(from Greek, hypo: under, thalamus: chamber) The hypothalamus is a structure in the DIENCEPHALON, sitting beneath the THALAMUS and is concerned with the control of body processes. It can regulate activity in the PERIPHERAL NERVOUS SYSTEM and controls the release of many HORMONES. It is best known for its role in the control of fundamental activities such as FEEDING, DRINKING, THERMOREGULATION, REPRODUCTION and SEXUAL BEHAVIOUR, It is a nucleated structure: that is, the hypothalamus en masse is divided into discrete parts (nuclei) that can be identified using a number of criteria: the nature and density of the NEURONS they contain, the connections they have within and without the hypothalamus, and by their development from embryonic tissue for example.
The division of the hypothalamus into its component nuclei can be systematized, divisions being made both along its anterior-posterior length and longwise, in parallel with the MIDLINE. Four regions are identified along its length: preoptic (the most anterior), anterior hypothalamic, tuberal hypothalamic and mammillary (or posterior hypothalamic) region. Longitudinally, three areas are identified: the periventricular (or midline) zone (the most
Notes: AV anteroventral; AD anterodorsal; MPO medial preoptic nucleus; PD posterodorsal; n nucleus
central), the medial (or core) zone and the lateral (or reticular) zone. The nuclei that can be identified within this matrix are listed in the table above; details of the specific functions of individual nuclei are contained in separate entries. The hypothalamus has major connections with: (1) the AUTONOMIC NERVOUS SYSTEM. The NUCLEUS OF THE SOLITARY TRACT and the PARABRACHIAL NUCLEI of the PONS, nuclei that receive afferent visceral information either directly or indirectly from the CRANIAL NERVES or SPINAL CORD, are the sensory nuclei of the autonomic nervous system. These project in turn to the hypothalamus as well as other parts of the LIMBIC SYSTEM and the CEREBRAL CORTEX (INSULA CORTEX and PREFRONTAL CORTEX). Output from the hypotha-lamus returns to these places and to sites such as the DORSAL MOTOR NUCLEUS OF THE VAGUS, the origin of the efferent limb of the VAGUS NERVE. These connections bring in a wealth of information about the state of the body’s tissues. In addition, BLOOD borne information, and information about the body’s fluid balance (see OSMOREGULATION) is received via the circumventricular organs. (2) The hypothalamus has extensive neuroendocrine connections, principally by virtue of its relationship with the PITUITARY GLAND: these connections allow the hypothalamus both to monitor hormonal activity in the body and to control the release of HORMONES. The hypothalamus appears also to receive information about the state of the immune system (via the blood) and is able to influence immune system activity (via the activity of hormones it controls). (3) The hypothalamus has sensory information from the external world, as well as the internal milieu.
It has been acknowledged for a long time that the hypothalamus is concerned with HOMEOSTASIS—the maintenance of a stable body state. For many years the prevailing view was that the hypothalamus was divided into discrete drive centres, some acting to turn behaviour on, some to turn it off (see DUAL CENTRES HYPOTHESIS). This is a view that is not generally accepted any longer, though it is still recognized that the hypothalamus is best seen as a coordinating structure: sensory information is received from the body: the autonomic nervous system, hormones, the immune system, sensory systems for monitoring the external environment, and from other parts of the central nervous system. Output is directed towards the goal of bodily maintenance: this is achieved by taking action on the body via hormone release and by the autonomic nervous system and by influencing processes of behavioural selection which in turn will serve to maintain the body. The precise way in which individual hypothalamic nuclei contribute to this is still a matter of active research interest.
Altman J. & Bayer S.A. (1986) The Development of the Rat Hypothalamus. Advances in Embryology and Cell Biology, vol. 100, Springer-Verlag: Berlin.
Risold P.Y., Thompson R.H. & Swanson L.W. (1997) The structural organization of connections between hypothalamus and cerebral cortex. Brain Research Reviews 24: 197–254.
Simerly R.B. (1995) Anatomical substrates of hypothalamic integration. In The Rat Nervous System, 2nd edn, ed. G.Paxinos, pp. 353–376, Academic Press: San Diego.
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