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oxytocin

cervical stretch receptors

contraction

uterus

FIGURE 6 Regulation of oxytocin secretion showing a positive feedback arrangement. Oxytocin stimulates the uterus to contract and causes the cervix to stretch. Increased cervical stretch is sensed by neurons in the cervix and transmitted to the hypothalamus, which signals more oxytocin secretion. Oxytocin secreted in response to suckling forms an open-loop feedback system in which positive input is interrupted when the infant is satisfied and stops suckling. Further details are given in Chapter 47.

FIGURE 6 Regulation of oxytocin secretion showing a positive feedback arrangement. Oxytocin stimulates the uterus to contract and causes the cervix to stretch. Increased cervical stretch is sensed by neurons in the cervix and transmitted to the hypothalamus, which signals more oxytocin secretion. Oxytocin secreted in response to suckling forms an open-loop feedback system in which positive input is interrupted when the infant is satisfied and stops suckling. Further details are given in Chapter 47.

hormone from nerve terminals in the posterior pituitary gland (Fig. 6). The importance of neural input to oxytocin secretion is underscored by the observation that it may also be secreted in a conditioned reflex. A nursing mother sometimes releases oxytocin in response to cries of her baby even before the infant begins to suckle. Oxytocin is also secreted at the same low basal rate in both men and women, but no physiologic role for oxytocin in men and nonlactating women has been established.

Signals for AVP secretion in response to increased osmolality of the blood are thought to originate in hypothalamic neurons and possibly in the AVP secretory cells themselves. Osmoreceptor cells are exquisitely sensitive to small increases in osmolality and signal cells in the supraoptic and paraventricular nuclei to secrete AVP. AVP is also secreted in response to decreased blood volume. Although the specific cells responsible for monitoring blood volume have not been identified, volume monitors appear to be located within the thorax and relay their information to the central nervous system in afferent neurons in the vagus nerves. The control of AVP secretion is shown in Fig. 7 and is discussed more fully in Chapter 28. Under basal conditions blood levels of AVP fluctuate in a diurnal rhythmic pattern that closely resembles that of ACTH.

Suggested Reading

Andersen B, Rosenfeld MG. POU domain factors in the neuroendocrine system: Lessons from developmental biology provide insights into human disease. Endocr Rev 2001;22:2-35.

Eipper BA, Mains RE. Structure and biosynthesis of Pro-ACTH/ endorphin and related peptides. Endocr Rev 1980;1:1-27.

Gharib SD, Wierman ME, Shupnik, MA, Chin WW. Molecular biology of the pituitary gonadotropins. Endocr Rev 1990;11: 177-199.

Ling N, Zeytin F, Bohlen P, Esch F, Brazeau P, Wehrenberg WB, Baird A, Guillemin R. Growth hormone releasing factors. Annu Rev Biochem 1985;54:404-424.

Miller WL, Eberhardt NL. Structure and evolution of the growth hormone gene family. Endocr Rev 1983;4:97-130.

Osorio M, Kopp P, Marui S, Latronico AC, Mendonca BB, Arnhold IJ. Combined pituitary hormone deficiency caused by a novel mutation

38. Pituitary Gland volume receptors

decreased plasma volume central nervous relays t supraoptic and paraventricular nuclei

posterior pituitary osmo-receptors

increased plasma osmolarity

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