Chapter 3 Summary & Outline

Electrical Signals Are the Vocabulary of the Nervous System

• Nerve cells are specialized for receiving, processing, and transmitting signals. Chemical signals transmit information between neurons; electrical signals transmit information within a neuron.
• Neurons exhibit a small electrical potential across the cell membrane; neural signals are changes in this potential.
• The different concentrations of ions inside and outside the neuron—especially potassium ions (K⁺), to which the resting membrane is selectively permeable—account for the resting potential. At equilibrium, the electrostatic pressure pulling K⁺ ions into the neuron is balanced by the concentration gradient pushing them out; at this point, the membrane potential is about –60 mV, the resting potential. Review Figure 3.3, Web Activity 3.1
• The brain uses a great deal of energy maintaining ionic gradients through the operation of sodium-potassium pumps. Review Web Activity 3.2
• Reduction of the resting potential (depolarization) in axons opens voltage-gated channels. If the neuronal membrane is depolarized until it reaches a threshold value, voltage-gated sodium (Na+) channels of the axonal membrane open and the membrane becomes completely permeable to Na⁺. As a result, Na⁺ ions rush in, and the axon becomes briefly more positive inside than outside. This event is called an action potential. Following the action potential, the resting membrane potential is quickly restored. Review Figure 3.6, Web Activity 3.3
• The action potential strongly depolarizes the adjacent patch of axonal membrane, causing it to generate its own action potential. In this regenerative manner, the action potential spreads down the axon. Review Figure 3.8, Web Activity 3.4
• Postsynaptic (local) potentials spread very rapidly, but they are not regenerated. They diminish in amplitude as they spread passively along dendrites and the cell body.
• Excitatory postsynaptic potentials (EPSPs) are depolarizing (they decrease the resting potential) and increase the likelihood that the neuron will generate an action potential. Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizing (they increase the resting potential) and decrease the likelihood that the neuron will fire. Review Figure 3.5
• Cell bodies process information by integrating (summing algebraically) the postsynaptic potentials moving across their surfaces. Postsynaptic potentials are integrated through both spatial summation (summing potentials that occur in different locations) and temporal summation (summing potentials across time). Review Figure 3.11, Web Activity 3.5
• An action potential is initiated at the axon hillock when the excess of EPSPs over IPSPs reaches threshold.
• During the action potential, the neuron cannot be excited by a second stimulus; it is absolutely refractory. For a few milliseconds afterward, the hyperpolarized neuron is relatively refractory, requiring a stronger stimulation than usual in order to fire.
• Some synapses use electrical transmission and do not require a chemical transmitter. At these electrical synapses, the cleft between presynaptic and postsynaptic cells is extremely small.

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Synaptic Transmission Requires a Sequence of Events

• At most synapses, the transmission of information from one neuron to another requires a chemical transmitter that diffuses across the synaptic cleft and binds to receptor molecules in the postsynaptic membrane. A substance that binds to a receptor is called a ligand. Review Box 3.2, Web Activity 3.6

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Neurons and Synapses Combine to Make Circuits

• At ionotropic synapses, the receptor molecule responds to recognition of a transmitter by opening an ion channel within its own structure. At metabotropic synapses, the binding of a transmitter molecule to a receptor molecule activates an intracellular second-messenger system that can have a variety of effects, including the opening of membrane channels. Review Figure 3.15, Web Activity 3.7
• Neurons and synapses are assembled into circuits that process information. Some circuits are very simple, involving only a few cells; others may be massive, involving millions of neurons. Review Figures 3.17 and 3.18

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Gross Electrical Activity of the Human Brain

• The summation of electrical activity over millions of nerve cells can be detected by electrodes on the scalp. Electroencephalograms (EEGs) can reveal rapid changes in brain function, especially in response to a brief, controlled stimulus that evokes an event-related potential (ERP). Review Figure 3.19

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