The Hypothalamus and Pituitary

A region the size of an almond that keeps the body in balance — temperature, hunger, thirst, sleep, sex and stress — by commanding the autonomic nervous system and the pituitary gland.

The hypothalamus weighs only about 4 grams — less than 1% of the brain — yet it is indispensable. It sits below the thalamus, around the bottom of the third ventricle, with the pituitary gland hanging beneath it on a stalk.

The body's thermostat (and much more)

The hypothalamus maintains homeostasis: keeping internal conditions within safe ranges. It contains clusters of neurons (nuclei) that monitor body temperature, blood salt concentration, blood sugar and hormone levels, and respond by adjusting behaviour, the autonomic nervous system and hormones. It controls:

Temperature — triggering sweating or shivering. Thirst and water balance — releasing vasopressin to make the kidneys conserve water. Hunger and fullness — the arcuate nucleus responds to hormones such as leptin and ghrelin. Sleep and daily rhythms — the suprachiasmatic nucleus is the body's master clock, and other hypothalamic neurons make orexin to stabilise wakefulness. Stress — starting the HPA axis. Reproductive behaviour and hormones — via gonadotropin-releasing hormone.

Commanding the pituitary

The pituitary gland, the size of a pea, has two parts. The anterior pituitary responds to releasing hormones from the hypothalamus delivered through a private blood supply, and secretes growth hormone, prolactin, ACTH, thyroid-stimulating hormone and the reproductive hormones LH and FSH. The posterior pituitary is actually an extension of the brain: hypothalamic neurons send axons into it and release oxytocin and vasopressin directly into the blood.

Disorders

Tumours of the pituitary can cause hormone excess (for instance gigantism or Cushing's disease) or deficiency, and can press on the optic chiasm just above, narrowing peripheral vision. Hypothalamic damage can cause extreme obesity, disrupted temperature control or sleep disorders.