An almond-sized patch of brain runs your temperature, hunger and sleep
Deep beneath the thalamus sits a structure roughly the size of an almond that steers body temperature, hunger, thirst, fatigue and daily rhythms. The hypothalamus is the brain's bridge to the hormone system, sending chemical instructions to the pituitary gland that ripple out through the entire body.
The name means under the chamber in Ancient Greek, a reference to its position below the thalamus. Every vertebrate brain has one. It forms the lowest part of the diencephalon, belongs to the limbic system, and is wired heavily to the brainstem as well as to limbic neighbours such as the amygdala and septum. Most of its nerve fibres carry traffic in both directions.
Its most important job is connecting nerves to hormones. The hypothalamus makes releasing hormones that tell the pituitary gland either to step up or to hold back its own secretions. Two groups of cells show how this works. Large neurosecretory cells in the paraventricular and supraoptic nuclei make oxytocin and vasopressin, which travel down nerve fibres and enter the blood at the posterior pituitary. Much smaller cells in the paraventricular nucleus release corticotropin-releasing hormone into a tiny private blood network that carries it to the anterior pituitary.
Beyond hormones, it governs a long list of basic drives: temperature, appetite, thirst, tiredness, sleep and circadian timing, along with parenting and maternal attachment, and social behaviours including sex and aggression. Anatomists map it into four regions from front to back, named preoptic, supraoptic, tuberal and mammillary, and three zones from the midline outward. A dedicated pathway runs to the spinal cord's sympathetic segments, from T1 down to about L2 or L3, switching on the body's fight-or-flight wiring.
Several of its nuclei differ between males and females. The best known is a sexually dimorphic nucleus in the preoptic area; when it is damaged in male animals, their usual preference for females weakens. In mice, growth hormone release follows sex-specific patterns, helping explain size differences between adults. Human hypothalami also show sex differences that affect thermoregulation and metabolism, but it remains uncertain which behaviours, if any, they actually cause.
Source: Hypothalamus
