hypothalamus

A region of the vertebrate brain that is derived from the forebrain; it is located on the ventral surface, in the floor of the third ventricle, below the thalamus and the cerebrum, and above the pituitary gland. The hypothalamus contains several important centers which control body temperature, thirst, hunger, water balance, and sexual function. It is also closely connected with emotional activity and sleep, and functions as a center for the integration of hormonal and autonomic nervous activity through its control of secretions by the pituitary gland.

The hypothalamus exerts overall control over the sympathetic nervous system (part of the autonomic nervous system). In response to sudden excitement or alarm, signals are sent from the higher regions of the brain to the hypothalamus, which initiates sympathetic nervous system activity. This increases the heart rate, breathing rate, and blood flow to the muscles, and causes the pupils of the eye to widen (all together known as the "fight or flight" response).

Other groups of nerve cells in the hypothalamus are concerned with the control of body temperature. Some are sensitive to heat and cold so that, when blood flowing to the brain is hotter or cooler than normal, the hypothalamus switches on temperature-regulating mechanisms (among them sweating or shivering). The hypothalamus receives information from internal sense organs regarding the body's water content and the level of glucose in the blood; if these are too low, the hypothalamus stimulates thirst and appetite for food. The hypothalamus is also involved in regulating sleep, in motivating sexual behavior, and in determining mood and the experience of emotions.

Another role of the hypothalamus is to coordinate the function of the nervous and endocrine (hormonal) systems of the entire body. The hypothalamus connects with the pituitary gland through a short stalk of nerve fibers and controls hormonal secretions from this gland. It does this in two ways – through direct nerve connections and through specialized nerve cells, which secrete hormones called releasing factors into the blood to travel to the pituitary gland. In this way, the hypothalamus can convert nerve signals into hormonal signals. Thus, the hypothalamus indirectly controls many of the glands of the endocrine system, including the pituitary gland, thyroid gland, cortex of the adrenal glands, ovaries, and testes.

Disorders of the hypothalamus

Disorders of the hypothalamus are usually caused by a brain hemorrhage within the hypothalamic region or by an expanding pituitary tumor. Loss of hypothalamic function can have diverse effects, ranging from hormonal disorders to disturbances in temperature regulation, and increased or decreased appetite for food, sex, and sleep.

Related category

• ANATOMY AND PHYSIOLOGY

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