The Hypothalamus

The hypothalamus is a small but functionally very important region forming the floor and part of the lateral wall of the third ventricle, at the base of the forebrain. Although it is anatomically and developmentally part of the brain, and so belongs to the nervous system, the hypothalamus also behaves as a true endocrine structure, because specialised nerve cells within it — neurosecretory cells — synthesise and release hormones directly into the blood, exactly as an ordinary endocrine gland's cells do. This dual nature makes the hypothalamus the crucial link between the nervous system, which continuously monitors conditions inside and outside the body, and the endocrine system, which brings about many of the body's slower, longer-term responses to those conditions. Below the hypothalamus, connected to it by a narrow stalk called the infundibulum, lies the pituitary gland, whose activity the hypothalamus closely regulates.

The hypothalamus regulates the pituitary in two quite different ways, corresponding to the pituitary's two distinct parts. For the anterior pituitary, the hypothalamus does not send hormones by nerve fibre at all; instead, its neurosecretory cells release a set of small peptide hormones, collectively called hypophysiotropic hormones, into a short, specialised network of blood vessels called the hypothalamo-hypophyseal portal system, which carries them the short distance directly to the anterior pituitary at a high local concentration, without first diluting them through the whole body's circulation. These hypophysiotropic hormones fall into two functional classes. Releasing hormones stimulate the anterior pituitary to secrete more of a particular hormone — for example, thyrotropin-releasing hormone (TRH) stimulates release of thyroid-stimulating hormone (TSH), gonadotropin-releasing hormone (GnRH) stimulates release of FSH and LH, corticotropin-releasing hormone (CRH) stimulates release of ACTH, and growth-hormone-releasing hormone (GHRH) stimulates release of growth hormone. Inhibiting hormones instead suppress the release of a particular anterior pituitary hormone — for example, somatostatin (growth-hormone-inhibiting hormone) suppresses growth hormone release, and prolactin-inhibiting hormone (essentially the neurotransmitter dopamine acting in this role) suppresses prolactin release. By varying the balance of releasing and inhibiting signals it sends, the hypothalamus can turn each anterior pituitary hormone up or down independently, in response to information it receives from elsewhere in the nervous system and from the levels of hormones already circulating in the blood. …