Hormone Release Is Regulated by a Hierarchy of Neuronal and Hormonal Signals

The changing levels of specific hormones regulate specific cellular processes, but what regulates the level of each hormone? The brief answer is that the central nervous system receives input from many internal and external sensors—signals about danger, hunger, dietary intake, blood composition and pressure, for example—and orchestrates the production of appropriate hormonal signals by the endocrine tissues. For a more complete answer, we must look at the hormone-producing systems of the human body and some of their functional interrelationships.

Figure 23-7 shows the anatomic location of the major endocrine glands in humans, and Figure 23-8 represents the "chain of command" in the hormonal signaling hierarchy. The hypothalamus, a small region of the brain (Fig. 23-9), is the coordination center of the

FIGURE 23-7 The major endocrine glands. The glands are shaded dark pink.

endocrine system; it receives and integrates messages from the central nervous system. In response to these messages, the hypothalamus produces regulatory hormones (releasing factors) that pass directly to the nearby pituitary gland, through special blood vessels and neurons that connect the two glands (Fig. 23-9b). The pituitary gland has two functionally distinct parts. The posterior pituitary contains the axonal endings of many neurons that originate in the hypothalamus. These neurons produce the short peptide hormones oxytocin and vasopressin (Fig. 23-10), which then move down the axon to the nerve endings in the pituitary, where they are stored in secretory granules to await the signal for their release.

The anterior pituitary responds to hypothalamic hormones carried in the blood, producing tropic hormones, or tropins (from the Greek tropos, "turn"). These relatively long polypeptides activate the next rank of endocrine glands (Fig. 23-8), which includes the adrenal cortex, thyroid gland, ovaries, and testes. These glands in turn secrete their specific hormones, which are carried in the bloodstream to the receptors of cells in the target tissues. For example, corticotropin-releasing hormone from the hypothalamus stimulates the anterior pituitary to release ACTH, which travels to the zona fas-ciculata of the adrenal cortex and triggers the release of

Neuroendocrine origins of signals

Sensory input from environment

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