What is hypothalamus ? What does the hypothalamus do ? What are the functions of hypothalamus ?
In the past the pituitary gland was considered the master endocrine gland. However, now the hypothalamus has superseded the pituitary gland because it regulates the secretion of pituitary hormones.
The hypothalamus is a complicated collection of nerve cells and fiber tracts which help regulate autonomic functions and body rhythms.
The ventral hypothalamus consists of three major systems.
- A classic neuronal pathway through the base of the brain and through the autonomic nervous system pathways that terminates in the liver, gastro-intestinal tract, pancreas, adrenal medulla and adipose tissue. This involves appetite control, temperature control and body fat stores.
- A neurosecretory pathway involved in osmoregulation through the production of vasopressin, and in parturition and nursing through the secretion of oxytocin.
- A neuroendocrine system involving clusters of peptide and mono-amine secreting cells in the ventral hypothalamus which regulates the secretions of anterior pituitary hormones.
The hypothalamus has specialized neurons called the neurosecretory cells or magnocellular neurons (squalled because of their large size). These function both as neurons and hormone secreting cells. The hypothalamus, along with the pituitary gland, thus acts as an intimate link between the nervous system and the endocrine system.
The neurosecretory cells of the supraoptic and paraventricular nuclei synthesize oxytocin and vasopressin respectively. Their unmyelinated tracts pass through the median eminence to terminate in the posterior lobe of the pituitary.
The other hormones of the endocrine hypothalamus regulate the adenohypophysis. These adeno- hypophysiotrophic releasing and inhibiting factors are synthesized in ill-defined areas of the hypothalamus. These neurosecretions are transported to the primary capillary network of the pituitary portal system. In addition to providing route for hypothalamic control, the portal vessels carry most of the blood supply to the anterior lobe of the pituitary.
Neurosecretory neurons of the hypothalamus are regulated both by neural inputs (positive control) and concentration of circulating hormones (negative feedback control). Circulating hormones also act at the pituitary level to determine pituitary sensitivity to hypothalamic hormones.
All of the hypothalamic hormones, whose function has been determined so far, have a stimulatory effect on the pituitary except prolactin. All of the hypothalamic hormones whose structure have been determined are peptides except dopamine. Dopamine whose major role is to function as a neurotransmitter is the most important physiologic Inhibitor of prolactin.
The hypothalamus also contains a large concentration of catecholamines-norepinephrine, dopamine, serotonin and peptides. These amines control the secretion of hypophysiotrophic hormones e.g. dopamine stimulates GH and inhibits prolactin in normal persons. Paradoxically in acromegaly it suppresses GH. This has a therapeutic implication.