The hypothalamus is a small area at the base of the brain, weighing about 4 gm out of the 1400 gm brain weight of an adult human, yet it performs a wide range of functions that are vital for the survival of the individual. In general, the hypothalamus acts as a integrator to regulate and coordinate basic functions necessary for life, such as fluid and electrolyte balance; feeding and energy metabolism; wake-sleep cycles; thermoregulation; stress responses; and sexual behavior and reproduction.
The hypothalamus controls the endocrine system in three ways. First, neurons in the paraventricular and supraoptic nuclei send their axons to form the posterior pituitary gland, where they secrete oxytocin and vasopressin. Second, neurons in the periventricular, paraventricular, and arcuate nuclei send axons to the median eminence, to secrete pituitary hormone releasing hormones, which regulate the anterior pituitary gland. Finally, the hypothalamus controls autonomic outputs to many peripheral endocrine tissues, which further regulate their secretion. Hypothalamic control of behavior is mediated in several ways. First, the lateral hypothalamic area and the histaminergic tuberomammillary nucleus play a major role in determining the overall level of wakefulness or arousal. Second, hypothalamic inputs to various motor pattern generators may increase the probability of specific behaviors. Third, there are hypothalamic descending outputs to sensory systems that may sensitize them (e.g., when hungry, food tastes better) or desensitize them (e.g., when under threat, pain is not perceived as readily).
Cellular biochemical reactions require that body temperature be tightly controlled. Thermoregulation is controlled mainly by neurons in the median and medial preoptic nuclei, as well as the lateral preoptic area. In general, these neurons tend to inhibit a thermogenic region in the dorsomedial nucleus and paraventricular nucleus. Thermoregulation interacts with feeding (as energy is required to produce heat and increase metabolic rate), reproduction (as body temperature is affected by menstrual cycles), and wake-sleep cycles (as body temperature falls during sleep). Neurons in the posterior half of the lateral hypothalamus as well as in the tuberomammillary nucleus, provide major inputs to the cerebral cortex and the basal forebrain that are concerned with alerting and arousal responses, and are critical for producing a fully awake state. These neurons, and others in the brainstem that promote wakefulness, are in turn under the influence of a master switch, the ventrolateral preoptic nucleus, which inhibits the components of the arousal system during sleep, and is necessary for normal sleep states to occur. The wake sleep system, including neurons in the lateral hypothalamus containing the peptide orexin, is in turn under the control of the circadian system. The dorsomedial nucleus, which receives circadian timing signals from the suprachiasmatic nucleus, seems to play critical role in coordinating the two. Sleep-wake regulation interacts with feeding, drinking, and sexual and defensive behavior, all of which, of course, require a waking state.