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The Hypothalamus

What is it?

Anatomy

The hypothalamus is an integral part of the substance of the brain. A small cone-shaped structure, it projects downward, ending in the pituitary (infundibular) stalk, a tubular connection to the pituitary gland. The round bony cavity containing the pituitary gland is called the sella turcica. The posterior portion of the hypothalamus, called themedian eminence, contains many neurosecretory cells. Important adjacent structures include the mammillary bodies, the third ventricle, and the optic chiasm, the last being of particular concern to physicians because pressure from expanding tumours or inflammations in the hypothalamus or pituitary gland may result in severe visual defects or total blindness. Above the hypothalamus is the thalamus. (Fordiscussion of the function of these surrounding structures, see the nervous system.)

Regulation of hormone secretion

The hypothalamus regulates homeostasis. It has regulatory areas for thirst, hunger, body temperature, water balance, and blood pressure, and links the nervous system to the endocrine system.

The hypothalamus, like the rest of the brain, consists of interconnecting nervecells ( neurons) with a rich blood supply. To understand hypothalamic function it is necessary to define the various forms of neurosecretion. First, there is neurotransmission, which occurs throughout the brain and is the process by which one nerve cell communicates with another at an intimate intermingling of projections from the two cells (a synapse). This transmission of an electrical impulse fromone cell to another requires the secretion of a chemical substance from a long projection from one nerve cell body (the axon) into the synaptic space. The chemical substance that is secreted is called a neurotransmitter. The process of synthesis and secretion of neurotransmitters is similar to that shown in Figure 1 with the exception that neurosecretory granules migrate through lengths of theaxon before being discharged into the synaptic space.

Figure 1: Intracellular structure of a typical endocrine cell.

Neurologists have long been aware of four classical neurotransmitters: epinephrine, norepinephrine, serotonin, and acetylcholine, but recently there have emerged a large number of additional neurotransmitters, of which an important group is the neuropeptides. While bioamines andneuropeptides function as neurotransmitters, some of them also perform the role of neuromodulators; they do not act directly as neurotransmitters but rather as inhibitors or stimulators of neurotransmission. Well-known examples are the opioids (for example, enkephalins), so named because they are the naturally occurring peptides with a strong affinity to the receptors that bind opiate drugs suchas morphine and heroin. In effect, they are the body's opiates.

Thus the brain, and indeed the entire central nervous system, consists of an extraordinary network of neurons interconnected by neurotransmitters. The secretion of specific neurotransmitters, modified by neuromodulators, lends an organized, directed function to the overall system. These neural connections extend upward from thehypothalamus into other key areas, including the cerebral cortex. The result is that intellectual and functional activities as well as external influences, including stresses, can be funneled into the hypothalamus and thence to the endocrine system, from which they may exert effects on the body.

In addition to secreting neurotransmitters and neuromodulators, the hypothalamus synthesizes andsecretes a number of neurohormones. The neurons secreting neurohormones are true endocrine (neurohemal) cells in the classical sense since secretory granules containing neurohormones travel from the cell body through the axon to be extruded, where they enter directly a capillary network, thence to be transported through the hypophyseal-portal circulation to the anterior pituitary gland.

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