Both of the posterior pituitary hormones—antidiuretic hormone and oxytocin—are actually produced in neuron cell bodies of the supraoptic nuclei and paraventricular nuclei of the hypothalamus. These nuclei within the hypothalamus are thus endocrine glands. The hormones they produce are transported along axons of the hypothalamo-hypophyseal tract (fig. 11.13) to the posterior pituitary, where they are stored and later released. The posterior pituitary is thus more a storage organ than a true gland.
The release of ADH and oxytocin from the posterior pituitary is controlled by neuroendocrine reflexes. In nursing mothers, for example, the mechanical stimulus of suckling acts, via sensory nerve impulses to the hypothalamus, to stimulate the reflex secretion of oxytocin (chapter 20). The secretion of ADH is stimulated by osmoreceptor neurons in the hypothalamus in response to a rise in blood osmotic pressure (chapter 6); its secretion is inhibited by sensory impulses from stretch receptors in the left atrium of the heart in response to a rise in blood volume (chapter 14).
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