Hypothalamus

I

Introduction

Hypothalamus, part of the brain, important in regulating the internal activities of the body. Although the hypothalamus constitutes less than 1 percent of the total volume of the brain, it has an important influence on many of the body's functions, including sexual behavior, emotions, hormone production, and the autonomic nervous system.

II

Anatomy

The human hypothalamus weighs about 4 g (0.14 oz) and is found behind the eyes, directly below the brain's thalamus and above the pituitary gland. The hypothalamus is divided into several distinct nuclei, that is, aggregates of nerve cell bodies. These nerves connect the brain with the hypothalamus and the hypothalamus to virtually all regions of the nervous system. The hypothalamus also receives nerve inputs from the erogenous zones (the genitalia and nipples), the viscera (internal organs), and the limbic system (concerned with motivation and drive).

III

Function

The hypothalamus controls a wide range of functions. It directs the “fight or flight” response of the autonomic nervous system. Fear or excitement causes signals to travel to the hypothalamus, which triggers a rapid heartbeat, faster breathing, widening of the pupils, and increased blood flow. The hypothalamus monitors blood glucose levels and the body's water content to regulate appetite for food or drink. It regulates sleep and sexual behavior.

The hypothalamus plays an important role in regulating feeding behavior. Experiments performed on rats demonstrate that if the middle of the hypothalamus is damaged, the rat overeats and becomes obese; damage in the lower part causes the rat to refuse to eat and starve. The role of the human hypothalamus is less important than in rodents because conscious decisions play a greater part in human processes such as eating and drinking. For example, it has been shown that custom and habit have a greater influence over the amount eaten than actual hunger.

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The hypothalamus has an effect on the cardiovascular system and the rest of the autonomic nervous system. This effect is vital for the coordination of mind and body; for example, it is responsible for the physical changes required before exercise.

The hypothalamus can be regarded as the thermostat controlling the temperature of the body. It initiates shivering and contraction or expansion of blood vessels. The hypothalamus triggers behaviors such as putting on or removing clothes, turning on the heat, or moving into the shade.

IV

Endocrine Functions of the Hypothalamus

The hypothalamus is responsible for controlling the hormones released from the pituitary gland. Two of these hormones are oxytocin and vasopressin (also called antidiuretic hormone or ADH).

Oxytocin plays a role in uterine contractions during childbirth. It also has a role in starting and maintaining the birth process. Breastfeeding also triggers the secretion of oxytocin via a nervous pathway that connects the nipple and the hypothalamus; oxytocin stimulates the flow of milk from the breast to the infant. Oxytocin secretion can also be caused by the sound of a baby crying—an example of the connections the hypothalamus has with the other parts of the brain.

The hormone vasopressin acts on the kidneys to increase reabsorption of water from urine, thereby maintaining the water level within the body. When the hypothalamus senses that blood concentration has increased, it stimulates the pituitary gland to produce more vasopressin. Likewise, if blood concentration is too dilute, the hypothalamus instructs the pituitary gland to release less vasopressin.

A part of the hypothalamus is involved in the regulation of circadian rhythms in the body. These rhythms are caused by hormone fluctuations in the bloodstream that occur during each 24-hour period, usually correlating with periods of light and darkness. These fluctuations ensure that the appropriate hormones are elevated when most needed in the body. Cortisol hormone levels, for example, routinely rise in the morning just before waking. This increases blood glucose levels to counterbalance the potentially harmful effects of not eating or drinking while asleep overnight.

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