Hormones of the Neurohypophysis

Two hormones, oxytocin and antidiuretic hor­mone, are released by the neurohypophysis.

Oxytocin is produced by the neurons of the paraventricular nucleus of the hypothalamus. When released into the bloodstream by their axon terminals, it induces contraction of target smooth muscle fibers of the mammary gland and the uterus.

Physical stimulation of the mammary gland (e.g., suckling) is a strong stimulus to secretion of oxytocin, but it is common for other sorts of stimuli to condition a reflex release of the hormone. For example, dairy cows frequently learn to associate the milking parlor and the preparations for milking with milk let-down. Even before the mammary gland is handled in any way, oxy­tocin is released and milk may be seen drip­ping from the teats.

in the pregnant uterus, oxytocin acts on the myometrium (muscle of the uterus) to produce uterine contractions for expulsion of the fetus at parturition. Stretching of the cervix by the fetus stimulates further secretion of oxytocin, which then stimulates greater uterine contrac­tions. oxytocin is sometimes administered by injection at parturition to enhance uterine contractions.

Antidiuretic hormone (ADH) is produced by neurons of the supraoptic nucleus in the hypothalamus and released from the neurohy­pophysis in response to increases in blood osmolality (concentration of dissolved sub­stances) or severe decreases in blood pressure, both of which are influenced by the animal’s hydration status. The water-retaining effects of ADH on the kidney are discussed in Chapter 23. ADH also produces constriction of blood vessels, an effect that gives the hormone its other name, vasopressin or arginine vasopres­sin (the form found in most mammals).