Oxytocin promotes contraction of smooth muscle of the uterus and myoepithelial cells of the breast.
Oxytocin is a more potent promoter of smooth muscle contraction than is ADH. Its primary effect includes promotion of contraction of  uterine smooth muscle during orgasm, menstruation, and parturition. As parturition approaches, uterine smooth muscle cells demonstrate about a 200-fold increase in the responsiveness to oxytocin. This is accompanied by increased formation of gap junctions between smooth muscle cells and increased density of oxytocin receptors.
 myoepithelial cells of the secretory alveoli and alveolar ducts of the mammary gland. Oxytocin secretion is triggered by neural stimuli that reach the hypothalamus. These stimuli initiate a neurohumoral reflex that resembles a simple sensorimotor reflex. In the uterus, the neurohumoral reﬂex is initiated by distension of the vagina and cervix. In the breast, the reﬂex is initiated by breastfeeding (suckling). Contraction of the myoepithelial cells that surround the base of the alveolar secretory cells and cells of the larger ducts cause milk to be released and pass through the ducts that open onto the nipple (i.e., milk ejection; see page 867).
Synthetic analogs of oxytocin are often used in intravenous infusion pumps to initiate and strengthen uterine contractions during active labor and delivery. Nasal preparations of oxytocin are also used to promote lactation in breastfeeding women.
The pituicyte is the only cell specific to the posterior lobe of the pituitary gland.
In addition to the numerous axons and terminals of the hypothalamic neurosecretory neurons, the posterior lobe of the pituitary gland contains fibroblasts, mast cells, and specialized glial cells called pituicytes associated with the fenestrated capillaries. These cells are irregular in shape, with many branches, and resemble astroglial cells. Their nuclei are round or oval, and pigment vesicles are present in the cytoplasm. Like astroglia, they possess specific intermediate filaments assembled from glial fibrillary acidic proteins (GFAP). Pituicytes often have processes that terminate in the perivascular space. Because of their many processes and relationships to the blood, the pituicyte serves a supporting role similar to that of astrocytes in the rest of the CNS (see page 367).
FIGURE 21.9 • Electron micrograph of Herring bodies of rat posterior lobe. Dilated portions of axons near their terminals called Herring bodies (HB) contain numerous neurosecretory vesicles filled with either oxytocin or ADH. They are surrounded by the specialized glial cells called pituicytes (P). Note that Herring bodies reside in a close proximity to blood vessels (BV), mainly fenestrated capillaries, lined by endothelial cells (En). 6,000. (Courtesy of Dr. Holger Jastrow).
The hypothalamus regulates pituitary gland activity.
The hypothalamus is located in the middle of the base of the brain, and it encapsulates the ventral portion of the third ventricle. It coordinates most endocrine functions of the body and serves as one of the major controlling centers of the autonomic nervous system. Some of the functions that it regulates include blood pressure, body temperature, fluid and electrolyte balance, body weight, and appetite. The hypothalamus produces numerous neurosecretory products. In addition to oxytocin and ADH, hypothalamic neurons secrete polypeptides that promote and inhibit the secretion and release of hormones from the anterior lobe of the pituitary gland (Table 21.5). These hypothalamic polypeptides also accumulate in nerve endings near the median eminence and infundibulum and are released into the capillary bed of the hypothalamohypophyseal portal system for transport to the pars distalis of the pituitary gland.