The Neurohypophysis Has Cell Bodies That Originate in the Hypothalamus, with Cell Endings That Secrete Oxytocin and Vasopressin
The neurohypophysis is composed of axons whose neural origin is largely within the supraoptic and paraventricular nuclei of the hypothalamus. The neurohypophysis is an extension of the hypothalamus into the pituitary; that is, the cell bodies are in the hypothalamus.
The axons form the stalk of the posterior lobe, and the nerve endings are in the lobe proper (Figure 33-11).The endocrine-secretory neurons that constitute the neurohypophysis differ from neurons involved in the transmission of neural signals in several ways: (1) neurosecretory neurons do not innervate other neurons, even though they are innervated; (2) the secretory product of neurosecretory neurons is secreted into the blood; and (3) the secretory product can act at distances greatly removed from the neuron. Also, in contrast to anterior pituitary hormones, which influence other tissues to produce hormones, posterior lobe hormones can directly cause the desired tissue response.
The first indication of the physiological activity of the neurohypophyseal lobe was the finding of Oliver and Schafer in 1895 that the injection of whole pituitary extracts caused a rise in blood pressure. This effect was soon found to be associated with the pars nervosa. This action represents the effects of one of the main neurohypophyseal hormones, vasopressin. The existence of the other main neurohypophyseal hormone, oxytocin, was first indicated in 1915, when Gaines showed that the injection of posterior pituitary gland extracts caused milk ejection. In 1941, Ely and Peterson showed that a denervated mammary gland could eject milk if the gland was perfused with blood that had been enriched with posterior pituitary extract. Both neurohypophyseal hormones were isolated and sequenced by du Vigneaud in 1954. These were some of the first proteins whose amino acid sequences were elucidated.