Hypothalamus

A section of the forebrain, connected to other parts of the forebrain and midbrain, that is involved in many complex behaviors.

The hypothalamus, which together with the thalamus makes up the section of the forebrain called the diencephalon, is involved in such aspects of behavior as motivation, emotion, eating, drinking, and sexuality. Lying under the thalamus, the hypothalamus weighs only a fraction of an ounce and is a little larger than the tip of the thumb. It is connected to the autonomic nervous system, and controls the entire endocrine system using the pituitary gland to direct the work of all the other endocrine glands. If a particular section of the hypothalamus is destroyed, an overwhelming urge to eat results; damage to another section of a male's hypothalamus can reduce the sex drive. Yet another part of the hypothalamus, the suprachiasmatic nuclei (SCN), is the site of a person's "internal clock" that regulates biological rhythms according to a cycle of roughly 24 hours. From the SCN, signals reach areas of the hindbrain that regulate sleep and wakefulness. With neurons firing on a 24-or 25-hour cycle, it determines the periods of greatest alertness—whether one is "morning person" or a "night person." Pathways from the SCN to the eyes connect its circadian rhythms to external cycles of light and dark.

Different roles have been identified for various sections of the hypothalamus in interpreting and acting on hunger signals. The ventromedial nucleus, whose neurons detect blood levels of glucose, signals when it is time to stop eating. Rats in whom this part of the hypothalamus has been destroyed will eat extremely large quantities of food, enough to triple their body weight. Similarly, the lateral hypothalamus signals when it is time to begin eating. Yet another area, the paraventricular nucleus, appears to motivate the desire for particular types of foods, depending on which neurotransmitters are acting on it at a particular time.