The pars nervosa, the neural lobe of the pituitary, contains the nonmyelinated axons and their nerve endings of approximately 100,000 neurosecretory neurons whose cell bodies lie in the supraoptic nuclei and paraventricular nuclei of the hypothalamus. The axons form the hypothalamohypophyseal tract and are unique in two respects. First, they do not terminate on other neurons or target cells but end in close proximity to the fenestrated capillary network of the pars nervosa. Second, they contain secretory vesicles in all parts of the cells, i.e., the cell body, axon, and axon terminal. Because of their intense secretory activity, the neurons have well-developed Nissl bodies and in this respect resemble ventral horn and ganglion cells.
The posterior lobe of the pituitary gland is not an endocrine gland. Rather, it is a storage site for neurosecretions of the neurons of the supraoptic and paraventricular nuclei of the hypothalamus. The nonmyelinated axons convey neurosecretory products to the pars nervosa. Other neurons from the hypothalamic nuclei (described below) also release their secretory products into the fenestrated capillary network of the infundibulum, the first capillary bed of the hypothalamohypophyseal portal system.
Electron microscopy reveals three morphologically distinct neurosecretory vesicles in the nerve endings of the pars nervosa.
Three sizes of membrane-bounded vesicles are present in the pars nervosa:  Neurosecretory vesicles with diameters ranging between 10 and 30 nm accumulate in the axon terminals. They also form accumulations that dilate portions of the axon near the terminals (Fig. 21.9). These dilations, called Herring bodies, are visible in the light microscope (Plate 81, page 774). In the electron microscope, Herring bodies in addition to abundant neurosecretory vesicles, contain mitochondria, few micro-tubules and profiles of SER (Fig. 21.10).
 Nerve terminals also contain 30-nm vesicles that contain acetylcholine. These vesicles may play a specific role in the release of neurosecretory vesicles.
 Larger 50to 80-nm vesicles that resemble the dense core vesicles of the adrenal medulla and adrenergic nerve endings are present in the same terminal as the other membrane-bound vesicles. The membrane-bound neurosecretory vesicles that aggregate to form Herring bodies contain either oxytocin or antidiuretic hormone (ADH; also called vasopressin; Table 21.4). Each hormone is a small peptide of nine amino acid residues. The two hormones differ in only two of these residues. Each vesicle also contains ATP and a neurophysin, a protein that binds to the hormone by noncovalent bonds. Oxytocin and ADH are synthesized as part of a large molecule that includes the hormone and its specific neurophysin. The large molecule is proteolytically cleaved into the hormone and neurophysin as it travels from the nerve cell body to the axon terminal. Immunocytochemical staining demonstrates that oxytocin and ADH are secreted by different cells in the hypothalamic nuclei.
ADH facilitates resorption of water from the distal tubules and collecting ducts of the kidney by altering the permeability of the cells to water.