CorpusHuman Anatomy
EndocrineBrain region

Hypothalamus

Hypothalamus

The small ventral part of the diencephalon (≈4 g) that links the nervous and endocrine systems, controlling the pituitary, the autonomic nervous system and homeostatic drives such as thirst, hunger and temperature.

Show on body map Brain — Midsagittal Section Hypothalamic–Pituitary Axis

Overview

Mass≈4 g (<1 % of brain)
GnRH pulse frequencyEvery ≈60–120 min

The hypothalamus forms the floor and lower lateral walls of the third ventricle, below the hypothalamic sulcus. It extends from the lamina terminalis and optic chiasm in front to the mammillary bodies behind, and its tuberal part (tuber cinereum) narrows into the median eminence and pituitary stalk. Although it weighs less than 1 % of the brain, it is the master integrator of homeostasis: it receives the limbic system (via the fornix and stria terminalis), the retina (retinohypothalamic tract), visceral afferents (via the nucleus of the solitary tract) and blood-borne signals sensed at circumventricular organs that lack a blood–brain barrier (OVLT, subfornical organ, median eminence).

Its output is threefold. Neuroendocrine parvocellular neurons release releasing and inhibiting hormones into the hypophyseal portal capillaries of the median eminence; magnocellular neurons of the supraoptic and paraventricular nuclei send axons to the posterior pituitary, where they release ADH and oxytocin; and pre-autonomic neurons project to brainstem and spinal autonomic centres, coordinating sympathetic and parasympathetic outflow.

Location

Ventral diencephalon, below the thalamus, forming the floor of the third ventricle; between the optic chiasm (anterior) and mammillary bodies (posterior), directly above the pituitary gland.

Parts & subdivisions

  1. Preoptic and anterior (supraoptic) region
    Preoptic area (GnRH neurons, thermoregulation — heat loss, sexually dimorphic nucleus), supraoptic, paraventricular, suprachiasmatic and anterior nuclei.
  2. Tuberal region
    Arcuate (infundibular), ventromedial (satiety) and dorsomedial nuclei; lateral hypothalamic area (hunger, orexin neurons that stabilise wakefulness).
  3. Mammillary region
    Mammillary bodies (memory circuit of Papez, via the mammillothalamic tract) and posterior nucleus (heat conservation, shivering).
  4. Paraventricular nucleus
    Magnocellular oxytocin/ADH neurons, parvocellular CRH and TRH neurons, and pre-autonomic neurons.
  5. Suprachiasmatic nucleus
    Master circadian pacemaker above the optic chiasm.
  6. Median eminence
    Circumventricular organ at the base of the stalk where parvocellular axon terminals release hormones into the primary capillary plexus of the portal system.

Anatomy

Arterial supply

Small perforating branches of the arterial circle (anterior cerebral, anterior and posterior communicating, and posterior cerebral arteries); superior hypophyseal arteries (from the internal carotid) supply the median eminence and form the primary portal plexus.

Relations

Anterior: lamina terminalis and optic chiasm. Superior: thalamus (separated by the hypothalamic sulcus). Medial: third ventricle. Inferior: tuber cinereum, median eminence and pituitary stalk; pituitary in the sella turcica. Posterior: mammillary bodies, then the midbrain tegmentum.

Function

  • Controls the anterior pituitary via releasing/inhibiting hormones (TRH, CRH, GnRH, GHRH, somatostatin, dopamine).
  • Synthesises ADH and oxytocin for release from the posterior pituitary.
  • Regulates water balance and thirst (osmoreceptors of the OVLT).
  • Regulates body temperature (anterior: heat loss; posterior: heat conservation) and generates fever via prostaglandin E2.
  • Controls food intake and energy balance (arcuate, ventromedial, lateral areas).
  • Generates circadian rhythms (suprachiasmatic nucleus) and contributes to sleep–wake regulation.
  • Coordinates autonomic and behavioural responses to stress, emotion, sexual and defensive behaviour.
Physiology

Hypothalamic neurons are true neurosecretory cells: they fire action potentials and release peptides by Ca²⁺-dependent exocytosis, but into blood rather than a synapse. Many are pulsatile (the GnRH pulse generator in the arcuate nucleus is driven by kisspeptin/neurokinin B/dynorphin "KNDy" neurons). Circumventricular organs let the hypothalamus sample plasma osmolality, angiotensin II, leptin, cytokines and glucose.

Clinical significance

Hypothalamic lesions

Tumours (craniopharyngioma, germinoma, glioma), trauma, radiotherapy and infiltrative disease (sarcoidosis, Langerhans cell histiocytosis) cause combinations of hypopituitarism with raised prolactin, central diabetes insipidus, hyperphagia and obesity (ventromedial damage), disordered temperature and sleep.

Kallmann syndrome

Failure of GnRH neurons to migrate from the olfactory placode into the hypothalamus (e.g. ANOS1/KAL1 mutations) → hypogonadotropic hypogonadism (absent puberty, low LH/FSH) with anosmia.

Wernicke encephalopathy

Thiamine deficiency damages the mammillary bodies (confusion, ophthalmoplegia, ataxia); Korsakoff amnesia may follow.

Mnemonics

Hypothalamic functions

TAN HATS — Thirst, Adenohypophysis, Neurohypophysis, Hunger, Autonomic, Temperature, Sexual behaviour.

Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.