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Chapter 5 Summary & Outline

Hormones Act in a Great Variety of Ways throughout the Body

Hormones are chemical compounds that act as signals in the body. Many are secreted by endocrine glands into the bloodstream and are taken up by receptor molecules in target cells. Review Figure 5.1, Web Activity 5.1
Neural communication differs from hormonal communication in that neural signals travel rapidly over fixed pathways, whereas hormonal signals spread more slowly and throughout the body.
Neural and hormonal communication systems have several characteristics in common: Both utilize chemical messages; some substances act as a hormone in some locations and as a synaptic transmitter in others. Both systems manufacture, store, and release chemical messengers. Both use specific receptors and may employ second messengers. Review Figure 5.6, Web Activity 5.2

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Some hormones have receptors in a wide variety of cells and can therefore influence the activity of most cells in the body. Other hormones have receptors in only certain special cells or organs. Review Figure 5.8, Web Activity 5.3
Hormones act by promoting the proliferation and differentiation of cells and by modulating the activity of cells that have already differentiated.
Protein and amine hormones typically bind to specific receptor molecules at the surface of the target cell membrane and activate second-messenger molecules inside the cell. Steroid hormones generally pass through the membrane and bind to receptor molecules inside the cell, ultimately regulating gene expression. However, some steroid actions involve other, more rapid signaling mechanisms that include cell surface receptors.
A negative feedback system monitors and controls the rate of secretion of each hormone. In the simplest case the hormone acts on target cells, leading them to change the amount of a substance they release; this change in turn regulates output of the endocrine gland. Review Figure 5.9

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Several hormones are controlled by a more complex feedback system: a releasing hormone from the hypothalamus regulates the release of an anterior pituitary tropic hormone, which in turn controls secretion by an endocrine gland. In these cases, negative feedback of the endocrine hormone acts mainly at the hypothalamus and anterior pituitary. Review Figures 5.10 and 5.15, Web Activity 5.4
Endocrine influences on structures and functions often involve more than one hormone, as in growth, homeostasis, metabolism, and learning and memory. Review Table 5.2

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Circulating hormones alter the probability of various behaviors, including regulating social behaviors. Abnormally high or low levels of hormones can result in symptoms of cognitive, emotional or psychiatric disorders. Review Table 5.3

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Many behaviors require the coordination of neural and hormonal components. Messages may be transmitted in the body via neural-to-neural, neural-to-endocrine, endocrine-to-endocrine, or endocrine-to-neural links. There are continuous, reciprocal influences between the endocrine system and the nervous system: experience affects hormone secretion, and hormones affect behavior and therefore future experiences. Review Figures 5.22 and 5.23

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