Anatomy of the Neurohypophysis

The neurohpophysis is composed of the median eminence, a highly vascular area beneath the third ventricle at the base of the hypothalamus. The infundibular stalk arises in the median eminence and connects the pituitary gland to the hypothalamus, the infundibular process, the pars nervosa or posterior lobe itself (see top figure). Antidiuretic hormone (ADH) or vasopressin, and oxytocin are synthesized in neurons (cell bodies) located in the supraoptica and paraventricular nuclei in the hypothalamus (see bottom figure). These neurohormones, which are stored in vesicles, are sent down the nonmylinated axons, through the infundibular stalk and end up in the axonic endbulb (bouton) in the neurolobe proper. Upon proper stimulation the vesicles release these neuropeptides into veins of the posterior lobe. Some neurons containing ADH and oxytocin end in capillaries in the median eminence and pass into the portal system to the anterior pituitary. Other neurohormones, hormones, and neurotransmitters have been isolated from the neurohypophysis but they have been of small quantity and their function in this lobe is unknown. The only cell found in the neural lobe is a neuroglial cell called a pituicyte. Blood supply to the median eminence is very extensive, as would be expected. Releasing and inhibitory hormones are secreted here from specific hypothalamic nuclei, then sent into the portal system to stimulate anterior pituitary hormone release.