Because the hormones of the posterior pituitary gland are synthesized and stored in nerve cells, it should not be surprising that their secretion is controlled in the same way as that of more conventional neurotransmitters. Action potentials that arise from synaptic input to the cell bodies within the hypothalamus course down the axons in the pituitary stalk, trigger an influx of calcium into nerve terminals, and release the contents of neurosecretory granules. Vasopressin and oxytocin are released along with their respective neurophysins, other segments of the precursor molecule, and presumably the enzymes responsible for cleavage of the precursor. Tight binding of AVP or oxytocin to their respective neuro-physins is favored by the acidic pH of the secretory granule, but upon secretion, the higher pH of the extracellular environment allows the hormones to dissociate from their neurophysins and circulate in an unbound form. Oxytocin and vasopressin are rapidly cleared from the blood with a half-life of about 2 min.
As discussed in Chapter 37, signals for the secretion of oxytocin originate in the periphery and are transmitted to the brain by sensory neurons. After appropriate processing in higher centers, cells in the supraoptic and paraventricular nuclei are signaled to release their
Regulation of Posterior Pituitary Function central nervous relays suckling supraoptic and paraventricular nuclei milk let down i
mammary glands supraoptic and paraventricular nuclei
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