The hypothalamus weighs 4-5 grams, and is bounded rostrally by the midbrain, anteriorly by the optic chiasm and superiorly by a sulcus separating it from the thalamus. It forms the floor and part of the lateral wall of the third ventricle. There are three morphological zones: periventricular; medial (rich in cells); and lateral (rich in axons). The major role of the hypothalamus in the control of autonomic, endocrine, visceral, affective and emotional behaviour is in contrast to its small size. The hypothalamus possesses three main groups of nuclei: Anterior: preoptic; supraoptic; paraventricular Middle: tuberal; arcuate; ventro-medial; dorso-medial Posterior: mammillary bodies; posterior hypothalamic nuclei The hypothalamus receives inputs from the nucleus of the tractus solitarius (visceral sensory information), the reticular formation (spinal cord), the circum-ventricular organs, the limbic and olfactory systems and the retina. Optic nerve input from the retina synchronises the body clock in the suprachiasmatic nucleus with the night-day cycle. The two main outputs are neuronal signals to the autonomic nervous system via the lateral medulla and endocrine signals to the pituitary gland. The functions of the hypothalamus include:

* The head ganglion of the autonomic nervous system, the anterior hypothalamus being parasympathomimetic and the posterior hypothalamus sym-pathomimetic. The hypothalamus is thereby responsible for the neural control of the internal viscera.

• Circadian and seasonal clock for behavioural and sleep-wake functions, in the suprachiasmatic nuclei. The body clock comprises the paired suprachiasmatic e nuclei, which rhythmically influence core temperature, hormonal activity and

I autonomic function. It is influenced by the light-dark cycle, and by melatonin p y signals from the pineal gland. It separates body functions associated with

l daytime from those at night. The basic oscillator in the suprachiasmatic nuclei

y is an autoregulatory negative feedback system involving cyclical synthesis of period proteins.

* Neural control centre of the endocrine system. Cells in the supraoptic and paraventricular nuclei produce the peptide hormones oxytocin and vasopres-sin respectively, which are transported to the posterior pituitary. Thirst is mediated by osmoreceptors. A variety of releasing and inhibiting hormones produced by the tuberal and arcuate nuclei affect the anterior pituitary release of hormones.

* Body temperature regulation: The neuronal groups in the anterior hypothalamus co-ordinate heat losing mechanisms, and in the posterior hypothalamus co-ordinate heat conservation. The hypothalamus co-ordinates autonomic (vasoconstriction) and somatic (shivering) responses.

* Hunger is mediated by the satiety centre in the ventro-medial nuclei and the feeding centre in the lateral nuclei.

* Sexual behaviour is controlled by the medial hypothalamus.

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