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The Hypothalamus

The Hypothalamus is the main output of the limbic system. It is located on lower part of the top end of the brain stem.

The limbic system is basic to motivation, emotion, and reward processes. It control emotional expression through the hypothalamus, which has control over the body's emotional responses systems:

  • the autonomic nervous system, which controls internal organs (e.g., gut, heart) and the release of adrenalin (epinephrine) by the adrenal medulla.
  • pituitary gland of the endocrine system just below it. The pituitary is often called "master gland of the body," because it regulates the action of many other glands, including the adrenal cortex (vital for response to stress and control of salt balance in the body), the gonads (sex glands: ovaries in females and the testicles in males), the thyroid gland (controls metabolism), etc.
  • skeletal muscle system, which shows emotion through facial expression and body posture

The hypothalamus is particularly important for maintaining homeostas is, the proper balance of the body's internal environment (e.g., body temperature, blood sugar, water and salt concentration, oxygen level, etc.). The hypothalamus helps maintain homeostasis by its control over the autonomic nervous system and the endocrine system.

Stimulating the hypothalamus can activate many functions related to motivation, emotion, and reward, whereas damage disrupts such functions. For example, electrodes implanted in the lateral hypothalamus can activate very strong reward. Rats and other animals with such electrodes will make thousands of responses per hour to turn on 0.5-second bursts of stimulation (Olds, 1969). Such stimulation can also elicit (trigger) vigorous eating in animals that have just finished a meal (Valenstein, 1976)

Repeated stimulation several times a day for several weeks will make animals eat so much that they put on a lot of fat. (Steinbaum & Miller, 1965). Damage in this same area depresses eating (Teitelbaum & Epstein, 1962). Stimulation in the hypothalamus can also trigger aggression indicating anger and/or fear, and many other functions related to motivation and emotion.