Hypothalamus

At a glance

English

Hypothalamus

Latin (TA2)

Hypothalamus

System

Brain and spinal cord

Region

Head

Side

Single, on the midline

About the hypothalamus

The hypothalamus (Greek for below the chamber) forms the lower part of the diencephalon, beneath the thalamus, and makes up the floor and the lower side walls of the third ventricle. Although it is only about the size of an almond, it is the chief link between the nervous and endocrine systems and the head of the autonomic nervous system. On the base of the brain it shows as the area between the optic chiasm and the mammillary bodies, with the tuber cinereum and the infundibulum, the stalk of the pituitary, in between.

It is made of many small nuclei. From front to back they are grouped into a supraoptic (chiasmatic) region, a tuberal region and a mammillary region; from medial to lateral into periventricular, medial and lateral zones. Important nuclei include the suprachiasmatic nucleus above the chiasm, the supraoptic and paraventricular nuclei, the arcuate (infundibular) nucleus at the base of the stalk, the ventromedial and dorsomedial nuclei, and the mammillary nuclei. Its many connections run in the fornix, the stria terminalis, the medial forebrain bundle, the mammillothalamic and mammillotegmental tracts and the dorsal longitudinal fasciculus; descending fibres reach the autonomic nuclei of the brainstem and spinal cord.

It controls the pituitary in two ways. Large cells of the supraoptic and paraventricular nuclei send axons down the stalk to the posterior pituitary, where they release antidiuretic hormone (vasopressin) and oxytocin straight into the blood. Smaller cells release releasing and inhibiting hormones into capillaries in the median eminence; the hypophyseal portal veins carry these to the anterior pituitary, which then governs the thyroid, adrenal cortex and other glands. Through its nuclei the hypothalamus also regulates body temperature, thirst and water balance, hunger and satiety, the sleep-wake cycle and daily rhythms set by light, and the bodily side of emotion and defence. Parts of its wall, the circumventricular organs, lack a blood-brain barrier and sense blood osmolality and hormones such as leptin and angiotensin.

Disorders show as disturbed balance rather than paralysis. Damage to the supraoptic system causes central diabetes insipidus, with large volumes of dilute urine. Tumours near it, such as craniopharyngioma or a large pituitary adenoma, may cause visual field loss from chiasm pressure, pituitary failure, obesity, disturbed sleep or temperature instability.

Function and supply

Function

Controls the pituitary gland through releasing hormones and the posterior pituitary hormones, and regulates temperature, water balance, hunger, sleep and daily rhythms, autonomic output and emotional responses.

Blood supply

Small central (perforating) branches from the cerebral arterial circle, chiefly from the anterior cerebral, anterior and posterior communicating and posterior cerebral arteries; the superior hypophyseal arteries supply the median eminence.