Pituitary gland

" pituitary gland

Posterior pituitary or neurohypophysis

The posterior pituitary gland or neurohypophysis functions as a "repository" for the hypothalamic hormones ADH and oxytocin, produced by hypothalamic neurons with their relative body located in the hypothalamus (Supraoptic Nuclei → ADH and Paraventricular → Oxytocin).

• ADH or antidiuretic hormone increases the permeability of the distal renal tubule of the nephron, making it permeable to water to reduce water loss; in addition, it vasoconstructs peripheral vessels, raising blood pressure. It is therefore secreted in response to many stimuli, especially an increase in electrolytes in the blood or a drop in blood volume or blood pressure. An ADH deficiency is responsible for so-called diabetes insipidus.
• Oxytocin is responsible for stimulating the uterine myometrium during labor (not the neck which is released instead ...). Outside pregnancy, in men it stimulates the smooth muscle cells of the prostate and the following ejaculatory duct, while in the woman favors menstruation and coitus.

The posterior lobe of the pituitary or neurohypophysis is made up of three parts: median eminence, infundibulum peduncle and pars nervosa. It is made up of glial cells, the pituicytes, and long nerve fibers, whose cell bodies - located in the hypothalamus - synthesize neurosecretes; these substances then flow into the axons that descend into the infundibulum (forming a nerve bundle called the hypothalamus-neurohypophyseal bundle) until they reach the neurohypophysis, where they accumulate in terminal swellings filled with granules (Hering's bodies); in this area the hypothalamic factors) they are poured directly into the blood capillaries (see figure) and from there they enter the general circulation.

Intermediate pituitary

The intermediate part of the pituitary gland, considered an integral part of the adenohypophysis (pars intermedia), produces the hormone intermedina or melanotropic (MSH), which regulates the synthesis and distribution of melanin granules in melanocytes, but only in the fetus, in the child. small, in pregnant women (nipples and linea nigra (below the navel) and in some diseases.

Pituitary and feedback mechanisms

Generally, the regulation of the secretory activity of the hypothalamus and pituitary gland is subject to forms of negative feedback:

1. pituitary and hypothalamic endocrine cells respond to homeostatic changes by pouring their own hormones into the circulation;
2. pituitary hormones stimulate the endocrine cells of the target organs;
3. the hormonal response of the latter restores homeostasis and eliminates the stimulus that activated them, inhibiting the secretion of the relative pituitary and hypothalamic hormones. A sort of physiological circuit is thus created, where the final product of a specific pathway metabolic inhibits the first stages of the same pathway that generated it. We are talking about the famous negative feedback circuits that preside over the homeostasis of our organism. The opposite regulations, those with positive feedback, are rare and limited to cases in which the action must be completed quickly; for example, still remaining on the subject of pituitary gland, during childbirth oxytocin causes the release of further oxytocin.