Physiological Considerations Of The Hypothalamus And Pituitary Gland In The Endocrine System
A General Overview
The main seat of neuroendocrine coordination is the hypothalamus. The higher neural centers influence it through the monoaminergic pathways. Physical and chemical stimuli also influence the hypothalamus through the blood perfusing it. The supraoptic and paraventricular nuclei respond to these stimuli by the production of hormones vasopressing and oxytocin. These are bound to carrier proteins- neurophysins and in this form they travel down the pituitary stalk to be stored in the posterior pituitary. The releasing hormones which help in releasing the trophic hormones of the anterior pituitary are secreted by the hypothalamus. These pass down the hypothalamo-hypophyseal portal circulation to exert their action on the anterior pituitary.
The pituitary gland weighs 0.5 to 1g. It consists of two portions:
- Adenohypophysis which is composed of columns of epithelial cells derived from the primitive foregut and
- Neurohypophysis which is formed from a diverticulum arising from the floor of the third ventricle. The neurohypophysis (posterior pituitary) stores the secretions of the supraoptic and paraventricular nucle of the hypothalamus.
Blood supply to the hypothalamus and pituitary is derived from the superior and inferior hypophyseal arteries which are branches of the internal carotid artery. The radicles of these vessels lie in close relation to the axons of the peptidinergic neurons of the hypothalamus which transfer their secretions into the blood that perfuses them. From these radicles, the hypothalamico-hypophyseal portal system arises and it termintates around the epithelial cells of the anterior pituitary. The releasing and inhibiting hormones secreted by the hypothalamus reach the pituitary through this system and exert their control. Blood returning from the pituitary reaches the hypothalamus and the pituitary secretions are able to influence the hypothalamus through this pathway.
The Pituitary Gland
Hormones Of Great Importance
Thyrotropin releasing hormone (TRH): Is a tripeptide found in the median eminence of the hypothalamus and in several other parts of the nervous system. This stimulates the pituitary to enhance the secretion of TSH and prolactin (PRL) in normal individuals. TRH only modulates the release of prolactin.
Gonadotropin- releasing hormone (GnRH): Is a decapeptide which releases both follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the adenohypophysis. This is also referred to as luteinizing hormone releasing hormone (LHRH). Sex steroids exert a negative feedback on the hypothalamus and pituitary and influence the secretion of FSH and LH. However estrogens from the developing Graafian follicles exert a positive feeback on the hypothalamus and pituitary and thereby enhance the secretion of GnRH and gonadotropins.
Growth hormone-releasing-inhibiting hormone (GHRIH): Or also called Somatotrophin release-inhibiting hormone or somatostatin (SS). It is a polypeptide hormone with 14 aminoacids. In addition to the hypothalamus, several other cells in other locations like the stomach, duodenum, upper jejunum and islets of Langerhans are found to contain SS. This hormone exerts a general inhibiting effect on the surrounding endocrine and exocrine tissues. Somatostatin has a short biological life and hence it is unlikely to exert its effect on distant tissues. The growth hormone response brought about by stimuli like exercise, hypoglycemia, arginine and sleep is inhibited by SS. It also reduces the thyroid stimulating hormone (TSH) response to injected TRH. In the stomach and intestine, the production of gastrin, gastric inhibirotry polypeptide, hydrochlorid acid and vasoactive intestinal polypeptide is diminished. The secretion of insulin and glucagon from islet cells is reduced. The exocrine pancreatic secretions like bicarbonate and enzymes are inhibited.
© 2014 Funom Theophilus Makama