the cell body toward the axon terminals. Compare retrograde transport.
anterolateral system: Also called spinothalamic system. A somatosensory system that carries most of the pain information from the body to the brain. See Figure 8.23. Compare dorsal column system.
antibody: Also called immunoglobulin. A large protein that recognizes and permanently binds to particular shapes, normally as part of the immune system attack on foreign particles.
antidepressants: A class of drugs that relieve the symptoms of depression. Major categories include monoamine oxidase inhibitors, tricyclics, and selective serotonin reuptake inhibitors.
antidiuretic hormone (ADH): See arginine vasopressin.
anti-müllerian hormone (AMH): Also called müllerian regression hormone (MRH). A protein hormone secreted by the fetal testis that inhibits müllerian duct development.
antipsychotics: See neuroleptics.
anxiety disorder: Any of a class of psychological disorders that include recurrent panic states, generalized persistent anxiety disorders, and posttraumatic stress disorders.
anxiolytics: A class of substances that are used to combat anxiety. Examples include alcohol, opiates, barbiturates, and the benzodiazepines.
aphagia: Refusal to eat; often related to damage to the lateral hypothalamus. Compare hyperphagia.
aphasia: An impairment in language understanding and/or production that is caused by brain injury.
apical dendrite: The dendrite that extends from a pyramidal cell to the outermost surface of the cortex. Compare basal dendrite.
ApoE: See apolipoprotein E.
apolipoprotein E (ApoE): A protein that may help break down amyloid. Individuals carrying the ApoE4 allele are more likely to develop Alzheimer’s disease.
apoptosis: See cell death.
APP: See amyloid precursor protein.
appetitive behavior: The second stage of mating behavior; helps establish or maintain sexual interaction. See Figure 12.1.
apraxia: An impairment in the ability to begin and execute skilled voluntary movements, even though there is no muscle paralysis. See also ideational apraxia and ideomotor apraxia.
aquaporins: Channels spanning the cell membrane that are specialized for conducting water molecules into or out of the cell.
arachnoid: The thin covering (one of the three meninges) of the brain that lies between the dura mater and pia mater.
arborization: The elaborate branching of the dendrites of some neurons.
archicortex: See allocortex.
arcuate fasciculus: A tract connecting Wernicke’s speech area to Broca’s speech area. See Figure 19.8.
arcuate nucleus: An arc-shaped hypothalamic nucleus implicated in appetite control. See Figure 13.22.
area 17: See primary visual cortex.
arginine vasopressin (AVP): Also called antidiuretic hormone (ADH) or simply vasopressin. A peptide hormone from the posterior pituitary that promotes water conservation. See Table 5.2.
aromatase: An enzyme that converts many androgens into estrogens.
aromatization: The chemical reaction that converts testosterone to estradiol, and other androgens to other estrogens.
aromatization hypothesis: The hypothesis that testicular androgens enter the brain and are converted there into estrogens to masculinize the developing nervous system of some rodents.
arousal: The global, nonselective level of alertness of an individual.
aspartate: An amino acid transmitter that is excitatory at many synapses.
Asperger’s syndrome: Sometimes called high-functioning autism. A syndrome characterized by difficulties in social cognitive processing; usually accompanied by strong language skills.
associative learning: A type of learning in which an association is formed between two stimuli or between a stimulus and a response; includes both classical and instrumental conditioning. Compare non-associative learning.
astereognosis: The inability to recognize objects by touching and feeling them.
astrocyte: A star-shaped glial cell with numerous processes (extensions) that run in all directions. Astrocyte extensions provide structural support for the brain and may isolate receptive surfaces. See Figure 2.6.
ataxia: An impairment in the direction, extent, and rate of muscular movement; often caused by cerebellar pathology.
atrial natriuretic peptide (ANP): A hormone, secreted by the heart, that normally reduces blood pressure, inhibits drinking, and promotes the excretion of water and salt at the kidneys.
attention: Also called selective attention. A state or condition of selective awareness or perceptual receptivity, by which specific stimuli are selected for enhanced processing. See Figure 8.12.
attention deficit hyperactivity disorder (ADHD): Syndrome of distractibility, impulsiveness, and hyperactivity that, in children, interferes with school performance.
attentional blink: The reduced ability of subjects to detect a target stimulus if it follows another target stimulus by about 200–450 ms.
attentional bottleneck: A filter that results from the limits intrinsic to our attentional processes, with the result that only the most important stimuli are selected for special processing.
attentional spotlight: The shifting of our limited selective attention around the environment to highlight stimuli for enhanced processing.
atypical neuroleptics: A class of antischizophrenic drugs that have actions other than the dopamine D₂ receptor antagonism that characterizes the typical neuroleptics. Atypical neuroleptics often feature selective and high-affinity antagonism of serotonin 5HT₂ receptors.
auditory brainstem implant (ABI): A type of auditory prosthesis in which implanted microphones directly stimulate the auditory nuclei of the brainstem rather than the cochlea. Compare cochlear implant.
auditory P300: See P3 effect.
aura: In epilepsy, the unusual sensations or premonition that may precede the beginning of a seizure. See Box 3.3.
australopithecine: Of or related to Australopithecus, a primate genus, known only from the fossil record, thought to be an ancestor to humans. See Figure 6.17.
autism: A disorder arising during childhood, characterized by social withdrawal and perseverative behavior.
autocrine: Referring to a signal that is secreted by a cell into its environment and that feeds back to the same cell. See Figure 5.3. Compare paracrine.
autoimmune disorder: A disorder caused when the immune system mistakenly attacks a person’s own body, thereby interfering with normal functioning.
autonomic ganglia: Collections of nerve cell bodies, belonging to the autonomic division of the peripheral nervous system, that are found in various locations and innervate the major organs.
autonomic nervous system: The part of the peripheral nervous system that supplies neural connections to glands and to smooth muscles of internal organs. Its two divisions (sympathetic and parasympathetic) act in opposite fashion. See Figure 2.11.
autoradiography: A histological technique that shows the distribution of radioactive chemicals in tissues. See Boxes 2.1, 5.1.
autoreceptor: A receptor for a synaptic transmitter that is located in the presynaptic membrane and tells the axon terminal how much transmitter has been released.
AVP: See arginine vasopressin.
axo-axonic: Referring to a synapse in which a presynaptic axon terminal synapses onto another axon’s terminal. Compare axo-dendritic, axo-somatic, and dendro-dendritic.
axo-dendritic: Referring to a synapse in which a presynaptic axon terminal synapses onto a dendrite of the postsynaptic neuron, either via a dendritic spine or directly onto the dendrite itself. Compare axo-axonic, axo-somatic, and dendro-dendritic.
axo-somatic: Referring to a synapse in which a presynaptic axon terminal synapses onto the cell body (soma) of the postsynaptic neuron. Compare axo-axonic, axo-dendritic, and dendro-dendritic.
axon: A single extension from the nerve cell that carries nerve impulses from the cell body to other neurons. See Figure 2.2.
axon collateral: A branch of an axon from a single neuron.
axon hillock: A cone-shaped area from which the axon originates out of the cell body. Functionally, the integration zone of the neuron. See Figure 2.7.
axon terminal: The end of an axon or axon collateral, which forms a synapse on a neuron or other target cell.
axonal transport: The transportation of materials from the neuronal cell body to distant regions in the dendrites and axons, and from the axon terminals back to the cell body.

B

B cell: See B lymphocyte.
B lymphocyte: Also called B cell. An immune system cell, formed in the bone marrow (hence the B), that mediates humoral immunity. Compare T lymphocyte. See Figure 15.22.
ballistic movement: A rapid muscular movement that is often organized or programmed in the cerebellum. Compare ramp movement.
bar detector: See simple cortical cell.
bariatric: Having to do with obesity.
baroreceptor: A pressure receptor in the heart or a major artery that detects a fall in blood pressure.
basal dendrite: One of several dendrites on a pyramidal cell that extend horizontally from the cell body. Compare apical dendrite.
basal forebrain: A ventral region in the forebrain that has been implicated in sleep and Alzheimer’s disease. See Figure 4.2.
basal ganglia: A group of forebrain nuclei, including caudate nucleus, globus pallidus, and putamen, found deep within the cerebral hemispheres. See Figures 2.13, 2.15, 11.17.
basal metabolism: The consumption of energy to fuel processes such as heat production, maintenance of membrane potentials, and all the other basic life-sustaining functions of the body.
basilar artery: An artery, formed by the fusion of the vertebral arteries, that supplies blood to the brainstem and to posterior cerebral arteries. See Figure 2.20.
basilar membrane: A membrane in the cochlea that contains the principal structures involved in auditory transduction. See Figures 9.2, 9.3.
batrachotoxin: A toxin, produced by poison arrow frogs, that selectively interferes with Na⁺ channels.
Bcl-2: A family of proteins that regulate apoptosis.
BDNF: See brain-derived neurotrophic factor.
behavioral intervention: An approach to finding relations between body variables and behavioral variables that involves intervening in the behavior of an organism and looking for resultant changes in body structure or function. See Figure 1.2. Compare somatic intervention.
behavioral medicine: See health psychology.
behavioral neuroscience: See biological psychology.
behavioral teratology: The study of impairments in behavior that are produced by embryonic or fetal exposure to toxic substances.
Bell’s palsy: A disorder, usually caused by viral infection, in which the facial nerve on one side stops conducting action potentials, resulting in paralysis on one side of the face. See Figure 15.7.
benzodiazepine agonists: A class of anti-anxiety drugs that bind to sites on GABA_A receptors.
benzodiazepines: A class of antianxiety drugs that bind with high affinity to receptor molecules in the central nervous system; one example is diazepam (Valium).
beta activity: See desynchronized EEG.
β-amyloid: A protein that accumulates in senile plaques in Alzheimer’s disease.
β-secretase: An enzyme that cleaves amyloid precursor protein, forming β-amyloid, which can lead to Alzheimer’s disease. See also presenilin.
bigamy: A mating system in which an individual has two mates or spouses. Compare monogamy and polygamy.
binaural: Pertaining to two ears. Compare monaural.
binding affinity: Also called simply affinity. The propensity of molecules of a drug (or other ligand) to bind to their corresponding receptors. Drugs with high affinity for their receptors are effective even at low doses.
binding problem: The question of how the brain understands which individual attributes blend together into a single object, when these different features are processed by different regions in the brain.
binge eating: The paroxysmal intake of large quantities of food, often of poor nutritional value and high calories.
binocular deprivation: Depriving both eyes of form vision, as by sealing the eyelids. Compare monocular deprivation.
bioavailable: Referring to a substance, usually a drug, that is present in the body in a form that is able to interact with physiological mechanisms.
biological psychology: Also called behavioral neuroscience. The study of the biological bases of psychological processes and behavior.
biotransformation: The process in which enzymes convert a drug into a metabolite that is itself active, possibly in ways that are substantially different from the actions of the original substance.
bipolar cells: A class of interneurons of the retina that receive information from rods and cones and pass the information to retinal ganglion cells. See Figure 10.6. See also amacrine cells.
bipolar disorder: Also called manic-depressive illness. A psychiatric disorder characterized by periods of depression that alternate with excessive, expansive moods. Compare unipolar depression.
bipolar neuron: A nerve cell that has a single dendrite at one end and a single axon at the other end; found in some vertebrate sensory systems. See Figure 2.5. Compare unipolar neuron and multipolar neuron.
blind spot: The portion of the visual field from which light falls on the optic disc. Because there are no receptors in this region, light striking it cannot be seen.
blob: Also called peg. A region of visual cortex distinguished by stains for the enzyme cytochrome oxidase.
blood-brain barrier: The mechanisms that make the movement of substances from blood vessels into brain cells more difficult than exchanges in other body organs, thus affording the brain greater protection from exposure to some substances found in the blood.
blotting: Transferring DNA, RNA, or protein fragments to nitrocellulose following separation via gel electrophoresis. The blotted substance can then be labeled.
border cell: A neuron that selectively fires when the animal arrives at the perimeter of the local spatial cognitive map.
bottom-up process: A process in which lower-order mechanisms, like sensory inputs, trigger further processing by higher-order systems. There may be no conscious awareness until late in the process. Exogenous attention is one example. Compare top-down process.
bovine spongiform encephalopathy (BSE): Mad cow disease, a disorder caused by improperly formed prion proteins, leading to dementia and death. See also Creutzfeldt-Jakob disease.
brain-derived neurotrophic factor (BDNF): A protein purified from the brains of animals that can keep some classes of neurons alive.
brain self-stimulation: The process in which animals will work to provide electrical stimulation to particular brain sites, presumably because the experience is very rewarding.
brainstem: The region of the brain that consists of the midbrain, the pons, and the medulla.
brightness: One of three basic dimensions (along with hue and saturation) of light perception. Brightness varies from dark to light. See Figure 10.23.
Broca’s aphasia: See nonfluent aphasia.
Broca’s area: A region of the left frontal lobe of the brain that is involved in the production of speech. See Figures 19.6, 19.7, 19.8.
Brodmann’s areas: A classification of cortical regions based on subtle variations in the relative appearance of the six layers of neocortex.
brown fat: Also called brown adipose tissue. A specialized type of fat tissue that generates heat through intense metabolism.
BSE: See bovine spongiform encephalopathy.
bulimia: Also called bulimia nervosa. A syndrome in which individuals periodically gorge themselves, usually with “junk food,” and then either vomit or take laxatives to avoid weight gain.
bungarotoxin: A neurotoxin, isolated from the venom of the banded krait, that selectively blocks acetylcholine receptors.

C

C fiber: A small, unmyelinated axon that conducts pain information slowly and adapts slowly. See Table 8.2. Compare Aδ fiber.
c-fos: An immediate early gene commonly used to identify activated neurons.See Box 2.1.
caffeine: A stimulant compound found in coffee, cacao, and other plants.
CAH: See congenital adrenal hyperplasia.
calcium ion (Ca²⁺): A calcium atom that carries a double positive charge because it has lost two electrons.
CAM: See cell adhesion molecule.
cAMP: See cyclic adenosine monophosphate.
cAMP responsive element–bindingprotein: See CREB.
capsaicin: A compound synthesized by various plants to deter predators by mimicking the experience of burning. Capsaicin is responsible for the burning sensation in chili peppers.
carotid arteries: The major arteries that ascend the left and right sides of the neck to the brain, supplying blood to the anterior and middle cerebral arteries. The branch that enters the brain is called the internal carotid artery. See Figure 2.20.
CART: See cocaine- and amphetamine-regulated transcript.
caspases: A family of proteins that regulate cell death (apoptosis).
castration: Removal of the gonads, usually the testes.
CAT or CT scan: See computerized axial tomography.
cataplexy: Sudden loss of muscle tone, leading to collapse of the body without loss of consciousness.
catecholamines: A class of monoamines that serve as neurotransmitters, including dopamine and norepinephrine. See Table 4.1.
cation: A positively charged ion, such as a potassium or sodium ion. Compare anion.
cauda equina: Literally “horse’s tail” (in Latin). The caudalmost spinal nerves, which extend beyond the spinal cord proper to exit the spinal column.
caudal: See posterior.
caudate nucleus: One of the basal ganglia; it has a long extension or tail. See Figure 2.15.
CBT: See cognitive behavioral therapy.
CCK: See cholecystokinin.
cell adhesion molecule (CAM): A protein found on the surface of a cell that guides cell migration and/or axonal pathfinding.
cell assembly: A large group of cells that tend to be active at the same time because they have been activated simultaneously or in close succession in the past.
cell-autonomous: Referring to cell processes that are directed by the cell itself rather than being under the influence of other cells.
cell body: Also called soma. The region of a neuron that is defined by the presence of the cell nucleus. See Figure 2.2.
cell-cell interactions: The general process during development in which one cell affects the differentiation of other, usually neighboring, cells.
cell death: Also called apoptosis. The developmental process during which “surplus” cells die. See Figure 7.3.
cell differentiation: The developmental stage in which cells acquire distinctive characteristics, such as those of neurons, as the result of expressing particular genes. See Figure 7.3.
cell membrane: The lipid bilayer that ensheathes a cell.
cell migration: The movement of cells from site of origin to final location. See Figure 7.3.
cell nucleus: The spherical central structure of a cell that contains the chromosomes.
central deafness: A hearing impairment that is related to lesions in auditory pathways or centers, including sites in the brainstem, thalamus, or cortex. Cortical deafness and word deafness are two examples of central deafness. Compare conduction deafness and sensorineural deafness.
central modulation of sensory information: The process in which higher brain centers, such as the cortex and thalamus, suppress some sources of sensory information and amplify others.
central nervous system (CNS): The portion of the nervous system that includes the brain and the spinal cord. See Figures 2.8, 2.14. Compare peripheral nervous system.
central pattern generator: Neural circuitry that is responsible for generating the rhythmic pattern of a behavior such as walking.
central sulcus: A fissure that divides the frontal lobe from the parietal lobe. See Figure 2.12.
cerebellum: A structure located at the back of the brain, dorsal to the pons, that is involved in the central regulation of movement. See Figures 2.12, 2.14, 2.16.
cerebral cortex: Often called simply cortex. The outer covering of the cerebral hemispheres, which consists largely of nerve cell bodies and their branches. In mammals, the cerebral cortex has the six distinct layers that are typical of neocortex. See Figure 2.17.
cerebral hemispheres: The right and left halves of the forebrain. See Figure 2.14.
cerebrocerebellum: The lowermost part of the cerebellum, consisting especially of the lateral parts of each cerebellar hemispehere. It is implicated in planning complex movements. Compare spinocerebellum and vestibulocerebellum.
cerebrospinal fluid (CSF): The fluid that fills the cerebral ventricles. See Figure 2.19.
cerveau isolé: See isolated forebrain.
cervical: Referring to topmost eight segments of the spinal cord, in the neck region. See Figures 2.10, 2.11.
cGMP: See cyclic guanosine monophosphate.
change blindness: A failure to notice changes in comparisons of two alternating static visual scenes.
channelopathy: A genetic abnormality of ion channels, causing a variety of symptoms.
ChAT: See choline acetyltransferase.
chemical neuroanatomy: The distribution of key chemicals, such as transmitters and enzymes, within the structure of the nervous system.
chemical transmitter: See neurotransmitter.
chemically gated ion channel: See ligand-gated ion channel.
chemoaffinity hypothesis: The notion that each cell has a chemical identity that directs it to synapse on the proper target cell during development. See Box 7.2.
chemoattractants: Compounds that attract particular classes of growth cones. Compare chemorepellents.
chemorepellents: Compounds that repel particular classes of growth cones. Compare chemoattractants.
chloride ion (Cl^–): A chlorine atom that carries a negative charge because it has gained one electron.
chlorpromazine: An antipsychotic drug, one of the class of phenothiazines.
cholecystokinin (CCK): A peptidehormone that is released by the gutafter ingestion of food high in protein and/or fat.
choline acetyltransferase (ChAT): An important enzyme involved in the synthesis of the neurotransmitter acetylcholine.
cholinergic: Referring to cells that use acetylcholine as their synaptic transmitter.
choroid plexus: A highly vascular portion of the lining of the ventricles that secretes cerebrospinal fluid.
chromosome: A complex of condensed strands of DNA and associated protein molecules; found in the nucleus of cells.
chronic traumatic encephalopathy (CTE): Also called dementia pugilistica or punch-drunk. The dementia that develops in boxers; it is especially prominent in successful boxers because they participate in more bouts.
ciliary muscle: One of the muscles that controls the shape of the lens inside the eye, focusing an image on the retina. See Figure 10.5.
cilium (pl. cilia): A hairlike extension. The extensions in the hair cells of the cochlea, for example, are cilia. See Figure 9.2.
cingulate cortex: Also called cingulate gyrus or cingulum. A region of medial cerebral cortex that lies dorsal to the corpus callosum. See Figures 2.15, 16.19.
cingulate gyrus: Also called cingulate cortex or cingulum. A cortical portion of the limbic system, found in the frontal and parietal midline. See Figures 2.12, 2.15.
cingulum (pl. cingula): See cingulate cortex or cingulate gyrus.
circadian rhythm: A pattern of behavioral, biochemical, or physiological fluctuation that has a 24-hour period.
circannual: Occurring on a roughly annual basis.
circle of Willis: A structure at the base of the brain that is formed by the joining of the carotid and basilar arteries. See Figure 2.20.
circumvallate papillae: One of three types of small structures on the tongue, located in the back, that contain taste receptors. See Figure 9.19. Compare foliate papillae and fungiform papillae.
circumventricular organ: An organ that lies in the wall of a cerebral ventricle and monitors the composition of the cerebrospinal fluid. See Figure 13.14.
CJD: See Creutzfeldt-Jakob disease.
classical conditioning: Also called Pavlovian conditioning. A type of associative learning in which an originally neutral stimulus (the conditioned stimulus, or CS)—through pairing with another stimulus (the unconditioned stimulus, or US) that elicits a particular response—acquires the power to elicit that response when presented alone. A response elicited by the US is called an unconditioned response (UR); a response elicited by the CS alone is called a conditioned response (CR). See Box 17.1. Compare instrumental conditioning.
cloaca: The sex organ in many birds, through which sperm are discharged (in the male) and eggs are laid (in the female). This is the same passage through which wastes are eliminated.
cloacal exstrophy: A rare medical condition in which XY individuals are born completely lacking a penis.
clones: Asexually produced organisms that are genetically identical.
closed-loop control mechanism: A control mechanism that provides a flow of information from whatever is being controlled to the device that controls it. See Figure 11.3. Compare open-loop control mechanism.
clozapine: An atypical neuroleptic.
CMR1: See cool-menthol receptor 1.
CNS: See central nervous system.
cocaine: A drug of abuse, derived from the coca plant, that acts by potentiating catecholamine stimulation.
cocaine- and amphetamine-regulated transcript (CART): A peptide produced in the brain when an animal is injected with either cocaine or amphetamine. It is also associated with the appetite control circuitry of the hypothalamus.
coccygeal: Referring to the lowest spinal vertebra (also known as the tailbone). See Figure 2.11.
cochlea: A snail-shaped structure in the inner ear that contains the primary receptor cells for hearing. See Figure 9.2.
cochlear amplifier: The mechanism by which the cochlea is physically distorted by outer hair cells in order to “tune” the cochlea to be particularly sensitive to some frequencies more than others.
cochlear implant: An electromechanical device that detects sounds and selectively stimulates nerves in different regions of the cochlea via surgically implanted electrodes. Compare auditory brainstem implant.
cochlear nuclei: Brainstem nuclei that receive input from auditory hair cells and send output to the superior olivary complex. See Figure 9.7.
cocktail party effect: The selective enhancement of attention in order to filter out distracters, such as while listening to one person talking in the midst of a noisy party.
coding: The rules by which action potentials in a sensory system reflect a physical stimulus.
codon: A set of three nucleotides that uniquely encodes one particular amino acid. A series of codons determines the structure of a peptide or protein.
cognitive behavioral therapy (CBT): Psychotherapy aimed at correcting negative thinking and improving interpersonal relationships.
cognitive map: A mental representation of a spatial relationship.
cognitively impenetrable: Referring to data-processing operations of the central nervous system that are unconscious.
coincidence detector: A device that senses the co-occurrence of two events.
coitus: See copulation.
collateral sprouting: The formation of a new branch on an axon, usually in response to the uncovering of unoccupied postsynaptic sites.
co-localization: Also called co-release. Here, the appearance of more than one neurotransmitter in a given presynaptic terminal.
combat fatigue: See posttraumatic stress disorder.
co-morbid: Referring to the tendency of certain diseases or disorders to occur together in individuals.
competitive ligand: A substance that directly competes with the endogenous ligand for binding to a receptor molecule. See Figure 4.7. Compare noncompetitive ligand.
complex cortical cell: A cell in the visual cortex that responds best to a bar of a particular size and orientation anywhere within a particular area of the visual field. Compare simple cortical cell.
complex environment: See enriched condition.
complex partial seizure: In epilepsy, a type of seizure that doesn’t involve the entire brain, and therefore can cause a wide variety of symptoms. See Box 3.3.
computerized axial tomography (CAT or CT): A noninvasive technique for examining brain structure in humans through computer analysis of X-ray absorption at several positions around the head. CT affords a virtual direct view of the brain. The resulting images are referred to as CAT scans or CT scans. See Figure 2.21.
concentration gradient: Variation of the concentration of a substance within a region. Molecules and ions tend to move down the concentration gradient from areas of high concentration to areas of low concentration. See Figure 3.2.
concordant: Referring to any trait that is seen in both individuals of a pair of twins. Compare discordant.
conditional knockout: A gene that can be selectively deactivated in adulthood in specific tissues.
conditioned response (CR): See classical conditioning.
conditioned stimulus (CS): See classical conditioning.
conditioning: A form of learning in which an organism comes to associate two stimuli, or a stimulus and a response. See Box 17.1. See also classical conditioning and instrumental conditioning.
conduction aphasia: An impairment in the repetition of words and sentences.
conduction deafness: A hearing impairment that is associated with pathology of the external-ear or middle-ear cavities. Compare central deafness and sensorineural deafness.
conduction velocity: The speed at which an action potential is propagated along the length of an axon (or section of peripheral nerve).
conduction zone: The part of the neuron over which the nerve’s electrical signal may be actively propagated. Usually corresponds to the cell’s axon.
cones: A class of photoreceptor cells in the retina that are responsible for color vision. See Figure 10.6. Compare rods.
confabulate: To fill in a gap in memory with a falsification; often seen in Korsakoff’s syndrome.
congenital adrenal hyperplasia (CAH): Any of several genetic mutations that can result in exposure of a female fetus to adrenal androgens, which results in a clitoris that is larger than normal at birth.
congenital hypothyroidism: See cretinism.
congenital insensitivity to pain: The condition of being born without the ability to perceive pain.
congenital prosopagnosia: See prosopagnosia.
conjunction search: A search for an item that is based on two or more features (e.g., size and color) that together distinguish the target from distracters that may share some of the same attributes. Compare feature search.
consciousness: The state of awareness of one’s own existence and experience.
conserved: In the context of evolution, referring to a trait that is passed on from a common ancestor to two or more descendant species.
consolidation: A stage of memory formation in which information in short-term or intermediate-term memory is transferred to long-term memory. See Figure 17.7.
constraint-induced movement therapy: A therapy for recovery of movement after stroke or injury in which the person’s unaffected limb is constrained while he is required to perform tasks with the affected limb.
contralateral: In anatomy, pertaining to a location on the opposite side of the body. See Box 2.2. Compare ipsilateral.
convergence: The phenomenon of neural connections in which many cells send signals to a single cell. Compare divergence.
convergent evolution: The evolutionary process by which responses to similar ecological features bring about similarities in behavior or structure among animals that are only distantly related (i.e., that differ in genetic heritage).
cool-menthol receptor 1 (CMR1): Also called TRP8. A sensory receptor, found in some free nerve endings, that opens an ion channel in response to a mild temperature drop or exposure to menthol. See Figure 8.22.
Coolidge effect: The propensity of an animal that has appeared sexually satiated with a present partner to resume sexual activity when provided with a novel partner.
copulation: Also called coitus. The sexual act.
copulatory lock: Reproductive behavior in which the male’s penis swells after ejaculation so that the male and female are forced to remain joined for 5–10 minutes; occurs in dogs and some rodents, but not in humans.
co-release: See co-localization.
cornea: The transparent outer layer of the eye, whose curvature is fixed. It bends light rays and is primarily responsible for forming the image on the retina. See Figure 10.5.
coronal plane: Also called frontal plane or transverse plane. The plane that divides the body or brain into front and back parts. See Box 2.2. Compare horizontal plane and sagittal plane.
corpora lutea (sing. corpus luteum): The structures formed from collapsed ovarian follicles subsequent to ovulation. The corpora lutea are a major source of progesterone.
corpus callosum: The main band of axons that connects the two cerebral hemispheres. See Figures 2.12, 2.18.
correlation: The covariation of two measures.
cortex (pl. cortices): The outer layer of a structure. See also cerebral cortex and neocortex.
cortical column: One of the vertical columns that constitute the basic organization of the neocortex.
cortical deafness: A hearing impairment that is caused by a fault or defect in the cortex.
corticospinal system: See pyramidal system.
cortisol: A glucocorticoid stress hormone of the adrenal cortex.
courtship: The period during which two potential sexual partners increase their attractiveness toward each other.
covert attention: Attention in which the focus can be directed independently of sensory orientation (e.g., you’re attending to one sensory stimulus while looking at another). Compare overt attention.
CR: See classical conditioning.
cranial nerve: A nerve that is connected directly to the brain. Composed of a set of pathways concerned mainly with sensory and motor systems associated with the head, the cranial nerves together constitute one of the three main subdivisions of the peripheral nervous system. There are 12 cranial nerves, typically designated by Roman numerals I–XII. See Figure 2.9.
CREB: cAMP responsive element–binding protein. A protein that is activated by cyclic AMP (cAMP) so that it now binds the promoter region of several genes involved in neural plasticity. See Figure 17.22.
cretinism: Also called congenital hypothyroidism. Reduced stature and intellectual disability caused by thyroid deficiency during early development.
Creutzfeldt-Jakob disease (CJD): A brain disorder in humans, leading to dementia and death, that is caused by improperly folded prion proteins. CJD is the human equivalent of bovine spongiform encephalopathy, or mad cow disease.
crib death: See sudden infant death syndrome.
cross-tolerance: A condition in which the development of tolerance for an administered drug causes an individual to develop tolerance for another drug.
crystallization: The final stage of birdsong formation, in which fully formed adult song is achieved.
CS: See classical conditioning.
CSF: See cerebrospinal fluid.
CT or CAT scan: See computerized axial tomography.
CTE: See chronic traumatic encephalopathy.
cue-induced drug use: An increased likelihood to use a drug (especially an addictive drug) because of the presence of environmental stimuli that were present during previous use of the same drug.
cupula: A small gelatinous column that forms part of the lateral-line system of aquatic animals and also occurs within the vestibular system of mammals. See Figure 9.16.
curare: An alkaloid neurotoxin that causes paralysis by blocking acetylcholine receptors in muscle.
Cushing’s syndrome: A condition in which levels of adrenal glucocorticoids are abnormally high.
cyclic adenosine monophosphate (cyclic AMP, or cAMP): A second messenger activated in target cells in response to synaptic or hormonal stimulation.
cyclic AMP: See cyclic adenosine monophosphate.
cyclic GMP: See cyclic guanosine monophosphate.
cyclic guanosine monophosphate (cyclic GMP, or cGMP): A second messenger activated in target cells in response to synaptic or hormonal stimulation.
cytokine: A protein that induces the proliferation of other cells, as in the immune system. Examples include interleukins and interferons.
cytoplasm: See intracellular fluid.
cytoskeleton: The lattice of specialized proteins that gives a cell its shape. Changes in the cytoskeleton allow neurons to change their shape and form new connections; therefore the cytoskeleton plays an important role in neural plasticity.

D

DA: See dopamine.
dB: See decibel.
DBS: See deep brain stimulation.
death gene: A gene that is expressed only when a cell becomes committed to natural cell death (apoptosis).
decibel (dB): A measure of sound intensity. See Box 9.1.
declarative memory: A memory that can be stated or described. See Figures 17.3, 17.5, 17.8. Compare nondeclarative memory.
decomposition of movement: Difficulty of movement in which gestures are broken up into individual segments instead of being executed smoothly; a symptom of cerebellar lesions.
decorticate rage: Also called sham rage. Sudden intense rage characterized by actions (such as snarling and biting in dogs) that lack clear direction.
deep brain stimulation (DBS): Mild electrical stimulation through an electrode that is surgically implanted deep in the brain.
deep dyslexia: Acquired dyslexia in which the patient reads a word as another word that is semantically related. Compare surface dyslexia.
degradation: The chemical breakdown of a neurotransmitter into inactive metabolites.
dehydration: Excessive loss of water.
delay conditioning: A form of conditioning in which only a brief delay separates the conditioned and unconditioned stimuli. Compare trace conditioning.
delayed non-matching-to-sample task: A test in which the subject must respond to the unfamiliar stimulus of a pair. See Figure 17.9.
delta wave: The slowest type of EEG wave, characteristic of stages 3 and 4 slow-wave sleep. See Figure 14.11.
Δ9-tetrahydrocannabinol (THC): The major active ingredient in marijuana.
delusion: A false belief strongly held in spite of contrary evidence.
dementia: Drastic failure of cognitive ability, including memory failure and loss of orientation.
dementia pugilistica: See chronic traumatic encephalopathy.
dendrite: One of the extensions of the cell body that are the receptive surfaces of the neuron. See Figure 2.5.
dendritic knob: A portion of olfactory receptor cells present in the olfactory epithelium. See Figure 9.22.
dendritic spine: An outgrowth along the dendrite of a neuron. See Figure 2.7.
dendro-dendritic: Referring to a type of synapse in which a synaptic connection forms between the dendrites of two neurons. Compare axo-axonic, axo-dendritic, and axo-somatic.
dentate gyrus: A strip of gray matter in the hippocampal formation. See Figure 17.21.
deoxyribonucleic acid (DNA): A nucleic acid that is present in the chromosomes of cells and codes hereditary information. Compare ribonucleic acid.
dependence: Also called addiction. In the context of substance-related disorders, the strong desire to self-administer a drug of abuse.
dependent variable: The factor that an experimenter measures to monitor a change in response to changes in an independent variable.
depolarization: A reduction in membrane potential (the interior of the neuron becomes less negative). See Figure 3.5. Compare hyperpolarization.
depressants: A class of drugs that act to reduce neural activity.
depression: A psychiatric condition characterized by such symptoms as an unhappy mood; loss of interests, energy, and appetite; and difficulty concentrating. See also bipolar disorder and unipolar depression.
dermatome: A strip of skin innervated by a particular spinal root. See Figure 8.16.
dermis: The middle layer of skin, between the epidermis and the hypodermis. See Figure 8.4.
desynchronized EEG: Also called beta activity. A pattern of EEG activity comprising a mix of many different high frequencies with low amplitude. Compare alpha rhythm.
developmental dyslexia: See dyslexia.
developmental prosopagnosia: See prosopagnosia.
dexamethasone suppression test: A test of pituitary-adrenal function in which the subject is given dexamethasone, a synthetic glucocorticoid hormone, which should cause a decline in the production of adrenal corticosteroids.
DHT: See dihydrotestosterone.
diabetes insipidus: Excessive urination, caused by the failure of vasopressin to induce the kidneys to conserve water.
diabetes mellitus: Excessive glucose in the urine, caused by the failure of insulin to induce glucose absorption by the body. Two types of diabetes mellitus are known: Type I (juvenile-onset) and Type II (adult-onset).
Diablo: A protein released by mitochondria, in response to high calcium levels, that activates apoptosis.
diaschisis: A temporary period of generalized impairment following brain injury.
dichotic presentation: The simultaneous delivery of different stimuli to both the right and the left ears at the same time. See Figure 19.16.
diencephalon: The posterior part of the forebrain, including the thalamus and hypothalamus. See Figure 2.14.
differentiation: See cell differentiation.
diffusion: The spontaneous spread of molecules of one substance among molecules of another substance until a uniform concentration is achieved. See Figure 3.2.
diffusion tensor imaging (DTI): A special use of MRI that takes advantage of the differences in how water molecules are constrained in myelin to reveal axonal tracts connecting brain regions.
digestion: The process by which food is broken down to provide energy and nutrients.
dihydrotestosterone (DHT): The 5α-reduced metabolite of testosterone; a potent androgen that is principally responsible for the masculinization of the external genitalia in mammalian sexual differentiation. See Figure 12.14.
dimer: A complex of two proteins that have bound together.
dioecious: Having distinct male and female sexes that specialize in making just one type of gamete, either ova or sperm.
discordant: Referring to any trait that is seen in only one individual of a pair of twins. Compare concordant.
dishabituation: The restoration of response amplitude following habituation.
dissociative drug: A type of drug that produces a dreamlike state in which consciousness is partly separated from sensory inputs.
dissociative thinking: A condition, seen in schizophrenia, that is characterized by disturbances of thought and difficulty relating events properly.
distal: In anatomy, toward the periphery of an organism or toward the end of a limb. See Box 2.2. Compare proximal.
diurnal: Active during the light periods of the daily cycle. Compare nocturnal.
divergence: The phenomenon of neural connections in which one cell sends signals to many other cells. See Figure 3.18. Compare convergence.
divided attention task: A task in which the subject is asked to simultaneously focus attention on two or more stimuli.
dizygotic: Referring to twins derived from separate eggs (fraternal twins). Such twins are no more closely related genetically than are other full siblings. Compare monozygotic.
DNA: See deoxyribonucleic acid.
DNA sequencing: The process by which the order of nucleotides in a gene, or amino acids in a protein, is determined.
dopamine (DA): A monoamine transmitter found in the midbrain—especially the substantia nigra—and basal forebrain. See Table 4.1; Figure 4.3.
dopamine hypothesis: The hypothesis that schizophrenia results from either excessive levels of synaptic dopamine or excessive postsynaptic sensitivity to dopamine.
dorsal: In anatomy, toward the back of the body or the top of the brain. See Box 2.2. Compare ventral.
dorsal column system: A somatosensory system that delivers most touch stimuli via the dorsal columns of spinal white matter to the brain. See Figure 8.15. Compare anterolateral system.
dorsal root: See roots.
dose-response curve (DRC): A formal plot of a drug’s effects (on the y-axis) versus the dose given (on the x-axis). Analysis of dose-response curves can provide a range of information about the drug, such as its efficacy, potency, and safety. See Figure 4.8.
double-blind test: A test of a drug or treatment in which neither the subjects nor the attending researchers know which subjects are receiving the drug (treatment) and which are receiving the placebo (control).
down-regulation: A compensatory decrease in receptor availability at the synapses of a neuron. Compare up-regulation.
Down syndrome: Intellectual disability that is associated with an extra copy of chromosome 21. See Figure 7.18.
DRC: See dose-response curve.
DTI: See diffusion tensor imaging.
dual dependence: Dependence for emergent drug effects that occur only when two drugs are taken simultaneously.
dualism: The notion, promoted by René Descartes, that the mind is subject only to spiritual interactions, while the body is subject only to material interactions.
duplex theory: A theory that we localize sound by combining information about intensity differences and latency differences between the two ears.
dura mater: The outermost of the three meninges that surround the brain and spinal cord. See also pia mater and arachnoid.
dynein: A protein “motor” that moves substances in axonal transport. See also kinesin.
dynorphins: One of three kinds of endogenous opioids. Enkephalins and endorphins are the other two. See Table 4.1.
dyskinesia: Difficulty or distortion in voluntary movement.
dyslexia: A reading disorder attributed to brain impairment. Acquired dyslexia occurs as a result of injury or disease. Developmental dyslexia is associated with brain abnormalities present from birth.
dysphoria: Unpleasant feelings; the opposite of euphoria.
dystrophin: A protein that is needed for normal muscle function. Dystrophin is defective in some forms of muscular dystrophy.

E

eardrum: See tympanic membrane.
early-selection model: A model of attention postulating that the attentional bottleneck imposed by the nervous system can exert control early in the processing pathway, filtering out stimuli before even preliminary perceptual analysis has occurred. Compare late-selection model.
easy problem of consciousness: The problem of how to read current conscious experiences directly from people’s brains as they’re happening. Compare hard problem of consciousness.
EC: See enriched condition.
ecological niche: The unique assortment of environmental opportunities and challenges to which each organism is adapted.
Ecstasy: See MDMA.
ECT: See electroconvulsive shock therapy.
ectoderm: The outer cellular layer of the developing fetus. The ectoderm gives rise to the skin and the nervous system.
ectopia: Something out of place—for example, clusters of neurons seen in unusual positions in the cortex of someone suffering from dyslexia. See Figure 19.10.
ectopic transmission: Cell-cell communication based on release of neurotransmitter in regions outside traditional synapses.
ectotherm: An animal whose body temperature is regulated by, and whose heat comes mainly from, the environment. Examples include snakes and bees. Compare endotherm.
ED₅₀: Effective dose 50%; the dose of a drug that is required to produce half of its maximal effect. See Figure 4.8.
edema: The swelling of tissue, especially in the brain, in response to injury.
edge detector: See simple cortical cell.
EEG: See electroencephalography.
efferent: In reference to an axon, carrying information from the nervous system to the periphery. See Box 2.2. Compare afferent.
efficacy: Also called intrinsic activity. The extent to which a drug activates a response when it binds to a receptor. Receptor antagonist drugs have low efficacy; receptor agonists have high efficacy. See Figure 4.8.
egg: See ovum.
ejaculation: The forceful expulsion of semen from the penis.
electrical synapse: Also called gap junction. The region between neurons where the presynaptic and postsynaptic membranes are so close that the action potential can jump to the postsynaptic membrane without first being translated into a chemical message. See Box 3.2.
electroconvulsive shock therapy (ECT): A last-resort treatment for intractable depression in which a strong electrical current is passed through the brain, causing a seizure. Rapid relief from depressive symptoms often results, associated with improved accumulation of monoamine neurotransmitters in the brain.
electroencephalography (EEG): The recording and study of gross electrical activity of the brain recorded from large electrodes placed on the scalp. The abbreviation EEG may refer either to the process of encephalography or to its product, the encephalogram. See Figures 3.19, 14.11.
electromyography (EMG): The electrical recording of muscle activity. See Figure 11.2.
electro-oculography (EOG): The electrical recording of eye movements, useful in determining sleep stages.
electrostatic pressure: The propensity of charged molecules or ions to move, via diffusion, toward areas with the opposite charge.
embryo: The earliest stage in a developing animal. Humans are considered to be embryos until 8–10 weeks after conception.
embryonic stem cell: A cell, derived from an embryo, that has the capacity to form any type of tissue that a donor might produce.
EMG: See electromyography.
emotional dyscontrol syndrome: A condition consisting of temporal lobe disorders that may underlie some forms of human violence.
encéphale isolé: See isolated brain.
encephalization factor: A measure of brain size relative to body size.
encoding: A stage of memory formation in which the information entering sensory channels is passed into short-term memory. See Figure 17.7.
endocannabinoid: An endogenous ligand of cannabinoid receptors; thus, an analog of marijuana that is produced by the brain.
endocast: A cast of the cranial cavity of a skull, especially useful for studying fossils of extinct species.
endocrine: Referring to glands that release chemicals to the interior of the body. These glands secrete the principal hormones. See Figure 5.3.
endocrine gland: A gland that secretes products into the bloodstream to act on distant targets. See Figure 5.1. Compare exocrine gland.
endogenous: Produced inside the body. Compare exogenous.
endogenous attention: Also called voluntary attention. The voluntary direction of attention toward specific aspects of the environment, in accordance with our interests and goals. Compare exogenous attention.
endogenous ligand: Any substance, produced within the body, that selectively binds to the type of receptor that is under study. Compare exogenous ligand.
endogenous opioids: A family of peptide transmitters that have been called the body’s own narcotics. The three kinds are enkephalins, endorphins, and dynorphins. See Table 4.1.
endorphins: One of three kinds of endogenous opioids. Enkephalins and dynorphins are the other two. See Table 4.1.
endotherm: An animal whose body temperature is regulated chiefly by internal metabolic processes. Examples include mammals and birds. Compare ectotherm.
enkephalins: One of three kinds of endogenous opioids. Endorphins and dynorphins are the other two. See Table 4.1.
enriched condition (EC): Also called complex environment. A condition in which laboratory rodents are group-housed with a wide variety of stimulus objects. See Figure 17.17. Compare impoverished condition and standard condition.
enteric nervous system: An extensive meshlike system of neurons that governs the functioning of the gut. This system is semiautonomous but is generally considered to be part of the autonomic nervous system.
entrainment: The process of synchronizing a biological rhythm to an environmental stimulus. See Figure 14.1.
enzyme: A complicated protein whose action increases the probability of a specific chemical reaction.
EOAE: See evoked otoacoustic emission.
EOG: See electro-oculography.
ependymal layer: See ventricular zone.
epidemiology: The statistical study of patterns of disease in a population.
epidermis: The outermost layer of skin, over the dermis. See Figure 8.4.
epigenetics: The study of factors that affect gene expression without making any changes in the nucleotide sequence of the genes themselves.
epilepsy: A brain disorder marked by major sudden changes in the electrophysiological state of the brain that are referred to as seizures. See Box 3.3.
epinephrine: Also called adrenaline. A compound that acts both as a hormone (secreted by the adrenal medulla under the control of the sympathetic nervous system) and as a synaptic transmitter. See Tables 4.1, 5.1.
episodic memory: Memory of a particular incident or a particular time and place.
EPSP: See excitatory postsynaptic potential.
equilibrium: In chemistry, the point at which all ongoing reactions are canceled or balanced by others, resulting in a stable, offset, or unchanging system.
ERK: See extracellular signal–regulated kinase.
ERP: See event-related potential.
estradiol: Also called 17β-estradiol. The primary type of estrogen that is secreted by the ovary. See Table 5.2.
estrogens: A class of steroid hormones produced by female gonads. See Figures 5.15, 5.19; Table 5.2.
estrus: The period during which female animals are sexually receptive.
eukaryote: Any organism whose cells have the genetic material contained within a nuclear envelope.
event-related potential (ERP): Also called evoked potential. Averaged EEG recordings measuring brain responses to repeated presentations of a stimulus. Components of the ERP tend to be reliable because the background noise of the cortex has been averaged out. See Figures 3.19, 18.8.
evoked otoacoustic emission (EOAE): A sound produced by the cochlea in response to acoustic stimulation. Compare spontaneous otoacoustic emission.
evoked potential: See event-related potential.
evolution: The process by which a population of interbreeding individuals changes over time.
evolution by natural selection: The Darwinian theory that evolution proceeds by differential success in reproduction.
evolutionary psychology: A field devoted to asking how natural selection has shaped behavior in humans.
excitatory postsynaptic potential (EPSP): A depolarizing potential in the postsynaptic neuron that is caused by excitatory presynaptic impulses. EPSPs increase the probability that the postsynaptic neuron will fire an action potential. See Figure 3.9. Compare inhibitory postsynaptic potential.
excitotoxicity: The property by which neurons die when overstimulated, as with large amounts of glutamate.
executive function: A neural and cognitive system that helps develop plans of action and organizes the activities of other high-level processing systems.
exhaustion stage: A stage in the response to stress that is caused by prolonged or frequently repeated stress and is characterized by increased susceptibility to disease.
exocrine gland: A gland whose secretions exit the body via ducts. Compare endocrine gland.
exogenous: Arising from outside the body. Compare endogenous.
exogenous attention: Also called reflexive attention. The involuntary reorienting of attention toward a specific stimulus source, cued by an object or event. Compare endogenous attention.
exogenous ligand: Any substance, originating from outside the body, that selectively binds to the type of receptor that is under study. Compare endogenous ligand.
expression: In the context of genetics, the process by which a cell makes an mRNA transcript of a particular gene.
external ear: The part of the ear that we readily see (the pinna) and the canal that leads to the eardrum. See Figure 9.2.
external fertilization: The process by which eggs are fertilized outside of the female’s body, as in many fishes and amphibians. Compare internal fertilization.
extinction: Short for extinction of simultaneous double stimulation, an inability to recognize the double nature of stimuli presented simultaneously to both sides of the body. People experiencing extinction report the stimulus from only one side.
extracellular compartment: The fluid space of the body that exists outside the cells. See Figure 13.11. Compare intracellular compartment.
extracellular fluid: The fluid in the spaces between cells (interstitial fluid) and in the vascular system. Compare intracellular fluid.
extracellular signal–regulated kinase (ERK): An important intracellular signal transduction system that can be activated by many different events that affect the cell surface.
extrafusal fiber: One of the ordinary muscle fibers that lie outside the spindles and provide most of the force for muscle contraction. See Figure 11.9. Compare intrafusal fiber.
extraocular muscle: One of the muscles attached to the eyeball that control its position and movements.
extrapyramidal system: A motor system that includes the basal ganglia and some closely related brainstem structures.
extrastriate cortex: Visual cortex outside of the primary visual (striate) cortex.

F

face blindness: See prosopagnosia.
FAS: See fetal alcohol syndrome.
fast-twitch muscle fiber: A type of striated muscle that contracts rapidly but fatigues readily. Compare slow-twitch muscle fiber.
fatal familial insomnia: An inherited disorder in which humans sleep normally at the beginning of their life but in midlife stop sleeping, and 7–24 months later die.
fear conditioning: A form of learning in which fear comes to be associated with a previously neutral stimulus.
feature detector model: A model of visual pattern analysis that emphasizes linear and angular components of the stimulus array. Compare spatial-frequency filter model.
feature integration theory: The idea that conjunction searches involve sequential shifts of attention that help coordinate multiple cognitive feature maps—overlapping representations of the search array based on individual stimulus attributes.
feature search: A search for an item in which the target, because it possesses a unique attribute, pops out right away, no matter how many distracters are present. Compare conjunction search.
FEF: See frontal eye field.
fetal alcohol syndrome (FAS): A disorder, including intellectual disability and characteristic facial anomalies, that affects children exposed to too much alcohol (through maternal ingestion) during fetal development.
fetus: A developing individual after the embryo stage. Humans are considered to be fetuses from 10 weeks after fertilization until birth.
filopodia (sing. filopodium): Very fine, tubular outgrowths from a cell. See Figure 7.8.
final common pathway: The information-processing pathway consisting of all the motoneurons in the body. Motoneurons are known by this collective term because they receive and integrate all motor signals from the brain and then direct movement accordingly.
fission: The process of splitting in two. Some unicellular organisms reproduce by fission; that is, they simply split into two daughter cells.
flaccid paralysis: A loss of reflexes below the level of transection of the spinal cord.
flavor neophobia: The avoidance of new foods.
flower spray ending: See secondary sensory ending.
fluent aphasia: Also called Wernicke’s aphasia. A language impairment characterized by fluent, meaningless speech and little language comprehension; related to damage in Wernicke’s area. See Figure 19.7. Compare nonfluent aphasia.
fMRI: See functional MRI.
foliate papillae: One of three types of small structures on the tongue, located along the sides, that contain taste receptors. See Figure 9.19. Compare circumvallate papillae and fungiform papillae.
follicle-stimulating hormone (FSH): A gonadotropin, named for its actions on ovarian follicles. See Figures 5.15, 5.19; Table 5.2.
follicles: Ovarian structures containing immature ova.
forebrain: Also called prosencephalon. The frontal division of the neural tube, containing the cerebral hemispheres, the thalamus, and the hypothalamus. See Figure 2.14.
fornix: A fiber tract that extends from the hippocampus to the mammillary body. See Figures 2.12, 2.15.
Fourier analysis: The analysis of a complex pattern into the sum of sine waves. See Box 9.1.
fourth ventricle: The passageway within the pons that receives cerebrospinal fluid from the third ventricle and releases it to surround the brain and spinal cord. See Figure 2.19.
fovea: The central portion of the retina, packed with the most photoreceptors and therefore the center of our gaze. See Figure 10.5.
fragile X syndrome: A condition that is a frequent cause of inherited intellectual disability; produced by a fragile site on the X chromosome that seems prone to breaking because the DNA there is unstable. See Figure 7.18.
free nerve ending: An axon that terminates in the skin without any specialized cell associated with it and that detects pain and/or changes in temperature. See Figure 8.4.
free-running: Referring to a rhythm of behavior shown by an animal deprived of external cues about time of day. See Figure 14.1.
frequency: The number of cycles per second in a sound wave; measured in hertz (Hz). See Box 9.1.
frontal eye field (FEF): An area in the frontal lobe of the brain containing neurons important for establishing gaze in accordance with cognitive goals (top-down processes) rather than with any characteristics of stimuli (bottom-up processes).
frontal lobe: The most anterior portion of the cerebral cortex. See Figure 2.12.
frontal plane: See coronal plane.
FSH: See follicle-stimulating hormone.
functional MRI (fMRI): Magnetic resonance imaging that detects changes in blood flow and therefore identifies regions of the brain that are particularly active during a given task.
functional tolerance: Decreased responding to a drug after repeated exposures, generally as a consequence of up- or down-regulation of receptors.
fundamental: Here, the predominant frequency of an auditory tone or a visual scene. Compare harmonic. See Box 9.1.
fungiform papillae: One of three types of small structures on the tongue, located in the front, that contain taste receptors. See Figure 9.19. Compare circumvallate papillae and foliate papillae.
fusiform gyrus: A region on the inferior surface of the cortex, at the junction of temporal and occipital lobes, that has been associated with recognition of faces. See Figure 19.19.

G

G proteins: A class of proteins that reside next to the intracellular portion of a receptor and that are activated when the receptor binds an appropriate ligand on the extracellular surface.
GABA: See gamma-aminobutyric acid.
gamete: A sex cell (sperm or ovum) that contains only unpaired chromosomes and therefore has only half of the usual total number of chromosomes.
gamma-aminobutyric acid (GABA): A widely distributed amino acid transmitter, and the main inhibitory transmitter in the mammalian nervous system. See Table 4.1.
gamma efferent: See gamma motoneuron.
gamma motoneuron: Also called gamma efferent. A motor neuron that innervates the contractile tissue in a muscle spindle. See Figure 11.9. Compare alpha motoneuron.
ganglion (pl. ganglia): A collection of nerve cell bodies outside the centralnervous system. Compare nucleus(definition 1).
ganglion cells: A class of cells in the retina whose axons form the optic nerve. See Figure 10.4. See also amacrine cells and bipolar cells.
gap junction: See electrical synapse.
gas neurotransmitter: A soluble gas, such as nitric oxide or carbon monoxide, that is produced and released by a neuron to alter the functioning of another neuron. Usually gas neurotransmitters act, in a retrograde fashion, on presynaptic neurons.
gastrin-releasing peptide (GRP): A neuropeptide that stimulates neurons in the dorsal horn to provide the sensation of itch.
gated: Referring to the property by which an ion channel may be opened or closed by factors such as chemicals, voltage changes, or mechanical actions. See Figure 3.6.
gel electrophoresis: A method of separating molecules of differing size or electrical charge by forcing them to flow through a gel. See Appendix Figure A.3.
gene: A length of DNA that encodes the information for constructing a particular protein.
gene amplification: See polymerase chain reaction.
general anesthetic: A drug that renders an individual unconscious.
generator potential: A local change in the resting potential of a receptor cell that mediates between the impact of stimuli and the initiation of nerve impulses.
genetics: The study of inheritance, including the genes encoded in DNA.
genome: See genotype.
genotype: Also called genome. All the genetic information that one specific individual has inherited. Compare phenotype.
genus (pl. genera): A group of species that resemble each other because of shared inheritance. See Figure 6.3.
GH: See growth hormone.
ghrelin: A peptide hormone emanating from the gut. See Figure 13.26. Compare obestatin.
giant axon: A large-diameter axon; found in some invertebrates. The size of giant axons facilitates research on the properties of neural membrane structure and function.
glia: See glial cells.
glial cells: Also sometimes called glia or neuroglia. Nonneuronal brain cells that provide structural, nutritional, and other types of support to the brain. See Figure 2.6.
global aphasia: The total loss of ability to understand language, or to speak, read, or write. See Figure 19.7.
globus pallidus: One of the basal ganglia. See Figure 2.15.
glomerulus (pl. glomeruli): A complex arbor of dendrites from a group of olfactory cells.
glucagon: A hormone, released by alpha cells in the islets of Langerhans, that increases blood glucose. See Table 5.2. Compare insulin.
glucocorticoids: A class of steroid hormones, released by the adrenal cortex, that affect carbohydrate metabolism and inflammation.
glucodetector: A cell that detects and informs the nervous system about levels of circulating glucose.
gluconeogenesis: The metabolism of body fats and proteins to create glucose.
glucose: An important sugar molecule used by the body and brain for energy.
glucose transporter: A molecule that spans the external membrane of a cell and transports glucose molecules from outside the cell to inside for use.
glutamate: An amino acid transmitter, the most common excitatory transmitter. See Table 4.1.
glutamate hypothesis: The hypothesis that schizophrenia may be caused, in part, by understimulation of glutamate receptors.
glutamatergic: Referring to cells that use glutamate as their synaptic transmitter.
glycine: An amino acid transmitter, often inhibitory. See Table 4.1.
glycogen: A complex carbohydrate made by the combining of glucose molecules for a short-term store of energy.
glycogenesis: The physiological process by which glycogen is produced.
glycogenolysis: The conversion of glycogen back into glucose, triggered when blood concentrations of glucose drop too low.
GnIH: See gonadotropin-inhibiting hormone.
GnRH: See gonadotropin-releasing hormone.
goiter: A swelling of the thyroid gland resulting from iodine deficiency.
Golgi stain: A histological stain that fills a small proportion of neurons with a dark, silver-based precipitate. See Box 2.1.
Golgi tendon organ: One of the receptors located in tendons that send impulses to the central nervous system when a muscle contracts. See Figure 11.9.
gonadotropin: An anterior pituitary hormone that selectively stimulates the cells of the gonads to produce sex steroids and gametes. See luteinizing hormone and follicle-stimulating hormone.
gonadotropin-inhibiting hormone (GnIH): A hypothalamic peptide hormone that reduces gonadotropin secretion by inhibiting the release of gonadotropin-releasing hormone. Compare kisspeptin.
gonadotropin-releasing hormone (GnRH): A hypothalamic hormone that controls the release of luteinizing hormone and follicle-stimulating hormone from the pituitary. See Figure 5.19.
gonads: The sexual organs (ovaries in females, testes in males), which produce gametes for reproduction. See Figure 5.1; Table 5.2.
graded response: A membrane electrical potential that spreads passively across the cell membrane, decreasing in strength with time and distance.
grammar: All of the rules for usage of a particular language.
grand mal seizure: A type of generalized epileptic seizure in which nerve cells fire in high-frequency bursts. Grand mal seizures cause loss of consciousness and sudden muscle contraction. See Box 3.3. Compare petit mal seizure.
granule cell: A type of small nerve cell. See Figure 2.16.
gray matter: Areas of the brain that are dominated by cell bodies and are devoid of myelin. See Figure 2.13. Compare white matter.
grid cell: A neuron that selectively fires when the animal crosses the intersection points of an abstract grid map of the local environment.
gross neuroanatomy: Anatomical features of the nervous system that are apparent to the naked eye.
growth cone: The growing tip of an axon or a dendrite. See Figure 7.8.
growth hormone (GH): Also called somatotropin or somatotropic hormone. A tropic hormone, secreted by the anterior pituitary, that influences the growth of cells and tissues. See Figure 5.15; Table 5.2.
GRP: See gastrin-releasing peptide.
guevedoces: Literally, “eggs at 12” (in Spanish). A nickname for individuals who are raised as girls but at puberty change appearance and begin behaving as boys.
gustatory system: The taste system. See Figure 9.21.
gyrus (pl. gyri): A ridged or raised portion of a convoluted brain surface. See Figure 2.12. Compare sulcus.

H

habituation: A form of nonassociative learning in which an organism becomes less responsive following repeated presentations of a stimulus. See Box 17.1. Compare sensitization (definition 1).
hair cell: One of the receptor cells for hearing in the cochlea. Displacement of hair cells by sound waves generates nerve impulses that travel to the brain. See Figure 9.2.
hallucinogens: A class of drugs that alter sensory perception and produce peculiar experiences.
hard problem of consciousness: The problem of how to read people’s subjective experience of consciousness and determine the qualia that accompany perception. Compare easy problem of consciousness.
harmonics: Multiples of a particular frequency called the fundamental. See Box 9.1.
health psychology: Also called behavioral medicine. A field that studies psychological influences on health-related processes, such as why people become ill or how they remain healthy.
Hebbian synapse: A synapse that is strengthened when it successfully drives the postsynaptic cell.
hemiparesis: Weakness of one side of the body.
hemiplegia: Partial paralysis involving one side of the body.
hemispatial neglect: A syndrome in which the patient fails to pay any attention to objects presented to one side of the body and may even deny connection with that side.
hermaphrodite: An individual possessing the reproductive organs of both sexes, either simultaneously or at different points in time.
heroin: Diacetylmorphine; an artificially modified, very potent form of morphine.
hertz (Hz): Cycles per second, as of an auditory stimulus. See Box 9.1.
high-functioning autism: See Asperger’s syndrome.
hindbrain: Also called rhombencephalon. The rear division of the brain, which, in the mature vertebrate, contains the cerebellum, pons, and medulla. See Figure 2.14.
hippocampal gyrus: See subiculum.
hippocampus (pl. hippocampi): A medial temporal lobe structure that is important for learning and memory. See Figures 2.15, 17.1, 17.21.
histology: The study of tissue structure.
homeostasis: The tendency for the internal environment to remain constant.
homeostatic: Referring to the process of maintaining a particular physiological parameter relatively constant.
homology: A physical resemblance that is based on common ancestry, such as the similarity in forelimb structures of different mammals. See Figure 6.1. Compare homoplasy and analogy.
homoplasy: A physical resemblance that is due to convergent evolution, such as the similar body form of tuna and dolphins. Compare homology.
horizontal cells: Specialized retinal cells that contact both the receptor cells and the bipolar cells.
horizontal plane: The plane that divides the body or brain into upper and lower parts. See Box 2.2. Compare coronal plane and sagittal plane.
hormone: A chemical secreted by an endocrine gland that is conveyed by the bloodstream and regulates target organs or tissues. See Tables 5.1, 5.2.
horseradish peroxidase (HRP): An enzyme found in horseradish and other plants that is used to determine the cells of origin of a particular set of axons. See Box 2.1.
HRP: See horseradish peroxidase.
hue: One of three basic dimensions (along with brightness and saturation) of light perception. Hue varies around the color circle through blue, green, yellow, orange, and red. See Figure 10.23.
hunger: The internal state of an animal seeking food. Compare satiety.
huntingtin: A protein produced by a gene (called HTT) that, when containing too many trinucleotide repeats, results in Huntington’s disease in a carrier.
Huntington’s disease: Also called Huntington’s chorea. A progressive genetic disorder characterized by abrupt, involuntary movements and profound changes in mental functioning.
hybridization: The process by which a string of nucleotides becomes linked to a complementary series of nucleotides.
hyperphagia: Excessive eating. Compare aphagia.
hyperpolarization: An increase in membrane potential (the interior of the neuron becomes even more negative). See Figure 3.5. Compare depolarization.
hypertonic: Referring to a solution with a higher concentration of salt than that found in interstitial fluid and blood plasma (more than about 0.9% salt). Compare hypotonic and isotonic.
hypocretins: Also called orexins. Neuropeptides produced in the hypothalamus that are involved in switching between sleep states, in narcolepsy, and in the control of appetite.
hypodermis: Also called subcutaneous tissue. The innermost layer of skin, under the dermis.
hypofrontality hypothesis: The hypothesis that schizophrenia may reflect underactivation of the frontal lobes.
hypophyseal portal system: A duplex system of capillaries spanning between the neurosecretory cells of the hypothalamus and the secretory tissue of the anterior pituitary.
hypophysis: See pituitary gland.
hypothalamus: Part of the diencephalon, lying ventral to the thalamus. See Figures 2.12, 2.14.
hypotonic: Referring to a solution with a lower concentration of salt than that found in interstitial fluid and blood plasma (less than about 0.9% salt).Compare hypertonic and isotonic.
hypovolemic thirst: A desire to ingest fluids that is stimulated by a reduced volume of extracellular fluid. Compare osmotic thirst.
hypoxia: A transient lack of oxygen.
Hz: See hertz.

I

IAPs: See inhibitors of apoptosis proteins.
IC: See impoverished condition.
ICC: See immunocytochemistry.
iconic memory: A very brief type of memory that stores the sensory impression of a scene. Compare short-term memory.
ideational apraxia: An impairment in the ability to carry out a sequence of actions, even though each element or step can be done correctly. Compare ideomotor apraxia.
identifiable neurons: Neurons that are large and similar from one individual to the next, enabling investigators to recognize them and give them names.
ideomotor apraxia: The inability to carry out a simple motor activity in response to a verbal command, even though this same activity is readily performed spontaneously. Compare ideational apraxia.
IEGs: See immediate early genes.
IHC: See inner hair cell.
immediate early genes (IEGs): A class of genes that show rapid but transient increases in expression in cells that have become activated. See Box 2.1.
immunocytochemistry (ICC): A method for detecting a particular protein in tissues in which an antibody recognizes and binds to the protein and then chemical methods are used to leave a visible reaction product around each antibody. See Boxes 2.1, 5.1.
immunoglobulin: See antibody.
impoverished condition (IC): Also called isolated condition. A condition in which laboratory rodents are housed singly in a small cage without complex stimuli. See Figure 17.17. Compare enriched condition and standard condition.
in situ hybridization: A method for detecting particular RNA transcripts in tissue sections by providing a nucleotide probe that is complementary to, and will therefore hybridize with, the transcript of interest. See Box 2.1; Appendix Figure A.4.
in vitro: Literally, “in glass” (in Latin). Usually, in a laboratory dish; outside the body.
inattentional blindness: The failure to perceive nonattended stimuli that seem so obvious as to be impossible to miss (e.g., a gorilla strolling across the screen).
incus (pl. incudes): Latin for “anvil.” A middle-ear bone situated between the malleus (attached to the tympanic membrane) and the stapes (attached to the cochlea); one of the three ossicles that conduct sound across the middle ear. See Figure 9.2.
independent variable: The factor that is manipulated by an experimenter. Compare dependent variable.
indifferent gonads: The undifferentiated gonads of the early mammalian fetus, which will eventually develop into either testes or ovaries. See Figure 12.13. See also gonads.
individual response stereotypy: The tendency of individuals to show the same response pattern to particular situations throughout their life span.
indoleamines: A class of monoamines that serve as neurotransmitters, including serotonin and melatonin. See Table 4.1.
induction: The process by which one set of cells influences the fate of neighboring cells, usually by secreting a chemical factor that changes gene expression in the target cells.
inferior colliculi (sing. colliculus): Paired gray matter structures of the dorsal midbrain that receive auditory information. See Figure 2.12. Compare superior colliculi.
infradian: Referring to a rhythmic biological event whose period is longer than that of a circadian rhythm—that is, longer than a day. Compare ultradian.
infrasound: Very low frequency sound; in general, below the threshold of human hearing, at about 20 Hz. Compare ultrasound.
infundibulum: See pituitary stalk.
inhibition of return: The phenomenon, observed in peripheral spatial cuing tasks and occurring when the interval between cue and target stimulus is 200 ms or more, in which detection of stimuli at the former location of the cue is increasingly impaired.
inhibitors of apoptosis proteins (IAPs): A family of proteins that inhibit caspases and thereby stave off apoptosis.
inhibitory postsynaptic potential (IPSP): A hyperpolarizing potential in the postsynaptic neuron that is caused by inhibitory connections. IPSPs decrease the probability that the postsynaptic neuron will fire an action potential. See Figure 3.9. Compare excitatory postsynaptic potential.
inner ear: The cochlea and vestibular apparatus. See Figure 9.2.
inner hair cell (IHC): One of the two types of receptor cells for hearing in the cochlea. See Figure 9.2. Compare outer hair cell.
innervate: To provide neural input.
innervation: The supply of neural input to an organ or a region of the nervous system.
innervation ratio: The ratio expressing the number of muscle fibers innervated by a single motor axon. The fewer muscle fibers an axon innervates (i.e., the lower the ratio), the finer the control of movements.
input zone: The part of a neuron that receives information, from other neurons or from specialized sensory structures. Usually corresponds to the cell’s dendrites. See Figure 2.5.
instrumental conditioning: Also called operant conditioning. A form of associative learning in which the likelihood that an act (instrumental response) will be performed depends on the consequences (reinforcing stimuli) that follow it. See Box 17.1. Compare classical conditioning.
instrumental response: See instrumental conditioning.
insula: A region of cortex lying below the surface, within the lateral sulcus, of the frontal, temporal, and parietal lobes.
insulin: A hormone, released by beta cells in the islets of Langerhans, that lowers blood glucose. See Table 5.2. Compare glucagon.
integration zone: The part of the neuron that initiates nerve electrical activity if the sum of all inhibitory and excitatory postsynaptic potentials exceeds a threshold value. Usually corresponds to the neuron’s axon hillock.
intensity differences: Perceived differences in loudness between the two ears, which can be used to localize a sound source. Compare latency differences.
intermale aggression: Aggression between males of the same species.
intermediate-term memory (ITM): A form of memory that lasts longer than short-term memory, but not as long as long-term memory.
internal carotid artery: See carotid arteries.
internal fertilization: The process by which sperm fertilize eggs inside of the female’s body, as in all mammals, birds, and reptiles. Compare external fertilization.
interneuron: A neuron that is neither a sensory neuron nor a motoneuron. Interneurons receive input from and send output to other neurons.
intersex: Referring to an individual with atypical genital development and sexual differentiation that generally resembles a form intermediate between typical male and typical female genitals.
intracellular compartment: The fluid space of the body that is contained within cells. See Figure 13.11. Compare extracellular compartment.
intracellular fluid: Also called cytoplasm. The watery solution found within cells. Compare extracellullar fluid.
intrafusal fiber: One of the small muscle fibers that lie within each muscle spindle. See Figure 11.9. Compare extrafusal fiber.
intraparietal sulcus (IPS): A region in the human parietal lobe, homologous to the monkey lateral intraparietal area, that is especially involved in voluntary, top-down control of attention.
intrinsic activity: See efficacy.
intromission: Insertion of the erect penis into the vagina during copulation.
inverse agonist: A substance that binds to a receptor and causes it to do the opposite of what the naturally occurring transmitter does.
ion: An atom or molecule that has acquired an electrical charge by gaining or losing one or more electrons.
ion channel: A pore in the cell membrane that permits the passage of certain ions through the membrane when the channels are open. See Figure 3.4.
ionotropic receptor: A receptor protein that includes an ion channel that is opened when the receptor is bound by an appropriate ligand. See Figures 3.15, 4.1. See also ligand-gated ion channel. Compare metabotropic receptor.
IPS: See intraparietal sulcus.
ipsilateral: In anatomy, pertaining to a location on the same side of the body. See Box 2.2. Compare contralateral.
IPSP: See inhibitory postsynaptic potential.
iris: The circular structure of the eye that provides an opening to form the pupil. See Figure 10.5.
islets of Langerhans: Clusters of cells in the pancreas that release two hormones (insulin and glucagon) with opposite effects on glucose utilization. See Figure 5.1.
isocortex: See neocortex.
isolated brain: Sometimes referred to by the French term, encéphale isolé. An experimental preparation in which an animal’s brainstem has been separated from the spinal cord by a cut below the medulla. See Figure 14.26. Compare isolated forebrain.
isolated forebrain: Sometimes referred to by the French term, cerveau isolé. An experimental preparation in which an animal’s nervous system has been cut in the upper midbrain, dividing the brain from the brainstem. See Figure 14.26. Compare isolated brain.
isolated condition: See impoverished condition.
isotonic: Referring to a solution with a concentration of salt that is the same as that found in interstitial fluid and blood plasma (about 0.9% salt). Compare hypertonic and hypotonic.
ITM: See intermediate-term memory.

K

K complex: A sharp negative EEG potential that is seen in stage 2 sleep.
kcal: See kilocalorie.
ketamine: A dissociative anesthetic drug, similar to PCP, that acts as an NMDA receptor antagonist.
ketones: A metabolic fuel source liberated by the breakdown of body fats and proteins.
khat: Also spelled qat. An African shrub that, when chewed, acts as a stimulant.
kilocalorie (kcal): A measure of energy commonly applied to food; formally defined as the quantity of heat required to raise the temperature of 1 kg of water by 1°C.
kindling: A method of experimentally inducing an epileptic seizure by repeatedly stimulating a brain region. See Box 3.3.
kinesin: A protein “motor” that moves substances in axonal transport. See also dynein.
kisspeptin: A hypothalamic peptide hormone that increases gonadotropin secretion by facilitating the release of gonadotropin-releasing hormone. Compare gonadotropin-inhibiting hormone.
Klüver-Bucy syndrome: A condition, brought about by bilateral amygdala damage, that is characterized by dramatic emotional changes including reduction in fear and anxiety.
knee jerk reflex: A variant of the stretch reflex in which stretching of the tendon beneath the knee leads to an upward kick of the leg. See Figure 3.17.
knockout organism: An individual in which a particular gene has been disabled by an experimenter. See Box 7.3.
Korsakoff’s syndrome: A memory disorder, related to a thiamine deficiency, that is generally associated with chronic alcoholism.

L

L-dopa: The immediate precursor of the transmitter dopamine.
labeled lines: The concept that each nerve input to the brain reports only a particular type of information.
lamellipodia (pl. lamellipodium): Sheetlike extensions of a growth cone. See Figure 7.8.
late-selection model: A model of attention postulating that the attentional bottleneck imposed by the nervous system exerts control late in the processing pathway, filtering out stimuli only after substantial analysis has occurred. Compare early-selection model.
latency differences: Differences between the two ears in the time of arrival of a sound, which can be employed by the nervous system to localize sound sources. Compare intensity differences.
latent learning: Learning that has taken place but has not (yet) been demonstrated by performance.
lateral: In anatomy, toward the side of the body. See Box 2.2. Compare medial.
lateral geniculate nucleus (LGN): The part of the thalamus that receives information from the optic tract and sends it to visual areas in the occipital cortex. See Figure 10.15.
lateral hypothalamus (LH): A hypothalamic region involved in the control of appetite and other functions. See Figure 13.22.
lateral inhibition: The phenomenon by which interconnected neurons inhibit their neighbors, producing contrast at the edges of regions. See Figure 10.4.
lateral intraparietal area (LIP): A region in the monkey parietal lobe, homologous to the human intraparietal sulcus, that is especially involved in voluntary, top-down control of attention.
lateral-line system: A sensory system, found in many kinds of fishes and some amphibians, that informs the animal of water motion in relation to the body surface.
lateral sulcus: See Sylvian fissure.
lateral ventricle: A complexly shaped lateral portion of the ventricular system within each hemisphere of the brain. See Figure 2.19.
lateralization: The tendency for the right and left halves of a system to differ from one another.
LD₅₀: Lethal dose 50%; the dose of a drug at which half the treated animals will die. See Figure 4.8.
learned helplessness: A learning paradigm in which individuals are subjected to inescapable, unpleasant conditions.
learning: The process of acquiring new and relatively enduring information, behavior patterns, or abilities, characterized by modifications of behavior as a result of practice, study, or experience.
lens: A structure in the eye that helps focus an image on the retina. The shape of the lens is controlled by the ciliary muscles inside the eye. See Figure 10.5.
leptin: A peptide hormone released by fat cells.
lesion momentum: The phenomenon in which the brain is impaired more by a lesion that develops quickly than by a lesion that develops slowly.
levels of analysis: The scope of experimental approaches. A scientist may try to understand behavior by monitoring molecules, nerve cells, brain regions, or social environments, or some combination of these levels of analysis.
LGN: See lateral geniculate nucleus.
LH: 1. See lateral hypothalamus. 2. See luteinizing hormone.
lie detector: See polygraph.
ligand: A substance that binds to receptor molecules, such as those at the surface of the cell.
ligand-gated ion channel: Also known as chemically gated ion channel. An ion channel that opens or closes in response to the presence of a particular chemical; an example is the ionotropic neurotransmitter receptor. Compare voltage-gated Na⁺ channel.
limbic system: A loosely defined, widespread group of brain nuclei that innervate each other to form a network. These nuclei are implicated in emotions. See Figure 2.15.
LIP: See lateral intraparietal area.
lipid bilayer: The structure of the neuronal cell membrane, which consists of two layers of lipid molecules, within which float various specialized proteins, such as receptors. See Figure 3.4.
lipids: Large molecules (commonly called fats) consisting of fatty acids and glycerol that are insoluble in water.
lithium: An element that, administered to patients, often relieves the symptoms of bipolar disorder.
lobotomy: The detachment of a portion of the frontal lobe from the rest of the brain, once used as a treatment for schizophrenia and many other ailments.
local anesthetic: A drug, such as procaine or lidocaine, that blocks sodium channels to stop neural transmission in pain fibers.
local potential: An electrical potential that is initiated by stimulation at a specific site, which is a graded response that spreads passively across the cell membrane, decreasing in strength with time and distance.
locus coeruleus: Literally, “blue spot.” A small nucleus in the brainstem whose neurons produce norepinephrine and modulate large areas of the forebrain.
long-term depression (LTD): A lasting decrease in the magnitude of responses of neurons after afferent cells have been stimulated with electrical stimuli of relatively low frequency. Compare long-term potentiation.
long-term memory (LTM): An enduring form of memory that lasts days, weeks, months, or years and has a very large capacity.
long-term potentiation (LTP): A stable and enduring increase in the effectiveness of synapses following repeated strong stimulation. See Figures 17.21, 17.22, 17.23. Compare long-term depression.
lordosis: A female receptive posture in quadrupeds in which the hindquarter is raised and the tail is turned to one side, facilitating intromission by the male. See Figures 12.3, 12.6.
Lou Gehrig’s disease: See amyotrophic lateral sclerosis.
loudness: The subjective experience of the pressure level of a sound. See Box 9.1.
LSD: Also called acid. Lysergic acid diethylamide, a hallucinogenic drug.
LTD: See long-term depression.
LTM: See long-term memory.
LTP: See long-term potentiation.
lumbar: Referring to the five spinal segments that make up the upper part of the lower back. See Figures 2.10, 2.11.
luteinizing hormone (LH): A gonadotropin, named for its stimulatory effects on the ovarian corpora lutea. See Figures 5.15, 5.19; Table 5.2.
lysergic acid diethylamide: See LSD.

M

M1: See primary motor cortex.
mad cow disease: See bovine spongiform encephalopathy.
magnetic resonance imaging (MRI): A noninvasive technique that uses magnetic energy to generate images that reveal some structural details in the living brain. See Figures 1.6, 2.21.
magnetoencephalography (MEG): A passive and noninvasive functional brain-imaging technique that measures the tiny magnetic fields produced by active neurons, in order to identify regions of the brain that are particularly active during a given task.
magnocellular: Of or consisting of relatively large cells. Compare parvocellular.
major histocompatibility complex (MHC): A large family of genes that identify an individual’s tissues (to aid in immune responses against foreign proteins).
malleus (pl. mallei): Latin for “hammer.” A middle-ear bone that is connected to the tympanic membrane; one of the three ossicles that conduct sound across the middle ear. See Figure 9.2.
mammillary body: One of a pair of nuclei at the base of the brain. See Figure 2.12.
manic-depressive illness: See bipolar disorder.
MAO: See monoamine oxidase.
marijuana: A dried preparation of the Cannabis sativa plant, usually smoked to obtain THC.
maximal response: In pharmacology, the strongest effect that a drug can have on a particular measured response, no matter how much of the drug is given.
MC4Rs: See melanocortin type-4 receptors.
MD: See muscular dystrophy.
MDMA: Also called Ecstasy. A drug of abuse, 3,4-methylenedioxymethamphetamine.
medial: In anatomy, toward the middle of an organ or organism. See Box 2.2. Compare lateral.
medial amygdala: A portion of the amygdala that receives olfactory and pheromonal information.
medial forebrain bundle: A collection of axons traveling in the midline region of the forebrain. See Figure 4.21.
medial geniculate nuclei: Nuclei in the thalamus that receive input from the inferior colliculi and send output to the auditory cortex. See Figure 9.7.
medial preoptic area (mPOA): A region of the anterior hypothalamus implicated in the control of many behaviors, including thermoregulation, sexual behavior, and gonadotropin secretion.
median eminence: Midline feature on the base of the brain marking the point at which the infundibulum exits the hypothalamus to connect to the pituitary. Contains elements of the hypophyseal portal system.
medulla: Also called myelencephalon. The posterior part of the hindbrain. See Figures 2.12, 2.14.
medullary reticular formation: The hind-most portion of the brainstem reticular formation, implicated in motor control and copulatory behavior. See Figure 12.6.
MEG: See magnetoencephalography.
Meissner’s corpuscle: A skin receptor cell type that detects light touch. See Figures 8.4, 8.13.
melanocortin type-4 receptors (MC4Rs): A specific subtype of melanocortinreceptor.
melanocortins: One category of endogenous opioid peptides.
melanopsin: A photopigment found within particular retinal ganglion cells that project to the suprachiasmatic nucleus. See Figure 14.6.
melatonin: An amine hormone that is released by the pineal gland. See Tables 4.1, 5.2.
memory: 1. The ability to retain information, based on the mental process of learning or encoding, retention across some interval of time, and retrieval or reactivation of the memory. 2. The specific information that is stored in the brain.
memory trace: A persistent change in the brain that reflects the storage of memory.
meninges: The three protective sheets of tissue—dura mater, pia mater, and arachnoid—that surround the brain and spinal cord. See Figure 2.10.
meningitis: An acute inflammation of the membranes covering the central nervous system—the meninges—usually caused by a viral or bacterial infection.
Merkel’s disc: A skin receptor cell type that detects light touch. See Figures 8.4, 8.13.
mesencephalon: See midbrain.
mesolimbocortical pathway: A set of dopaminergic axons arising in the midbrain and innervating the limbic system and cortex. See Figure 4.3. Compare mesostriatal pathway.
mesostriatal pathway: A set of dopaminergic axons arising from the midbrain and innervating the basal ganglia, including those from the substantia nigra to the striatum. See Figure 4.3. Compare mesolimbocortical pathway.
messenger RNA (mRNA): A strand of RNA that carries the code of a section of a DNA strand to the cytoplasm. See the Appendix.
metabolic tolerance: The form of drug tolerance that arises when the metabolic machinery of the body becomes more efficient at clearing the drug, as a consequence of repeated exposure.
metabolism: The breakdown of complex molecules into smaller molecules.
metabotropic receptor: A receptor protein that does not contain an ion channel but may, when activated, use a G protein system to alter the functioning of the postsynaptic cell. See Figures 3.13, 4.1. Compare ionotropic receptor.
metencephalon: A subdivision of the hindbrain that includes the cerebellum and the pons. See Figure 2.14.
methylation: A chemical modification of DNA that does not affect the nucleotide sequence of a gene but makes that gene less likely to be expressed.
MHC: See major histocompatibility complex.
microelectrode: An especially small electrode used to record electrical potentials from living cells.
microfilament: A very small filament (7 nm in diameter) found within all cells. Microfilaments determine cell shape.
microglial cells: Also called microglia. Extremely small glial cells that remove cellular debris from injured or dead cells.
micropolygyria: A condition of the brain in which small regions are characterized by more gyri than usual. See Figure 19.10.
microtubule: A small, hollow, cylindrical structure (20–26 nm in diameter) in axons that is involved in axonal transport.
midbrain: Also called mesencephalon. The middle division of the brain. See Figure 2.14.
middle canal: See scala media.
middle cerebral arteries: Two large arteries, arising from the internal carotids, that provide blood to most of the lateral surfaces of the cerebral hemispheres.
middle ear: The cavity between the tympanic membrane and the cochlea. See Figure 9.2.
milk letdown reflex: The reflexive release of milk in response to suckling, or to stimuli associated with suckling. The mechanism involves release of the hormone oxytocin. See Figure 5.12.
millivolt (mV): A thousandth of a volt.
mineralocorticoids: A class of steroid hormones, released by the adrenal cortex, that affect ion concentrations in body tissues.
minimal discriminable frequency difference: The smallest change in frequency that can be detected reliably between two tones.
mirror neuron: A neuron that is active both when an individual makes a particular movement and when that individual sees another individual make that same movement.
mitochondrion (pl. mitochondria): A cellular organelle that provides metabolic energy for the cell’s processes. See Figure 2.7.
mitosis: The process of division of somatic cells that involves duplication of DNA.
mitral cell: A type of cell in the olfactory bulb that conducts smell information from the glomeruli to the rest of the brain. See Figure 9.22.
modulatory circuit: See superordinate circuit.
modulatory site: A portion of a receptor that, when bound by a compound, alters the receptor’s response to its transmitter.
monaural: Pertaining to one ear. Compare binaural.
monoamine hormones: See amine hormones.
monoamine oxidase (MAO): An enzyme that breaks down and thereby inactivates monoamine transmitters.
monocular deprivation: Depriving one eye of light. Compare binocular deprivation.
monogamy: A mating system in which a female and a male form a breeding pair that may last for one breeding period or for a lifetime. A durable and exclusive relationship between a male and a female is called a pair bond. Compare polygamy and bigamy.
monopolar neuron: See unipolar neuron.
monotreme: An egg-laying mammal belonging to an order that contains only the echidnas and the platypus.
monozygotic: Referring to twins derived from a single fertilized egg (identical twins). Such individuals have the same genotype. Compare dizygotic.
morpheme: The smallest grammatical unit of a language; a word or meaningful part of a word.
morphine: An opiate compound derived from the poppy flower.
motion sickness: The experience of nausea brought on by unnatural passive movement, as in a car or boat.
motoneuron: Also called motor neuron. A nerve cell that transmits motor messages, stimulating a muscle or gland. See Figure 11.8.
motor neuron: See motoneuron.
motor plan: Also called motor program. A plan for action in the nervous system.
motor theory of language: The theory proposing that the left-hemisphere language zones are motor control systems that are concerned with both the precise production and the perception of the extremely complex movements that go into speech.
motor unit: A single motor axon and all the muscle fibers that it innervates.
movement: A brief, unitary activity of a muscle or body part; less complex than an act.
mPOA: See medial preoptic area.
MRH: See anti-müllerian hormone.
MRI: See magnetic resonance imaging.
mRNA: See messenger RNA.
müllerian duct: A duct system in the embryo that will develop into female reproductive structures (fallopian tubes, uterus, and upper vagina) if testes are not present. See Figures 12.13, 12.14. Compare wolffian duct.
müllerian regression hormone (MRH): See anti-müllerian hormone.
multiple sclerosis: Literally, “many scars”; a disorder characterized by widespread degeneration of myelin.
multipolar neuron: A nerve cell that has many dendrites and a single axon. See Figure 2.5. Compare bipolar neuron and unipolar neuron.
multisensory: See polymodal.
muscarinic: Referring to cholinergic receptors that respond to the chemical muscarine as well as to acetylcholine. Muscarinic receptors mediate chiefly the inhibitory activities of acetylcholine. Compare nicotinic.
muscle fiber: A collection large, cylindrical cells, making up most of a muscle, that can contract in response to neurotransmitter released from a motoneuron. See Figure 11.7. See also extrafusal fiber and intrafusal fiber.
muscle spindle: A muscle receptor that lies parallel to a muscle and sends impulses to the central nervous system when the muscle is stretched. See Figure 11.9.
muscular dystrophy (MD): A disease that leads to degeneration of and functional changes in muscles.
musth: An annual period of heightened aggressiveness and sexual activity in male elephants.
mutant: An animal carrying a gene that differs from the norm or from the alleles carried by its parents.
mutation: A change in the nucleotide sequence of a gene as a result of unfaithful replication.
mV: See millivolt.
myasthenia gravis: A disorder characterized by a profound weakness of skeletal muscles; caused by a loss of acetylcholine receptors.
myelencephalon: See medulla.
myelin: The fatty insulation around an axon, formed by glial cells. This myelin sheath improves the speed of conduction of nerve impulses. See Figures 2.6, 3.8.
myelination: The process of myelin formation. See Figures 2.6, 7.15.
myopia: Nearsightedness; the inability to focus the retinal image of objects that are far away.
myosin: A protein that, along with actin, mediates the contraction of muscle fibers. See Figure 11.7.

N

N1 effect: A negative deflection of the event-related potential, occurring about 100 ms after stimulus presentation, that is enhanced for selectively attended input compared to ignored input.
naloxone: A potent antagonist of opiates that is often administered to people who have taken drug overdoses. Naloxone binds to receptors for endogenousopioids.
narcolepsy: A disorder that involves frequent, intense episodes of sleep, which last from 5 to 30 minutes and can occur anytime during the usual waking hours.
natural selection: See evolution by natural selection.
naturalist: A student of animal life and structure.
NE: See norepinephrine.
negative feedback: The property by which some of the output of a system feeds back to reduce the effect of input signals. Compare positive feedback.
negative polarity: A negative electrical-potential difference relative to a reference electrode. A neuron at rest exhibits a greater concentration of negatively charged ions in its interior than in its immediate surrounds; thus it is said to be negatively polarized. See Figure 3.1.
negative symptom: In psychiatry, a symptom that reflects insufficient functioning. Examples include emotional and social withdrawal, blunted affect, and slowness and impoverishment of thought and speech. Compare positive symptom.
neocortex: Also called isocortex or simply cortex. Cerebral cortex that is made up of six distinct layers. Compare allocortex.
neologism: An entirely novel word, sometimes produced by a patient with aphasia.
neonatal: Referring to newborns.
Nernst equation: An equation predicting the voltage needed to just counterbalance the diffusion force pushing an ion across a semipermeable membrane from the side with a high concentration to the side with a low concentration.
nerve: A collection of axons bundled together outside the central nervous system. See Figures 2.8, 2.9. Compare tract.
nerve cell: See neuron.
nerve growth factor (NGF): A substance that markedly affects the growth of neurons in spinal ganglia and in the ganglia of the sympathetic nervous system. See Figure 7.12.
nerve impulse: See action potential.
neural chain: A simple kind of neural circuit in which neurons are attached linearly, end-to-end.
neural groove: In the developing embryo, the groove between the neural folds. See Figure 7.1.
neural plasticity: See neuroplasticity.
neural tube: An embryonic structure with subdivisions that correspond to the future forebrain, midbrain, and hindbrain. The cavity of this tube will include the cerebral ventricles and the passages that connect them. See Figure 7.1.
neurochemistry: The branch of neuroscience concerned with the fundamental chemical composition and processes of the nervous system.
neurocrine: Referring to secretory functions of neurons, especially pertaining to synaptic transmission. See Figure 5.3.
neuroeconomics: The study of brain mechanisms at work during economic decision making.
neuroendocrine cell: See neurosecretory cell.
neurofibrillary tangle: An abnormal whorl of neurofilaments within nerve cells. Neurofibrillary tangles are especially apparent in people suffering from dementia. See Figure 7.28.
neurofilament: A small, rodlike structure found in axons. Neurofilaments are involved in the transport of materials. See Figure 2.22.
neurogenesis: The mitotic division of nonneuronal cells to produce neurons. See Figures 7.2, 7.3.
neuroglia: See glial cells.
neurohypophysis: See posterior pituitary.
neuroleptics: Also called antipsychotics. A class of drugs that alleviate symptoms of schizophrenia, typically by blocking dopamine receptors.
neuromodulator: A substance that influences the activity of synaptic transmitters.
neuromuscular junction (NMJ): The region where the motoneuron terminal and the adjoining muscle fiber meet; the point where the nerve transmits its message to the muscle fiber.
neuron: Also called nerve cell. The basic unit of the nervous system. Each neuron is composed of a cell body, receptive extension(s) (dendrites), and a transmitting extension (axon). See Figures 2.4, 2.5.
neuron doctrine: The hypothesis that the brain is composed of separate cells that are distinct structurally, metabolically, and functionally.
neuropathic pain: Pain caused by damage to peripheral nerves; often difficult to treat.
neuropeptide: See peptide neurotransmitter.
neuropeptide Y (NPY): A peptide neurotransmitter that may carry some of the signals for feeding.
neuropharmacology: Also called psychopharmacology. The scientific field concerned with the discovery and study of compounds that selectively affect the functioning of the nervous system.
neurophysiology: The study of the life processes of neurons.
neuropil: The conglomeration of dendrites and the synapses upon them.
neuroplasticity: Also called neural plasticity. The ability of the nervous system to change in response to experience or the environment.
neuroscience: The study of the nervous system.
neurosecretory cell: Also called neuroendocrine cell. A neuron that releases hormones into local or systemic circulation.
neurosteroids: Steroids produced in the brain.
neurotransmitter: Also called synaptic transmitter, chemical transmitter, or simply transmitter. The chemical released from the presynaptic axon terminal that serves as the basis of communication between neurons. See Figure 3.12; Table 4.1.
neurotrophic factor: A target-derived chemical that acts as if it “feeds” certain neurons to help them survive. See also trophic factor.
neurotrophin: A chemical that prevents neurons from dying.
NGF: See nerve growth factor.
nicotine: A compound found in plants, including tobacco, that acts as an agonist on a large class of cholinergic receptors.
nicotinic: Referring to cholinergic receptors that respond to nicotine as well as to acetylcholine. Nicotinic receptors mediate chiefly the excitatory activities of acetylcholine, including at the neuromuscular junction. Compare muscarinic.
night terror: A sudden arousal from stage 3 or stage 4 slow-wave sleep that is marked by intense fear and autonomic activation. Compare nightmare.
nightmare: A long, frightening dream that awakens the sleeper from REM sleep. Compare night terror.
Nissl stain: A histological stain that outlines all cell bodies because the dyes are attracted to RNA, which encircles the nucleus. See Box 2.1.
nitric oxide (NO): A soluble gas that serves as a retrograde gas neurotransmitter in the nervous system.
NMDA receptor: A glutamate receptor that also binds the glutamate agonist NMDA (N-methyl-D-aspartate). The NMDA receptor is both ligand-gated and voltage-sensitive, so it can participate in a wide variety of information processing. See Figure 17.22.
NMJ: See neuromuscular junction.
NO: See nitric oxide.
nociceptor: A receptor that responds to stimuli that produce tissue damage or pose the threat of damage.
nocturnal: Active during the dark periods of the daily cycle. Compare diurnal.
node of Ranvier: A gap between successive segments of the myelin sheath where the axon membrane is exposed. See Figures 2.6, 3.8.
nonassociative learning: A type of learning in which presentation of a particular stimulus alters the strength or probability of a response according to the strength and temporal spacing of that stimulus; includes habituation and sensitization. See Box 17.1. Compare associative learning.
noncompetitive ligand: A drug that affects a transmitter receptor while binding at a site other than that bound by the endogenous ligand. See Figure 4.7. Compare competitive ligand.
nondeclarative memory: Also called procedural memory. A memory that is shown by performance rather than by conscious recollection. See Figures 17.3, 17.5. Compare declarative memory.
nondirected synapse: A type of synapse in which the presynaptic and postsynaptic cells are not in close apposition; instead, neurotransmitter is released by axonal varicosities and diffuses away to affect wide regions of tissue.
nonfluent aphasia: Also called Broca’s aphasia. A language impairment characterized by difficulty with speech production but not with language comprehension; related to damage in Broca’s area. See Figure 19.7. Compare fluent aphasia.
nonfluent speech: Talking with considerable effort, short sentences, and the absence of the usual melodic character of conversational speech.
nongenomic effect: An effect of a steroid hormone that is not mediated by direct changes in gene expression.
nonprimary motor cortex: Frontal lobe regions adjacent to the primary motor cortex that contribute to motor control and modulate the activity of the primary motor cortex. See Figure 11.15.
nonprimary sensory cortex: See secondary sensory cortex.
nootropics: A class of drugs that enhance cognitive function.
noradrenaline: See norepinephrine.
noradrenergic: Referring to systems using norepinephrine (noradrenaline) as a transmitter.
norepinephrine (NE): Also called noradrenaline. A neurotransmitter produced and released by sympathetic postganglionic neurons to accelerate organ activity. Also produced in the brainstem and found in projections throughout the brain. See Table 4.1.
Northern blot: A method of detecting a particular RNA transcript in a tissue or organ, by separating RNA from that source with gel electrophoresis, blotting the separated RNAs onto nitrocellulose, and then using a nucleotide probe to hybridize with, and highlight, the transcript of interest. Compare Southern blot and Western blot.
notochord: A midline structure arising early in the embryonic development of vertebrates. See Figure 7.1.
NPY: See neuropeptide Y.
NPY/AgRP neurons: Neurons involved in the hypothalamic appetite control system, so named because they produce both neuropeptide Y and agouti-related peptide. Compare POMC/CART neurons.
NST: See nucleus of the solitary tract.
nucleotide: A portion of a DNA or RNA molecule that is composed of a single base and the adjoining sugar-phosphate unit of the strand. See Appendix Figure A.2.
nucleus (pl. nuclei): 1. A collection of neurons within the central nervous system (e.g., the caudate nucleus). Compare ganglion. 2. See cell nucleus.
nucleus accumbens: A region of the forebrain that receives dopaminergic innervation from the ventral tegmental area. Dopamine release in this region may mediate the reinforcing qualities of many activities, including drug abuse.
nucleus of the solitary tract (NST): A complicated brainstem nucleus that receives visceral and taste information via several cranial nerves. See Figure 13.16.
nutrient: A chemical that is needed for growth, maintenance, and repair of the body but is not used as a source of energy.

O

obestatin: A peptide hormone emanating from the gut that acts probably on the appetite controller of the hypothalamus to decrease appetite. Compare ghrelin.
obsessive-compulsive disorder (OCD): A syndrome in which the affected individual engages in recurring, repetitive acts that are carried out without rhyme, reason, or the ability to stop.
occipital cortex: Also called visual cortex. The cortex of the occipital lobe of the brain. See Figure 10.11.
occipital lobes: Large regions of cortex covering much of the posterior part of each cerebral hemisphere, and specialized for visual processing. See Figure 2.12.
OCD: See obsessive-compulsive disorder.
ocular dominance column: A region of cortex in which one eye or the other provides a greater degree of synaptic input. See Figure 10.22.
ocular dominance histogram: A graph that portrays the strength of response of a brain neuron to stimuli presented to either the left eye or the right eye. Ocular dominance histograms are used to determine the effects of manipulating visual experience. See Figure 7.21.
ocular dominance slab: A slab of visual cortex, about 0.5 mm wide, in which the neurons of all layers respond preferentially to stimulation of one eye. See Figure 10.23.
oculomotor apraxia: A severe difficulty in voluntarily steering visual gaze toward specific targets.
off-center bipolar cell: A retinal bipolar cell that is inhibited by light in the center of its receptive field. See Figure 10.14. Compare on-center bipolar cell.
off-center ganglion cell: A retinal ganglion cell that is activated when light is presented to the periphery, rather than the center, of the cell’s receptive field. See Figure 10.14. Compare on-center ganglion cell.
off-center/on-surround: Referring to a concentric receptive field in which the center inhibits the cell of interest while the surround excites it. See Figure 10.14. Compare on-center/off-surround.
OHC: See outer hair cell.
olfactory bulb: An anterior projection of the brain that terminates in the upper nasal passages and, through small openings in the skull, provides receptors for smell. See Figures 2.12, 9.18.
olfactory epithelium (pl. epithelia): A sheet of cells, including olfactory receptors, that lines the dorsal portion of the nasal cavities and adjacent regions, including the septum that separates the left and right nasal cavities. See Figures 9.22, 9.24, 9.25.
oligodendrocyte: A type of glial cell that forms myelin in the central nervous system. See Figure 2.6.
on-center bipolar cell: A retinal bipolar cell that is excited by light in the center of its receptive field. See Figure 10.14. Compare off-center bipolar cell.
on-center ganglion cell: A retinal ganglion cell that is activated when light is presented to the center, rather than the periphery, of the cell’s receptive field. See Figure 10.15. Compare off-centerganglion cell.
on-center/off-surround: Referring to a concentric receptive field in which the center excites the cell of interest while the surround inhibits it. See Figure 10.14. Compare off-center/on-surround.
ontogeny: The process by which an individual changes in the course of its lifetime—that is, grows up and grows old.
Onuf’s nucleus: The human homolog of the spinal nucleus of the bulbocavernosus (SNB) in rats.
open-loop control mechanism: A control mechanism in which feedback from the output of the system is not provided to the input control. Compare closed-loop control mechanism.
operant conditioning: See instrumental conditioning.
opiates: A class of compounds that exert an effect like that of opium, including reduced pain sensitivity. See Table 4.1, under “Opioid peptides.” Compare opioids.
opioid peptide: A type of endogenous peptide that mimics the effects of morphine in binding to opioid receptors and producing marked analgesia and reward. See Table 4.1.
opioid receptor: A receptor that responds to endogenous and/or exogenous opioids.
opioids: A class of peptides produced in various regions of the brain that bind to opioid receptors and act like opiates. See Table 4.1.
opium: A heterogeneous extract of the seedpod juice of the opium poppy,Papaver somniferum.
opponent-process hypothesis: The theory that color vision depends on systems that produce opposite responses to light of different wavelengths. See Figures 10.24, 10.25.
opsin: One of the two components of photopigments in the retina. The other component is RETINAL.
optic ataxia: A spatial disorientation in which the patient is unable to accurately reach for objects using visual guidance.
optic chiasm: The point at which the two optic nerves meet. See Figures 2.12, 10.11.
optic disc: The region of the retina devoid of receptor cells because ganglion cell axons and blood vessels exit the eyeball there. See Figure 10.7.
optic nerve: Cranial nerve II; the collection of ganglion cell axons that extend from the retina to the optic chiasm. See Figures 2.9, 10.7.
optic radiation: Axons from the lateral geniculate nucleus that terminate in the primary visual areas of the occipital cortex. See Figure 10.11.
optic tract: The axons of retinal ganglion cells after they have passed the optic chiasm; most terminate in the lateral geniculate nucleus. See Figure 10.11.
optical imaging: A method for visualizing brain activity in which near-infrared light is passed through the scalp and skull. The reflected light contains information about blood flow and electrical activity of the cortical surface.
oral contraceptive: A birth control pill, typically consisting of steroid hormones to prevent ovulation.
orexins: Also called hypocretins. Neuropeptides produced in the hypothalamus that are involved in switching between sleep states, in narcolepsy, and in the control of appetite.
organ of Corti: A structure in the inner ear that lies on the basilar membrane of the cochlea and contains the hair cells and terminations of the auditory nerve. See Figure 9.1.
organizational effect: A permanent alteration of the nervous system, and thus permanent change in behavior, resulting from the action of a steroid hormone on an animal early in its development. Compare activational effect.
orgasm: The climax of sexual experience, marked by extremely pleasurable sensations.
orientation column: A column of visual cortex that responds to rod-shaped stimuli of a particular orientation. See Figure 10.22.
orphan receptor: Any receptor for which no endogenous ligand has yet been discovered.
oscillator circuit: A neural circuit that produces a recurring, repeating pattern of output.
osmolality: The number of solute particles per unit volume of solvent.
osmosensory neuron: A specialized neuron that measures the movement of water into and out of the intracellular compartment. See Figures 13.11, 13.16.
osmosis: The passive movement of molecules from one place to another.
osmotic pressure: The tendency of a solvent to move through a membrane in order to equalize the concentration of a solute.
osmotic thirst: A desire to ingest fluids that is stimulated by excessive loss of water from the extracellular compartment. Compare hypovolemic thirst.
ossicles: Three small bones (incus, malleus, and stapes) that transmit sound across the middle ear, from the tympanic membrane to the oval window. See Figure 9.2.
otoacoustic emission: A sound produced by the cochlea itself, either spontaneously or in response to an environmental noise.
otolith: A small crystal on the gelatinous membrane in the vestibular system. See Figure 9.16.
ototoxic: Toxic to the ears, especially the middle or inner ear.
outer hair cell (OHC): One of the two types of receptor cells for hearing in the cochlea. See Figure 9.2. Compare inner hair cell.
output zone: The part of a neuron, usually corresponding to the axon terminals, at which the cell sends information to another cell. See Figure 2.5.
oval window: The opening from the middle ear to the inner ear. See Figure 9.2.
ovaries: The female gonads, which produce eggs for reproduction. See Figures 5.1, 12.8, 12.14; Table 5.2.
overt attention: Attention in which the focus coincides with sensory orientation (e.g., you’re attending to the same thing you’re looking at). Compare covert attention.
oviparous: Of or relating to oviparity, reproduction through egg laying. Compare viviparous and ovoviviparous.
ovoviviparous: Of or relating to ovoviviparity, reproduction in which eggs remain inside the mother’s body until they hatch or are about to hatch. Compare oviparous and viviparous.
ovulation: The production and release of an egg (ovum).
ovulatory cycle: The periodic occurrence of ovulation. See Figure 12.5.
ovum (pl. ova): An egg, the female gamete.
oxytocin: A hormone, released from the posterior pituitary, that triggers milk letdown in the nursing female. See Figures 5.11, 5.12; Table 5.2.

P

P1 effect: A positive deflection of the event-related potential, occurring 70–100 ms after stimulus presentation, that is enhanced for selectively attended visual input compared to ignored input.
P20–50 effect: A positive deflection of the event-related potential, occurring about 20–50 ms after stimulus presentation, that is enhanced for selectively attended input compared to ignored input.
P3 effect: Also called auditory P300. A positive deflection of the event-related potential, occurring about 300 ms after stimulus presentation, that is associated with higher-order auditory stimulus processing and late attentional selection.
Pacinian corpuscle: A skin receptor cell type that detects vibration. See Figures 8.4, 8.5, 8.13.
pain: The discomfort normally associated with tissue damage.
pair bond: A durable and exclusive relationship between a male and a female.
paleocortex: See allocortex.
pancreas: An endocrine gland, located near the posterior wall of the abdominal cavity, that secretes insulin and glucagon. See Figure 5.1; Table 5.2.
Papez circuit: A group of brain regions within the limbic system.
papilla (pl. papillae): A small bump that projects from the surface of the tongue. Papillae contain most of the taste receptor cells. See Figure 9.18.
parabiotic: Referring to a surgical preparation that joins two animals to share a single blood supply.
paracrine: Referring to cellular communication in which a chemical signal diffuses to nearby target cells through the intermediate extracellular space. See Figure 5.3. Compare autocrine.
paradoxical sleep: See rapid-eye-movement sleep.
paragigantocellular nucleus (PGN): A region of the brainstem reticular formation implicated in sleep and modulation of spinal reflexes.
parallel fiber: One of the axons of the granule cells that form the outermost layer of the cerebellar cortex. See Figure 2.16.
paraphasia: A symptom of aphasia that is distinguished by the substitution of a word by a sound, an incorrect word, an unintended word, or a neologism (a meaningless word).
parasympathetic nervous system: A component of the autonomic nervous system that arises from both the cranial nerves and the sacral spinal cord. Compare sympathetic nervous system. See Figure 2.11.
paraventricular nucleus (PVN): A nucleus of the hypothalamus. See Figures 5.11, 13.22.
parental behavior: Behavior of adult animals with the goal of enhancing the well-being of their own offspring, often at some cost to the parents.
paresis: Partial paralysis. Compare plegia.
parietal lobes: Large regions of cortex lying between the frontal and occipital lobes of each cerebral hemisphere. See Figure 2.12.
parkin: A protein that has been implicated in Parkinson’s disease.
Parkinson’s disease: A degenerative neurological disorder, characterized by tremors at rest, muscular rigidity, and reduction in voluntary movement, that involves dopaminergic neurons of the substantia nigra.
parthenogenesis: Literally, “virgin birth.” The production of offspring without the contribution of a male or sperm.
partial agonist: A drug that, when bound to a receptor, has less effect than the endogenous ligand would. The term partial antagonist is equivalent. See Figure 4.7.
parvocellular: Of or consisting of relatively small cells. Compare magnocellular.
patient H.M.: A patient who, because of damage to medial temporal lobe structures, was unable to encode new declarative memories. Upon his death we learned his name was Henry Molaison. See Figure 17.1.
patient K.C.: A patient who sustained damage to the cortex that renders him unable to form and retrieve new episodic memories, especially autobiographical memories.
patient N.A.: A patient who is unable to encode new declarative memories, because of damage to the dorsal thalamus and the mammillary bodies.
pattern coding: Coding of information in sensory systems based on the temporal pattern of action potentials.
Pavlovian conditioning: See classical conditioning.
PCP: See phencyclidine.
PCR: See polymerase chain reaction.
peg: See blob.
peptide: A short string of amino acids. Longer strings of amino acids are called proteins.
peptide hormones: See protein hormones.
peptide neurotransmitter: Also called neuropeptide. A neurotransmitter consisting of a short chain of amino acids. See Table 4.1.
perceptual load: The immediate processing challenge presented by a stimulus.
periaqueductal gray: The neuronal body–rich region of the midbrain surrounding the cerebral aqueduct that connects the third and fourth ventricles; involved in pain perception.
period: The interval of time between two similar points of successive cycles, such as sunset to sunset.
peripheral nervous system: The portion of the nervous system that includes all the nerves and neurons outside the brain and spinal cord. See Figures 2.8, 2.14. Compare central nervous system.
peripheral spatial cuing task: A task that tests exogenoous attention, using latency to detect a visual stimulus, preceded by a simple task-irrelevant sensory stimulus in the location where the stimulus will appear. Compare symbolic cuing task.
perseverate: To continue to show a behavior repeatedly.
PET: See positron emission tomography.
petit mal seizure: Also called absence attack. A seizure that is characterized by a spike-and-wave EEG and often involves a loss of awareness and inability to recall events surrounding the seizure. See Box 3.3. Compare grand mal seizure.
PGN: See paragigantocellular nucleus.
phagocyte: An immune system cell that engulfs invading molecules or microbes.
phallus: The clitoris or penis.
pharmacodynamics: Collective name for the factors that affect the relationship between a drug and its target receptors, such as affinity and efficacy.
pharmacokinetics: Collective name for all the factors that affect the movement of a drug into, through, and out of the body.
phase shift: A shift in the activity of a biological rhythm, typically provided by a synchronizing environmental stimulus.
phasic receptor: A receptor in which the frequency of action potentials drops rapidly as stimulation is maintained. Compare tonic receptor.
phencyclidine (PCP): Also called angel dust. An anesthetic agent that is also a psychedelic drug. PCP makes many people feel dissociated from themselves and their environment.
phenothiazines: A class of antipsychotic drugs that reduce the positive symptoms of schizophrenia.
phenotype: The sum of an individual’s physical characteristics at one particular time. Compare genotype.
phenotype matching: In general, processes by which an individual can assess the genetic relatedness of another individual on the basis of shared traits.
phenylketonuria (PKU): An inherited disorder of protein metabolism in which the absence of an enzyme leads to a toxic buildup of certain compounds, causing intellectual disability.
pheromone: A chemical signal that is released outside the body of an animal and affects other members of the same species. See Figure 5.3. Compare allomone.
phobic disorder: An intense, irrational fear that becomes centered on a specific object, activity, or situation that a person feels compelled to avoid.
phoneme: A sound that is produced for language.
phosphoinositides: A class of common second-messenger compounds in postsynaptic cells.
photon: A quantum of light energy.
photopic system: A system in the retina that operates at high levels of light, shows sensitivity to color, and involves the cones. See Table 10.1. Compare scotopic system.
photoreceptor adaptation: The tendency of rods and cones to adjust their light sensitivity to match ambient levels of illumination.
photoreceptors: Neural cells in the retina that respond to light.
phrenology: The belief that bumps on the skull reflect enlargements of brain regions responsible for certain behavioral faculties. See Figure 1.11.
phylogeny: The evolutionary history of a particular group of organisms. See Figure 6.4.
pia mater: The innermost of the three meninges that surround the brain and spinal cord. See also dura mater and arachnoid.
pineal gland: A secretory gland in the brain midline; the source of melatonin release. See Figures 2.12, 5.1; Table 5.2.
pinna (pl. pinnae): The external part of the ear.
pinocytosis: The process by which synaptic neurotransmitter is repackaged into synaptic vesicles. See Figure 3.12.
pitch: A dimension of auditory experience in which sounds vary from low to high.
pituitary gland: Also called hypophysis. A small, complex endocrine gland located in a socket at the base of the skull. The anterior pituitary and posterior pituitary are separate in function. See Figures 2.12, 5.11, 5.14, 5.15.
pituitary stalk: Also called infundibulum. A thin piece of tissue that connects the pituitary gland to the hypothalamus.
PKU: See phenylketonuria.
place cell: A neuron within the hippocampus that selectively fires when the animal is in a particular location.
place theory: A theory of frequency discrimination stating that pitch perception depends on the place of maximal displacement of the basilar membrane produced by a sound. Compare volley theory.
placebo: A substance, given to a patient, that is known to be ineffective or inert but that sometimes brings relief.
planum temporale: A region of superior temporal cortex adjacent to the primary auditory area. See Figure 19.17.
plegia: Paralysis, the loss of the ability to move. Compare paresis.
polioviruses: A class of viruses that destroy motoneurons of the spinal cord and brainstem.
polyandry: A mating system in which one female mates with more than one male. Compare polygyny.
polygamy: A mating system in which an individual mates with more than one other animal. Compare monogamy and bigamy.
polygraph: Popularly known as a lie detector. A device that measures several bodily responses, such as heart rate and blood pressure.
polygyny: A mating system in which one male mates with more than one female. Compare polyandry.
polymerase chain reaction (PCR): Also called gene amplification. A method for reproducing a particular RNA or DNA sequence manyfold, allowing amplification for sequencing or manipulating the sequence.
polymodal: Also called multisensory. Involving several sensory modalities.
POMC: See pro-opiomelanocortin.
POMC/CART neurons: Neurons involved in the hypothalamic appetite control system, so named because they produce both pro-opiomelanocortin and cocaine- and amphetamine-related transcript. Compare NPY/AgRP neurons.
pons: A portion of the metencephalon; part of the brainstem connecting midbrain to medulla. See Figures 2.12, 2.14.
positive feedback: The property by which some of the output of a system feeds back to increase the effect of input signals. Positive feedback is rare in biological systems. Compare negative feedback.
positive symptom: In psychiatry, an abnormal state. Examples include hallucinations, delusions, and excited motor behavior. Compare negative symptom.
positron emission tomography (PET): A technique for examining brain function by combining tomography with injections of radioactive substances used by the brain. Analysis of the metabolism of these substances reflects regional differences in brain activity. See Figure 2.21.
postcentral gyrus: The strip of parietal cortex, just behind the central sulcus, that receives somatosensory information from the entire body. See Figure 2.12. Compare precentral gyrus.
postcopulatory behavior: The final stage in mating behavior. Species-specific postcopulatory behaviors include rolling (in the cat) and grooming (in the rat). See Figure 12.1.
posterior: Also called caudal. In anatomy, toward the tail end of an organism. See Box 2.2. Compare anterior.
posterior cerebral arteries: Two large arteries, arising from the basilar artery, that provide blood to posterior aspects of the cerebral hemispheres, cerebellum, and brainstem.
posterior pituitary: Also called neurohypophysis. The rear division of the pituitary gland. See Figures 5.1, 5.11; Table 5.2. Compare anterior pituitary.
postganglionic: Literally, “after the ganglion.” Referring to neurons in the autonomic nervous system that run from the autonomic ganglia to various targets in the body. See Figure 2.11. Compare preganglionic.
postpartum depression: A bout of depression that afflicts a woman either immediately before or after giving birth.
postsynaptic: Referring to the region of a synapse that receives and responds to neurotransmitter. See Figure 2.7. Compare presynaptic.
postsynaptic membrane: The specialized membrane on the surface of the cell that receives information from a presynaptic neuron. This membrane contains specialized receptor proteins that allow it to respond to neurotransmitter molecules. Compare presynaptic membrane. See Figure 2.7.
postsynaptic potential: A local potential that is initiated by stimulation at a synapse, can vary in amplitude, and spreads passively across the cell membrane, decreasing in strength with time and distance. Compare all-or-none property.
posttraumatic stress disorder (PTSD)
potassium ion (K⁺): A potassium atom that carries a positive charge because it has lost one electron.
precentral gyrus: The strip of frontal cortex, just in front of the central sulcus, that is crucial for motor control. See Figure 2.12. Compare postcentral gyrus.
precocial: Referring to animals that are born in a relatively developed state and that are able to survive without maternal care. Compare altricial.
prefrontal cortex: The anteriormost region of the frontal lobe.
preganglionic: Literally, “before the ganglion.” Referring to neurons in the autonomic nervous system that run from the central nervous system to the autonomic ganglia. See Figure 2.11. Compare postganglionic.
premotor cortex: A region of nonprimary motor cortex just anterior to the primary motor cortex. See Figure 11.16.
presenilin: An enzyme that cleaves amyloid precursor protein, forming β-amyloid, which can lead to Alzheimer’s disease. See also β-secretase.
presynaptic: Referring to the region of a synapse that releases neurotransmitter. See Figure 2.7. Compare postsynaptic.
presynaptic membrane: The specialized membrane of the axon terminal of the neuron that transmits information by releasing neurotransmitter. Vesicles bearing neurotransmitter can bind to this membrane and release their contents, thus affecting the postsynaptic membrane. See Figure 2.7.
primacy effect: The superior performance seen in a memory task for items at the start of a list; usually attributed to long-term memory. Compare recency effect.
primary motor cortex (M1): The apparent executive region for the initiation of movement; primarily the precentral gyrus.
primary sensory cortex: For a given sensory modality, the region of cortex that receives most of the information about that modality from the thalamus or, in the case of olfaction, directly from the secondary sensory neurons. Compare secondary sensory cortex.
primary sensory ending: Also called annulospiral ending. The axon that transmits information from the central portion of a muscle spindle. See Figure 11.9. Compare secondary sensory ending.
primary somatosensory cortex (S1): Also called somatosensory 1. The gyrus just posterior to the central sulcus where sensory receptors on the body surface are mapped. Primary cortex for receiving touch and pain information, in the parietal lobe. See Figures 8.10, 8.15. Compare secondary somatosensory cortex.
primary visual cortex (V1): Also called striate cortex or area 17. The region of the occipital cortex where most visual information first arrives. See Figures 10.11, 10.12, 10.19.
priming: Also called repetition priming. In memory, the phenomenon by which exposure to a stimulus facilitates subsequent responses to the same or a similar stimulus.
prion: A protein that can become improperly folded and thereby can induce other proteins to follow suit, leading to long protein chains that impair neural function.
probe: Here, a manufactured sequence of DNA that is made to include a label (a colorful or radioactive molecule) that lets us track its location.
procedural memory: See nondeclarative memory.
proceptive: Referring to a state in which an animal advertises its readiness to mate through species-typical behaviors, such as ear wiggling in the female rat.
process outgrowth: The extensive growth of axons and dendrites.
progesterone: The primary type of progestin secreted by the ovary. See Figure 5.19; Table 5.2.
progestins: A major class of steroid hormones that are produced by the ovary, including progesterone. See Figure 5.15, 5.19; Table 5.2.
progressive supranuclear palsy (PSP): A rare, degenerative disease of the brain that begins with marked, persistent visual symptoms and leads to more widespread intellectual deterioration.
prolactin: A protein hormone, produced by the anterior pituitary, that promotes mammary development for lactation in female mammals. See Table 5.2; Figure 5.15.
promiscuity: A mating system in which animals mate with several members of the opposite sex and do not establish durable associations with sex partners.
pro-opiomelanocortin (POMC): A pro-hormone that can be cleaved to produce the melanocortins, which also participate in feeding control. See Figure 13.26.
proprioception: Body sense; information about the position and movement of the body that is sent to the brain.
prosencephalon: See forebrain.
prosody: The perception of emotional tone-of-voice aspects of language.
prosopagnosia: Also called face blindness. A condition characterized by the inability to recognize faces. Acquired prosopagnosia is caused by damage to the brain, particularly the fusiform gyrus. Developmental (or congenital) prosopagnosia is the result of brain defects present from birth.
protein: A long string of amino acids. The basic building material of organisms. Compare peptide.
protein hormones: Also called peptide hormones. A class of hormones, molecules of which consist of a string of amino acids.
protein kinase: An enzyme that adds phosphate groups (PO₄) to protein molecules.
proximal: In anatomy, near the trunk or center of an organism. See Box 2.2. Compare distal.
PSP: See progressive supranuclear palsy.
psychoneuroimmunology: The study of the immune system and its interaction with the nervous system and behavior.
psychopath: An individual incapable of experiencing remorse.
psychopharmacology: See neuropharmacology.
psychosocial dwarfism: Reduced stature caused by stress early in life that inhibits deep sleep. See Box 5.2.
psychosomatic medicine: A field of study that emphasizes the role of psychological factors in disease.
psychosurgery: Surgery in which brain lesions are produced to modify severe psychiatric disorders.
psychotomimetic: A drug that induces a state resembling schizophrenia.
PTSD: See posttraumatic stress disorder.
pulvinar: In humans, the posterior portion of the thalamus, heavily involved in visual processing and direction of attention.
punch-drunk: See chronic traumatic encephalopathy.
pupil: The aperture, formed by the iris, that allows light to enter the eye. See Figure 10.11.
pure tone: A tone with a single frequency of vibration. See Box 9.1.
Purkinje cell: A type of large nerve cell in the cerebellar cortex. See Figure 2.16.
putamen: One of the basal ganglia. See Figure 2.15.
PVN: See paraventricular nucleus.
pyramidal cell: A type of large nerve cell that has a roughly pyramid-shaped cell body. Pyramidal cells are found in the cerebral cortex. See Figure 2.17.
pyramidal system: Also called corticospinal system. The motor system that includes neurons within the cerebral cortex and their axons, which form the pyramidal tract. See Figure 11.12.
PYY_3-36: A peptide hormone, secreted by the intestines, that probably acts on hypothalamic appetite control mechanisms to suppress appetite.

Q

qat: See khat.
quale (pl. qualia): A purely subjective experience of perception.
quantum (pl. quanta): A unit of radiant energy.

R

radial glial cells: Glial cells that form early in development, spanning the width of the emerging cerebral hemispheres, and guide migrating neurons. See Figure 7.5.
radioimmunoassay (RIA): A technique that uses antibodies to measure the concentration of a substance, such as a hormone, in blood. See Box 5.1.
ramp movement: Also called smooth movement. A slow, sustained motion that is often controlled by the basal ganglia. Compare ballistic movement.
range fractionation: A hypothesis of stimulus intensity perception stating that a wide range of intensity values can be encoded by a group of cells, each of which is a specialist for a particular range of stimulus intensities. See Figure 8.6.
raphe nuclei: A string of nuclei in the midline of the midbrain and brainstem that contain most of the serotonergic neurons of the brain.
rapid-eye-movement (REM) sleep: Also called paradoxical sleep. A stage of sleep characterized by small-amplitude, fast-EEG waves, no postural tension, and rapid eye movements. REM rhymes with “gem.” See Figure 14.11. Compare slow-wave sleep.
RBD: See REM behavior disorder.
recency effect: The superior performance seen in a memory task for items at the end of a list; attributed to short-term memory. Compare primacy effect.
receptive field: The stimulus region and features that affect the activity of a cell in a sensory system. See Figures 8.9, 10.14, 10.16.
receptor: 1. The initial element in a sensory system, responsible for stimulus transduction. Examples include the hair cells in the cochlea, and the rods and cones in the retina. 2. Also called receptor molecule. A protein that captures and reacts to molecules of a neurotransmitter or hormone.
receptor cell: A specialized cell that responds to a particular energy or substance in the internal or external environment, and converts this energy into a change in the electrical potential across its membrane.
receptor isoform: A version of a receptor protein (in this context, a hormone receptor) with slight differences in structure that give it different functional properties. Conceptually similar to a receptor subtype.
receptor molecule: See receptor (definition 2).
receptor subtype: Any type of receptor having functional characteristics that distinguish it from other types of receptors for the same neurotransmitter. For example, at least 15 different subtypes of receptor molecules respond to serotonin.
reconsolidation: The return of a memory trace to stable long-term storage after it has been temporarily made volatile during the process of recall.
recovery of function: The recovery of behavioral capacity following brain damage from stroke or injury.
red nucleus: A brainstem structure related to motor control.
reductionism: The scientific strategy of breaking a system down into increasingly smaller parts in order to understand it.
redundancy: The property of having a particular process, usually an important one, monitored and regulated by more than one mechanism.
reflex: A simple, highly stereotyped, and unlearned response to a particular stimulus (e.g., an eye blink in response to a puff of air). See Figures 3.17, 11.10.
reflexive attention: See endogenous attention.
refraction: The bending of light rays by a change in the density of a medium, such as the cornea and the lens of the eyes.
refractory: Transiently inactivated or exhausted.
refractory phase: 1. A period during and after a nerve impulse in which the responsiveness of the axonal membrane is reduced. A brief period of complete insensitivity to stimuli (absolute refractory phase) is followed by a longer period of reduced sensitivity (relative refractory phase) during which only strong stimulation produces an action potential. 2. A period following copulation during which an individual cannot recommence copulation. The absolute refractory phase of the male sexual response is illustrated in Figure 12.9.
regulation: An adaptive response to early injury, as when developing individuals compensate for missing or injured cells.
reinforcing stimulus: See instrumental conditioning.
relative refractory phase: See refractory phase (definition 1).
releasing hormones: A class of hormones, produced in the hypothalamus, that traverse the hypothalamic-pituitary portal system to control the pituitary’s release of tropic hormones. See Figure 5.15.
REM behavior disorder (RBD): A sleep disorder in which a person physically acts out a dream.
REM sleep: See rapid-eye-movement sleep.
repetition priming: See priming.
repetitive transcranial magnetic stimulation (rTMS): See transcranial magnetic stimulation.
reserpine: A drug that causes the depletion of monoamines and can lead to depression.
resting membrane potential: A difference in electrical potential across the membrane of a nerve cell during an inactive period. See Figures 3.1, 3.5.
reticular formation: An extensive region of the brainstem (extending from the medulla through the thalamus) that is involved in arousal (waking). See Figure 14.26.
reticulospinal tract: A tract of axons arising from the brainstem reticular formation and descending to the spinal cord to modulate movement. Compare rubrospinal tract.
retina: The receptive surface inside the eye that contains photoreceptors and other neurons. See Figures 10.5, 10.6.
RETINAL: One of the two components of photopigments in the retina. The other component is opsin. (The term is printed in small capital letters in this text to distinguish it from the adjective retinal, meaning “pertaining to the retina.”)
retinohypothalamic pathway: The projection of retinal ganglion cells to the suprachiasmatic nuclei.
retrieval: A process in memory during which a stored memory is used by an organism. See Figure 17.7.
retrograde amnesia: Difficulty in retrieving memories formed before the onset of amnesia. Compare anterograde amnesia.
retrograde degeneration: Destruction of the nerve cell body following injury to its axon. See Box 7.1. Compare anterograde degeneration.
retrograde messenger: Transmitter that is released by the postsynaptic region, nd travels back across the synapse, and alters the functioning of the presynaptic neuron.
retrograde synapse: A synapse in which a signal (usually a gas neurotransmitter) flows from the postsynaptic neuron to the presynaptic neuron, thus counter to the usual direction of synaptic communication.
retrograde transmitter: A neurotransmitter that diffuses from the postsynaptic neuron back to the presynaptic neuron.
retrograde transport: Movement of cellular substances toward the cell body from the axon terminals. Compare anterograde transport.
reuptake: The process by which released synaptic transmitter molecules are taken up and reused by the presynaptic neuron, thus stopping synaptic activity.
rhodopsin: The photopigment in rods that responds to light.
rhombencephalon: See hindbrain.
RIA: See radioimmunoassay.
ribonucleic acid (RNA): A nucleic acid that implements information found in DNA. Compare deoxyribonucleic acid.
ribosomes: Structures in the cell body where genetic information is translated to produce proteins.
RNA: See ribonucleic acid.
rods: A class of light-sensitive receptor cells (photoreceptors) in the retina that are most active at low levels of light. See Figure 10.6. Compare cones.
roots: The two distinct branches of a spinal nerve, each of which serves a separate function. The dorsal root enters the dorsal horn of the spinal cord and carries sensory information from the peripheral nervous system to the spinal cord. The ventral root arises from the ventral horn of the spinal cord and carries motor messages from the spinal cord to the peripheral nervous system. See Figure 2.10.
rostral: See anterior.
round window: A membrane separating the cochlear duct from the middle-ear cavity. See Figure 9.2.
rTMS: See transcranial magnetic stimulation.
rubrospinal tract: A tract of axons arising from the red nucleus in the midbrain and innervating neurons of the spinal cord. See Figure 11.17. Compare reticulospinal tract.
Ruffini’s ending: A skin receptor cell type that detects stretching of the skin. See Figures 8.4, 8.13.

S

S1: See primary somatosensory cortex.
S2: See secondary somatosensory cortex.
saccule: A small, fluid-filled sac under the utricle in the vestibular system that responds to static positions of the head. See Figure 9.16.
sacral: Referring to the five spinal segments that make up the lower part of the lower back. See Figures 2.10, 2.11.
SAD: See seasonal affective disorder.
sagittal plane: The plane that bisects the body or brain into right and left portions. See Box 2.2. Compare coronal plane and horizontal plane.
saltatory conduction: The form of conduction that is characteristic of myelinated axons, in which the action potential jumps from one node of Ranvier to the next.
satiety: A feeling of fulfillment or satisfaction. Compare hunger.
saturation: One of three basic dimensions (along with brightness and hue) of light perception. Saturation varies from rich to pale (e.g., from red to pink to gray in the color solid of Figure 10.23).
saturated: Referring to the condition in which a maximal number of receptors of one type have been bound by molecules of a drug; additional doses of drug cannot produce additional binding.
saxitoxin (STX): An animal toxin that blocks sodium channels when applied to the outer surface of the cell membrane.
SC: See standard condition.
scala media: Also called middle canal. The central of the three spiraling canals inside the cochlea, situated between the scala vestibuli and scala tympani. See Figure 9.2.
scala tympani: Also called tympanic canal. One of three principal canals running along the length of the cochlea. The other two are the scala media and scala vestibuli. See Figure 9.2.
scala vestibuli: Also called vestibular canal. One of three principal canals running along the length of the cochlea. The other two are the scala media and scala tympani. See Figure 9.2.
schizophrenia: A severe psychopathology characterized by negative symptoms such as emotional withdrawal and impoverished thought, and by positive symptoms such as hallucinations and delusions.
Schwann cell: The glial cell that forms myelin in the peripheral nervous system.
SCN: See suprachiasmatic nucleus.
scotoma: A region of blindness caused by injury to the visual pathway or brain.
scotopic system: A system in the retina that operates at low levels of light and involves the rods. See Table 10.1. Compare photopic system.
SDN-POA: See sexually dimorphic nucleus of the preoptic area.
seasonal affective disorder (SAD): A putative depression brought about by the short days of winter.
second messenger: A slow-acting substance in the postsynaptic cell that amplifies the effects of synaptic activity and signals synaptic activity within the postsynaptic cell.
secondary sensory cortex: Also called non-primary sensory cortex. For a given sensory modality, the cortical regions receiving direct projections from primary sensory cortex for that modality. Compare primary sensory cortex.
secondary sensory ending: Also called flower spray ending. The axon that transmits information from the ends of a muscle spindle. Compare primary sensory ending.
secondary somatosensory cortex (S2): Also called somatosensory 2. The region of cortex that receives direct projections from primary somatosensory cortex. See Figure 8.12. Compare primary somatosensory cortex.
seizure: An epileptic episode. See Box 3.3.
selective attention: See attention.
selective permeability: The property of a membrane that allows some substances to pass through, but not others.
selective serotonin reuptake inhibitor (SSRI): A drug that blocks the reuptake of transmitter at serotonergic synapses; commonly used to treat depression.
semantic memory: Generalized memory—for instance, knowing the meaning of a word without knowing where or when you learned that word.
semantics: The meanings or interpretation of words and sentences in a language.
semen: A mixture of fluid, including sperm, that is released during ejaculation.
semicircular canal: One of the three fluid-filled tubes in the inner ear that are part of the vestibular system. Each of the tubes, which are at right angles to each other, detects angular acceleration. See Figure 9.16.
senile dementia: A neurological disorder of the aged that is characterized by progressive behavioral deterioration, including personality change and profound intellectual decline. It includes, but is not limited to, Alzheimer’s disease.
senile plaques: Also called amyloid plaques. Senile plaques are small areas of the brain that have abnormal cellular and chemical patterns. Senile plaques correlate with senile dementia. See Figure 7.28.
sensitive period: The period during development in which an organism can be permanently altered by a particular experience or treatment.
sensitization: 1. A form of nonassociative learning in which an organism becomes more responsive to most stimuli after being exposed to unusually strong or painful stimulation. See Box 17.1. Compare habituation. 2. A process in which the body shows an enhanced response to a given drug after repeated doses. Compare tolerance.
sensorineural deafness: A hearing impairment that originates from cochlear or auditory nerve lesions. Compare central deafness and conduction deafness.
sensory conflict theory: A theory of motion sickness suggesting that discrepancies between vestibular information and visual information simulate food poisoning and therefore trigger nausea.
sensory neuron: A neuron that is directly affected by changes in the environment, such as light, odor, or touch.
sensory pathway: The chain of neural connections from sensory receptor cells to the cortex.
sensory receptor organ: An organ specialized to receive particular stimuli. Examples include the eye and the ear.
sensory transduction: The process in which a receptor cell converts the energy in a stimulus into a change in the electrical potential across its membrane.
septal complex: A brain region that provides subcortical input to the hippocampal formation.
sequential hermaphrodites: Species in which individuals may be exclusively of one sex, and then switch to the other sex. Compare simultaneous hermaphrodites.
serotonergic: Referring to neurons that use serotonin as their synaptic transmitter.
serotonin (5-HT): A synaptic transmitter that is produced in the raphe nuclei and is active in structures throughout the cerebral hemispheres. See Table 4.1; Figure 4.5.
set point: The point of reference in a feedback system. An example is the setting of a thermostat.
set zone: The range of a variable that a feedback system tries to maintain.
sex determination: The process by which the decision is made for a fetus to develop as a male or a female. In mammals this is under genetic control, but in some groups of animals, environmental variables like incubation temperature determine the sex of the offspring.
sex-determining region on the Y chromosome: See SRY gene.
sex steroids: Steroid hormones secreted by the gonads: androgens, estrogens, and progestins.
sexual attraction: The first step in the mating behavior of many animals, in which animals emit stimuli that attract members of the opposite sex. See Figure 12.1.
sexual differentiation: The process by which individuals develop either malelike or femalelike bodies and behavior.
sexual dimorphism: The condition in which males and females show pronounced sex differences in appearance.
sexual selection: Darwin’s theoretical mechanism for the evolution of anatomical and behavioral differences between males and females.
sexually dimorphic nucleus of the preoptic area (SDN-POA): A region of the preoptic area that is five to six times larger in volume in male rats than in females. See Figure 12.18.
sexually receptive: Referring to the state in which an individual (in mammals, typically the female) is willing to copulate. In many species, no sexual activity is possible other than during the period of sexual receptivity in the female, which generally corresponds to ovulation.
shadowing: A task in which the subject is asked to focus attention on one ear or the other while stimuli are being presented separately to both ears, and to repeat aloud the material presented to the attended ear.
sham rage: See decorticate rage.
shell shock: See posttraumatic stress disorder.
shivering: Rapid involuntary muscle contractions that generate heat in hypothermic animals.
short-term memory (STM): A form of memory that usually lasts only for seconds, or as long as rehearsal continues. Compare iconic memory.
SIDS: See sudden infant death syndrome.
simple cortical cell: Also called bar detector or edge detector. A cell in the visual cortex that responds best to an edge or a bar that has a particular width, as well as a particular orientation and location in the visual field. Compare complex cortical cell.
simultagnosia: A profound restriction of attention, often limited to a single item or feature.
simultaneous hermaphrodites: Species in which individuals have both male and female reproductive organs at the same time. Compare sequential hermaphrodites.
site-directed mutagenesis: A technique in molecular biology that changes the sequence of nucleotides in an existing gene.
size principle: The idea that, as increasing numbers of motor neurons are recruited to produce muscle responses of increasing strength, small, low-threshold neurons are recruited first, followed by large, high-threshold neurons.
skill learning: Learning to perform a task that requires motor coordination.
sleep apnea: A sleep disorder in which respiration slows or stops periodically, waking the patient. Excessive daytime somnolence results from the frequent nocturnal awakening.
sleep cycle: A period of slow-wave sleep followed by a period of REM sleep. In humans, a sleep cycle lasts 90–110 minutes.
sleep deprivation: The partial or total prevention of sleep.
sleep enuresis: Bed-wetting.
sleep-maintenance insomnia: Difficulty in staying asleep. Compare sleep-onset insomnia.
sleep-onset insomnia: Difficulty in falling sleep. Compare sleep-maintenance insomnia.
sleep paralysis: A state during the transition to or from sleep, in which the ability to move or talk is temporarily lost.
sleep recovery: The process of sleeping more than normally after a period of sleep deprivation, as though in compensation.
sleep spindle: A characteristic 14- to 18-Hz wave in the EEG of a person said to be in stage 2 sleep. See Figure 14.11.
sleep state misperception: Commonly, a person’s perception that he has not been asleep when in fact he was. Typically occurs at the start of a sleep episode.
slow-twitch muscle fiber: A type of striated muscle fiber that contracts slowly but does not fatigue readily. Compare fast-twitch muscle fiber.
slow-wave sleep (SWS): Sleep, divided into stages 1–4, that is defined by the presence of slow-wave EEG activity. See Figure 14.11. Compare rapid-eye-movement (REM) sleep.
SMA: See supplementary motor area.
smooth movement: See ramp movement.
smooth muscle: A type of muscle fiber, as in the heart, that is controlled by the autonomic nervous system rather than by voluntary control. Compare striated muscle.
SNB: See spinal nucleus of the bulbocavernosus.
SOAE: See spontaneous otoacoustic emission.
sodium ion (Na⁺): A sodium atom that carries a positive charge because it has lost one electron.
sodium-potassium pump: The energetically expensive mechanism that pushes sodium ions out of a cell, and potassium ions in.
solute: A solid compound that is dissolved in a liquid. Compare solvent.
solvent: The liquid (often water) in which a compound is dissolved. Compare solute.
soma (pl. somata): See cell body.
somatic intervention: An approach to finding relations between body variables and behavioral variables that involves manipulating body structure or function and looking for resultant changes in behavior. See Figure 1.2. Compare behavioral intervention.
somatic nerve: See spinal nerve.
somatomedins: A group of proteins, released from the liver in response to growth hormone, that aid body growth and maintenance.
somatosensory: Referring to body sensation, particularly touch and pain sensation.
somatosensory 1: See primary somatosensory cortex.
somatosensory 2: See secondary somato-sensory cortex.
somatotropic hormone: See growth hormone.
somatotropin: See growth hormone.
somnambulism: Sleepwalking.
Southern blot: A method of detecting a particular DNA sequence in the genome of an organism, by separating DNA with gel electrophoresis, blotting the separated DNAs onto nitrocellulose, and then using a nucleotide probe to hybridize with, and highlight, the gene of interest. See Appendix Figure A.3. Compare Northern blot and Western blot.
spasticity: Markedly increased rigidity in response to forced movement of the limbs.
spatial-frequency filter model: A model of pattern analysis that emphasizes Fourier analysis of visual stimuli. Compare feature detector model.
spatial resolution: The ability to observe the detailed structure of the brain. Compare temporal resolution.
spatial summation: The summation at the axon hillock of postsynaptic potentials from across the cell body. If this summation reaches threshold, an action potential is triggered. See Figure 3.10. Compare temporal summation.
species: A group of individuals that can readily interbreed to produce fertile offspring. Individuals of different species produce either no offspring or infertile offspring. See Figure 6.3.
specific nerve energies: The doctrine that the receptors and neural channels for the different senses are independent and operate in their own special ways, and can produce only one particular sensation each.
spectral filtering: Alteration of the amplitude of some, but not all, frequencies in a sound. When performed by the irregular shapes of the external ear, this process is a source of information that assists in the localization of sound sources.
spectrally opponent cell: A visual receptor cell that has opposite firing responses to different regions of the spectrum. See Figures 10.25, 10.26.
sperm: The gamete produced by males for fertilization of eggs (ova).
sperm competition: The selective pressure that males of promiscuous species exert on each other to produce gametes that can outcompete the sperm of other males, because sperm from multiple males may be present in the genital tract of a single female.
spinal animal: An animal whose spinal cord has been surgically disconnected from the brain to enable the study of behaviors that do not require brain control.
spinal nerve: Also called somatic nerve. A nerve that emerges from the spinal cord. There are 31 pairs of spinal nerves. See Figure 2.10.
spinal nucleus of the bulbocavernosus (SNB): A group of motoneurons in the spinal cord of rats that innervate striated muscles controlling the penis. See Figure 12.20. See also Onuf’s nucleus.
spinocerebellum: The uppermost part of the cerebellum, consisting mostly of the vermis and anterior lobe. It receives sensory information about the current spatial location of the parts of the body and anticipates subsequent movement. Compare cerebrocerebellum and vestibulocerebellum.
spinothalamic system: See anterolateral system.
split-brain individual: An individual whose corpus callosum has been severed, halting communication between the right and left hemispheres.
spontaneous otoacoustic emission (SOAE): A sound produced by the ears of many normal people. Compare evoked otoacoustic emission.
SRY gene: A gene on the Y chromosome that directs the developing gonads to become testes. The name SRY stands for sex-determining region on the Y chromosome.
SSRI: See selective serotonin reuptake inhibitor.
stage 1 sleep: The initial stage of slow-wave sleep, which is characterized by small-amplitude EEG waves of irregular frequency, slow heart rate, and reduced muscle tension. See Figure 14.11.
stage 2 sleep: A stage of slow-wave sleep that is defined by bursts of regular 14- to 18-Hz EEG waves called sleep spindles. See Figure 14.11.
stage 3 sleep: A stage of slow-wave sleep that is defined by the spindles seen in stage 2 sleep, mixed with larger-amplitude slow waves. See Figure 14.11.
stage 4 sleep: A stage of slow-wave sleep that is defined by the presence of delta waves at least half the time. See Figure 14.11.
standard condition (SC): The usual environment for laboratory rodents, with a few animals in a cage and adequate food and water, but no complex stimulation. See Figure 17.17. Compare enriched condition and impoverished condition.
stapedius: A middle-ear muscle that is attached to the stapes. See Figure 9.2.
stapes: Latin for “stirrup.” A middle-ear bone that is connected to the oval window; one of the three ossicles that conduct sounds across the middle ear. See Figure 9.2.
stem cell: A cell that is undifferentiated and therefore can take on the fate of any cell that a donor organism can produce.
stereocilium (pl. stereocilia): A relatively stiff hair that protrudes from a hair cell in the auditory or vestibular system. See Figure 9.2.
steroid hormones: A class of hormones, each of which is composed of four interconnected rings of carbon atoms.
steroid receptor cofactors: Proteins that affect the cell’s response when a steroid hormone binds its receptor.
stimulus (pl. stimuli): A physical event that triggers a sensory response.
stimulus cuing: A technique for testing reaction time to sensory stimuli in which a cue to the location in which the stimulus will be presented is provided before the stimulus itself.
STM: See short-term memory.
stress: Any circumstance that upsets homeostatic balance. Examples include exposure to extreme cold or heat or an array of threatening psychological states.
stress immunization: The concept that mild stress early in life makes an individual better able to handle stress later in life.
stretch reflex: The contraction of a muscle in response to stretch of that muscle. See Figure 11.10.
striate cortex: See primary visual cortex.
striated muscle: A type of muscle with a striped appearance, generally under voluntary control. Compare smooth muscle.
striatum: The caudate nucleus and putamen together.
stroke: Damage to a region of brain tissue that results from blockage or rupture of vessels that supply blood to that region.
STX: See saxitoxin.
subcutaneous tissue: See hypodermis.
subfornical organ: One of the circumventricular organs. See Figure 13.14.
subicular complex: See subiculum.
subiculum (pl. subicula): Also called subicular complex or hippocampal gyrus. A region adjacent to the hippocampus that contributes to the hippocampal formation. See Figure 17.21.
substance abuse: A maladaptive pattern of substance use that has lasted more than a month but does not fully meet the criteria for dependence.
substance P: A peptide transmitter implicated in pain transmission.
substantia nigra: Literally, “black spot.” A group of pigmented neurons in the midbrain that provides dopaminergic projections to areas of the forebrain, especially the basal ganglia.
sudden infant death syndrome (SIDS): Also called crib death. The sudden, unexpected death of an apparently healthy human infant who simply stops breathing, usually during sleep. SIDS is not well understood.
sulcus (pl. sulci): A furrow of a convoluted brain surface. See Figure 2.12. Compare gyrus.
superior colliculi (sing. colliculus): Paired gray matter structures of the dorsal midbrain that receive visual information and are involved in direction of visual gaze and visual attention to intended stimuli. See Figures 2.12, 10.11. Compare inferior colliculi.
superior olivary nuclei: Brainstem nuclei that receive input from both right and left cochlear nuclei, and provide the first binaural analysis of auditory information. See Figure 9.7.
superordinate circuit: Also called modulatory circuit. A neural circuit that is hierarchically superior to other, simple circuits.
supersensitivity psychosis: An exaggerated psychosis that may emerge when doses of antipsychotic medication are reduced, probably as a consequence of the up-regulation of receptors that occurred during drug treatment.
supplementary motor area (SMA): A region of nonprimary motor cortex that receives input from the basal ganglia and modulates the activity of the primary motor cortex. See Figure 11.16.
suprachiasmatic nucleus (SCN): A small region of the hypothalamus above the optic chiasm that is the location of a circadian oscillator. See Figure 14.5.
surface dyslexia: Acquired dyslexia in which the patient seems to attend only to the fine details of reading. Compare deep dyslexia.
sustained attention task: A task in which a single stimulus source or location must be held in the attentional spotlight for a protracted period.
SWS: See slow-wave sleep.
Sylvian fissure: Also called lateral sulcus. A deep fissure that demarcates the temporal lobe. See Figure 2.12.
symbolic cuing task: A task that tests endogenous attention by presenting a visual stimulus and asking subjects to respond as soon as the stimulus appears on a screen. Each trial is preceded by a meaningful symbol used as a cue to hint at where the stimulus will appear. Compare peripheral spatial cuing task.
sympathetic chain: A chain of ganglia that runs along each side of the spinal column; part of the sympathetic nervous system. See Figure 2.11.
sympathetic nervous system: A component of the autonomic nervous system that arises from the thoracic and lumbar spinal cord. Compare parasympathetic nervous system. See Figure 2.11.
synapse: The cellular location at which information is transmitted from a neuron to another cell. See Figure 2.7.
synapse rearrangement: Also called synaptic remodeling. The loss of some synapses and the development of others; a refinement of synaptic connections that is often seen in development. See Figure 7.13.
synaptic cleft: The space between the presynaptic and postsynaptic elements. This gap measures about 20–40 nm. See Figures 2.7, 3.12.
synaptic delay: The brief delay between the arrival of an action potential at the axon terminal and the creation of a postsynaptic potential. The delay is caused by the translation of an electrical event into a secretory event, and back to an electrical event on the postsynaptic side.
synaptic remodeling: See synapse re-arrangement.
synaptic transmitter: See neurotransmitter.
synaptic vesicle: A small, spherical structure that contains molecules of neurotransmitter. See Figure 2.7.
synaptogenesis: The establishment of synaptic connections as axons and dendrites grow. See Figure 7.8.
synergist: A muscle that acts together with another muscle. See also agonist (definition 2). Compare antagonist (definition 2).
synesthesia: A condition in which stimuli in one modality evoke the involuntary experience of an additional sensation in another modality.
syntax: The grammatical rules for constructing phrases and sentences in a language.
syrinx: The vocal organ in birds.

T

T cell: See T lymphocyte.
T lymphocyte: Also called T cell. An immune system cell, formed in the thymus (hence the T), that attacks foreign microbes or tissue; “killer cell.” See Figure 15.22. Compare B lymphocyte.
T1R: A family of taste receptor proteins that, when particular members heterodimerize, form taste receptors for sweet flavors and umami flavors. Compare T2R.
T2R: A family of bitter taste receptors. Compare T1R.
TAARs: See trace amine-associated receptors.
tachistoscope test: A test in which stimuli are very briefly exposed in either the left or right visual half-field.
tactile: Of or relating to touch.
tardive dyskinesia: A disorder characterized by involuntary movements, especially involving the face, mouth, lips, and tongue; related to prolonged use of antipsychotic drugs, such as chlorpromazine. See Box 16.1.
tastant: A substance that can be tasted.
taste aversion: The conditioned avoidance of a particular food due to a previous pairing between the taste of that food and physical illness.
taste bud: A cluster of 50–150 cells that detects tastes. Taste buds are found in papillae. See Figure 9.19.
taste pore: The small aperture through which tastant molecules are able to access the sensory receptors of the taste bud. See Figure 9.19.
tau: 1. A protein associated with neurofibrillary tangles in Alzheimer’s disease. 2. A mutation (tau) in hamsters that causes a shorter circadian period in free-running conditions.
tauopathy: Any disease that is associated with abnormal accumulations of the protein Tau, forming neurofibrillary tangles that impair the normal function of neurons. Examples include Alzheimer’s disease, progressive supranuclear palsy, and chronic traumatic encephalopathy (punch-drunk).
taxonomy: The classification of organisms. See Figure 6.3.
tectorial membrane: A membrane that sitsatop the organ of Corti in the cochlear duct. See Figure 9.2.
tectum: The dorsal portion of the midbrain, including the inferior and superior colliculi.
telencephalon: The frontal subdivision of the forebrain that includes the cerebral hemispheres when fully developed. See Figure 2.14.
temporal lobes: Large lateral cortical regions of each cerebral hemisphere, continuous with the parietal lobes posteriorly, and separated from the frontal lobe by the Sylvian fissure. The temporal lobes contain the hippocampus and amygdala, and are involved in a variety of functions, including memory, emotional processing, and the olfactory and auditory senses. See Figure 2.12.
temporal resolution: The ability to track changes in the brain that occur very quickly. Compare spatial resolution.
temporal summation: The summation of postsynaptic potentials that reach the axon hillock at different times. The closer in time that the potentials occur, the more complete the summation. Compare spatial summation.
temporoparietal junction (TPJ): The point in the brain where the temporal and parietal lobes meet; plays a role in shifting attention to a new location after target onset.
tendon: Strong tissue that connects muscles to bone.
TENS: See transcutaneous electrical nerve stimulation.
tensor tympani: The muscle attached to the malleus that modulates mechanical linkage to protect the delicate receptor cells of the inner ear from damaging sounds. See Figure 9.2.
testes (sing. testis): The male gonads, which produce sperm and androgenic steroid hormones. See Figures 5.1, 12.8, 12.13; Table 5.2.
testosterone: A hormone, produced by male gonads, that controls a variety of bodily changes that become visible at puberty. See Figures 5.15, 5.19; Table 5.2.
tetanus: An intense volley of action potentials. See Figure 17.21.
tetrahydrocannabinol (THC): See Δ9-tetrahydrocannabinol.
tetraiodothyronine: See thyroid hormones.
tetrodotoxin (TTX): A toxin from puffer fish ovaries that blocks the voltage-gated sodium channel, preventing action potential conduction.
thalamus (pl. thalami): The brain regions at the top of the brainstem that trade information with the cortex. See Figures 2.12, 2.14.
THC: See Δ9-tetrahydrocannabinol.
therapeutic index: The margin of safety for a given drug, expressed as the distance between effective doses and toxic doses. See Figure 4.8.
third ventricle: The midline ventricle that conducts cerebrospinal fluid from the lateral ventricles to the fourth ventricle. See Figure 2.19.
thoracic: Referring to the 12 spinal segments below the cervical (neck) portion of the spinal cord, corresponding to the chest. See Figures 2.10, 2.11.
threshold: The stimulus intensity that is just adequate to trigger an action potential at the axon hillock.
thrombolytics: A class of substances that are used to unblock blood vessels and restore circulation.
thyroid gland: An endocrine gland, located in the throat, that regulates cellular metabolism throughout the body. See Figure 5.1; Table 5.2.
thyroid hormones: Two hormones, triiodothyronine and thyroxine (also called tetraiodothyronine), released from the thyroid gland that have widespread effects, including growth and maintenance of the brain.
thyroid-stimulating hormone (TSH): A tropic hormone, released by the anterior pituitary gland, that signals the thyroid gland to secrete its hormones. See Figures 5.10, 5.15.
thyrotropin-releasing hormone (TRH): A hypothalamic hormone that regulates the release of thyroid-stimulating hormone from the anterior pituitary. See Figure 5.10.
thyroxine: See thyroid hormones.
timbre: The characteristic sound quality of a musical instrument, as determined by the relative intensities of its various harmonics.
tinnitus: A sensation of noises or ringing in the ears.
tip link: A fine, threadlike fiber that runs along and connects the tips of stereocilia. See Figure 9.5.
TMS: See transcranial magnetic stimulation.
tolerance: A condition in which, with repeated exposure to a drug, an individual becomes less responsive to a constant dose. Compare sensitization (definition 2).
tonic receptor: A receptor in which the frequency of action potentials declines slowly or not at all as stimulation is maintained. Compare phasic receptor.
tonotopic organization: A major organizational feature in auditory systems in which neurons are arranged as an orderly map of stimulus frequency, with cells responsive to high frequencies located at a distance from those responsive to low frequencies.
top-down process: A process in which higher-order cognitive processes control lower-order systems, often reflecting conscious control. Endogenous attention is one example. Compare bottom-up process.
torpor: The condition in which an animal allows its body temperature to fall drastically. During torpor, animals are unresponsive to most stimuli.
Tourette’s syndrome: A heightened sensitivity to tactile, auditory, and visual stimuli that may be accompanied by the buildup of an urge to emit verbal or phonic tics. See Box 16.3.
TPJ: See temporoparietal junction.
trace amine-associated receptors (TAARs): A family of probable pheromone receptors produced by neurons in the main olfactory epithelium. TAARs are candidate pheromone receptors, despite being situated outside the vomeronasal organ.
trace conditioning: A form of conditioning in which a longer delay separates the conditioned and unconditioned stimuli. Compare delay conditioning.
tract: A bundle of axons found within the central nervous system. Compare nerve.
transcranial magnetic stimulation (TMS): Localized, noninvasive stimulation of cortical neurons through the application of strong magnetic fields. In repetitive TMS (rTMS), this focal magnetic stimulation of the brain is cycled several times per second, producing transient but measurable changes in behavior that may be of use in clinical settings, as well as in research.
transcript: The mRNA strand that is produced when a stretch of DNA is “read.”
transcription: The process during which mRNA forms bases complementary to a strand of DNA. The resulting message (called a transcript) is then used to translate the DNA code into protein molecules. See Appendix Figure A.2.
transcription factor: A substance that binds to recognition sites on DNA and alters the rate of expression of particular genes.
transcutaneous electrical nerve stimulation (TENS): The delivery of electrical pulses through electrodes attached to the skin, which excite nerves that supply the region to which pain is referred. TENS can relieve the pain in some instances.
transduction: The conversion of one form of energy to another.
transgenic: Referring to an animal in which a new or altered gene has been deliberately introduced into the genome. See Box 7.3.
transient receptor potential 2 (TRP2): A receptor, found in some free nerve endings, that opens its channel in response to rising temperatures. See Figure 8.22.
transient receptor potential vanilloid type 1 (TRPV1): Also called vanilloid receptor 1. A receptor that binds capsaicin to transmit the burning sensation from chili peppers and normally detects sudden increases in temperature. See Figure 8.22.
translation: The process by which amino acids are linked together (directed by an mRNA molecule) to form protein molecules. See Appendix Figure A.2.
transmitter: See neurotransmitter.
transport vesicle: A spheroid intracellular structure that contains molecules of important substances. Transport vesicles are moved between locations within the cell—for example, along the axons of neurons—in order to deliver substances to the locations where they are needed.
transporters: Specialized receptors in the presynaptic membrane that recognize transmitter molecules and return them to the presynaptic neuron for reuse.
transverse plane: See coronal plane.
TRH: See thyrotropin-releasing hormone.
trichromatic hypothesis: A hypothesis of color perception stating that there are three different types of cones, each excited by a different region of the spectrum and each having a separate pathway to the brain.
tricyclic antidepressants: A class of drugs that act by increasing the synaptic accumulation of serotonin and norepinephrine.
triiodothyronine: See thyroid hormones.
trinucleotide repeat: Repetition of the same three nucleotides within a gene, which can lead to dysfunction, as in the cases of Huntington’s disease and fragile X syndrome.
trophic factor: A substance that promotes cell growth and survival. See also neurotrophic factor.
tropic hormones: A class of anterior pituitary hormones that affect the secretion of other endocrine glands. See Figure 5.15.
TRP2: See transient receptor potential 2.
TRP8: See cool-menthol receptor 1.
TRPV1: See transient receptor potential vanilloid type 1.
TSH: See thyroid-stimulating hormone.
TTX: See tetrodotoxin.
tuberomammillary nucleus: A region of the basal hypothalamus, near the pituitary stalk, that plays a role in generating SWS.
tuning curve: A graph of the responses of a single auditory nerve fiber or neuron to sounds that vary in frequency and intensity.
turbinates: Complex shapes underlying the olfactory mucosa that direct inspired air over receptor cells. See Figure 9.22.
Turner’s syndrome: A condition seen in individuals carrying a single X chromosome but no other sex chromosome.
tympanic canal: See scala tympani.
tympanic membrane: Also called eardrum. The partition between the external ear and the middle ear. See Figure 9.2.
typical neuroleptics: A major class of antischizophrenic drugs that share antagonist activity at dopamine D₂ receptors. Compare atypical neuroleptics.

U

ultradian: Referring to a rhythmic biological event whose period is shorter than that of a circadian rhythm, usually from several minutes to several hours long. Compare infradian.
ultrasound: High-frequency sound; in general, above the threshold for human hearing, at about 20,000 Hz. Compare infrasound.
umami: One of the five basic tastes (along with salty, sour, sweet, and bitter), probably mediated by amino acids in foods.
unconditioned response (UR): See classical conditioning.
unconditioned stimulus (US): See classical conditioning.
unipolar depression: Depression that alternates with normal emotional states. Compare bipolar disorder.
unipolar neuron: Also called monopolar neuron. A nerve cell with a single branch that leaves the cell body and then extends in two directions; one end is the receptive pole, the other end the output zone. See Figure 2.5. Compare bipolar neuron and multipolar neuron.
up-regulation: A compensatory increase in receptor availability at the synapses of a neuron. Compare down-regulation.
UR: See classical conditioning.
US: See classical conditioning.
utricle: A small, fluid-filled sac in the vestibular system above the saccule that responds to static positions of the head. See Figure 9.16.

V

V1: See primary visual cortex.
vagus nerve: Cranial nerve X, which provides extensive innervation of the viscera (organs). The vagus both regulates visceral activity and transmits signals from the viscera to the brain. See Figures 2.9, 13.26.
vanilloid receptor 1: See transient receptor potential vanilloid type 1.
varicosity: The axonal swelling from which neurotransmitter diffuses in a nondirected synapse.
vasopressin: See arginine vasopressin.
ventral: In anatomy, toward the belly or front of the body, or the bottom of the brain. See Box 2.2. Compare dorsal.
ventral root: See roots.
ventral tegmental area (VTA): A portion of the midbrain that projects dopaminergic fibers to the nucleus accumbens.
ventricular system: A system of fluid-filled cavities inside the brain. See Figure 2.19.
ventricular zone: Also called ependymal layer. A region lining the cerebral ventricles that displays mitosis, providing neurons early in development and glial cells throughout life. See Figure 7.5.
ventromedial hypothalamus (VMH): A hypothalamic region involved in eating and sexual behaviors. See Figures 12.6, 13.22.
vertebral arteries: Arteries that ascend the vertebrae, enter the base of the skull, and join together to form the basilar artery. See Figure 2.20.
vertex spike: An sharp-wave EEG pattern that is seen during stage 1 slow-wave sleep. See Figure 14.11.
vestibular canal: See scala vestibuli.
vestibular nuclei: Brainstem nuclei that receive information from the vestibular organs through cranial nerve VIII (the vestibulocochlear nerve).
vestibulocerebellum: The middle portion of the cerebellum, sandwiched between the spinocerebellum and the cerebrocerebellum and consisting of the nodule and the flocculus. It helps the motor systems to maintain posture and appropriate orientation toward the external world. See Figure 11.24.
vestibulocochlear nerve: Cranial nerve VIII, which runs from the cochlea to the brainstem auditory nuclei. See Figures 2.9, 9.1.
vestibulo-ocular reflex (VOR): The brainstem mechanism that maintains gaze on a visual object despite movements of the head.
visual acuity: Sharpness of vision.
visual cortex: See occipital cortex.
visual field: The whole area that you can see without moving your head or eyes.
viviparous: Of or relating to viviparity (literally, “live birth”), reproduction in which the zygote develops extensively within the female until a well-formed individual emerges. Compare oviparous and ovoviviparous.
VMH: See ventromedial hypothalamus.
VNO: See vomeronasal organ.
volley theory: A theory of frequency discrimination that emphasizes the relation between sound frequency and the firing pattern of nerve cells. For example, a 500-Hz tone would produce 500 neural discharges per second by a nerve cell or group of nerve cells. Compare place theory.
voltage-gated Na⁺ channel: An Na⁺-selective channel that opens or closes in response to changes in the voltage of the local membrane potential. Voltage-gated Na⁺ channels mediate the action potential. Compare ligand-gated ion channel.
voluntary attention: See endogenous attention.
vomeronasal organ (VNO): A collection of specialized receptor cells, near to but separate from the olfactory epithelium, that detect pheromones and send electrical signals to the accessory olfactory bulb in the brain.
vomeronasal system: A specialized chemical detection system that detects pheromones and transmits information to the brain.
VOR: See vestibulo-ocular reflex.
VTA: See ventral tegmental area.

W

Wada test: A test in which a short-lasting anesthetic is delivered into one carotid artery to determine which cerebral hemisphere principally mediates language. See Box 19.1.
Wallerian degeneration: See anterograde degeneration.
war neurosis: See posttraumatic stress disorder.
wavelength: Here, the length between two peaks in a repeated stimulus such as a wave, light, or sound. See Box 9.1.
Wernicke’s aphasia: See fluent aphasia.
Wernicke’s area: A region of temporoparietal cortex in the brain that is involved in the perception and production of speech. See Figures 19.6, 19.7, 19.8.
Western blot: A method of detecting a particular protein molecule in a tissue or organ, by separating proteins from that source with gel electrophoresis, blotting the separated proteins onto nitrocellulose, and then using an antibody that binds, and highlights, the protein of interest. Compare Northern blot and Southern blot.
whisker barrel: A barrel-shaped column of somatosensory cortex in rodents that receives information from a particular whisker.
white matter: A shiny layer underneath the cortex that consists largely of axons with white myelin sheaths. See Figure 2.13. Compare gray matter.
Williams syndrome: A disorder characterized by fluent linguistic function, but poor performance on standard IQ tests and great difficulty with spatial processing. See Figure 19.14.
withdrawal symptom: An uncomfortable symptom that arises when a person stops taking a drug that he or she has used frequently, especially at high doses.
wolffian duct: A duct system in the embryo that will develop into male structures (the epididymis, vas deferens, and seminal vesicles) if testes are present in the embryo. See Figure 12.13. Compare müllerian duct.
word deafness: The specific inability to hear words, although other sounds can be detected.
working memory: A buffer that holds memories available for ready access during performance of a task. See Figure 17.17.

Z

zeitgeber: Literally, “time-giver” (in German). The stimulus (usually the light-dark cycle) that entrains circadian rhythms.
zygote: The fertilized egg.