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The Hypothalamus lies deep within the cranium below the thalamus ("hypo"; below the thalamus) and on either side of the inferior bed of the third ventricle. This very small, yet very important structure is composed of multiple nuclei and axonal tracts that carry out specific functions. The hypothalamus is included in the limbic system. Its complex functions include: 1) control over the synthesis and secretion of hormones of the endocrine system, 2) control over the responses of the sympathetic division of the autonomic nervous system, and 3) control of behaviors essential to survival such as eating, sleeping and drinking.
It was discovered in the earlier part of the 19th century that the mating season of birds is influenced by seasonal variations in the duration of daylight. This evidence led to the search for a structure in the nervous system that might mediate this environmental influence on reproductive hormones. Because of its physical proximity to the pituitary gland, the hypothalamus was the first to be considered for this role. Early studies involving the effects of stimulation and damage to the hypothalamus provided support for this hypothesis, resulting in the focus of vigorous attention on this important structure.
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The hypothalamus is divided into three main regions, each with nuclei that are differentiated based on both structure and function (Parent, 1996). The suprachiasmatic region is located in the anterior (rostral) hypothalamus just dorsal to the optic chiasm. Located in this region are the supraoptic, paraventricular, suprachiasmatic, and the anterior nuclei. The tuberal region is located in the middle hypothalamus and includes the ventromedial, dorsomedial, and the infundibular nuclei. The mammillary region is located in the posterior (caudal) hypothalamus and includes the mammillary bodies and the posterior nuclei. Most nuclei of the hypothalamus are reciprocally connected to a number of regions of the limbic system including the amygdala, hippocampus, and the septal area.
The suprachiasmatic nuclei, which receive a projection from the retina and the geniculate nuclei of the thalamus, maintain the biological clock by controlling the circadian (daily) cycle of sleep-wakefulness (Ibata et. al., 1999). In addition, the suprachiasmatic nuclei interact in a reciprocal fashion with the pineal gland. The pineal gland synthesizes and releases a hormone called melatonin during the hours of darkness. Melatonin informs other structures about the changing length of night associated with the changing seasons. Melatonin receptors have been identified in the tuberal region of the hypothalamus, suggesting this region may be involved in controlling the effects that photoperiods have on endocrine functions as described in other studies (Wittkowski, 1999). For example, the duration of the nighttime secretion of melatonin is inversely related to the length of day (Goldman, 1999). This information is used to adjust the timing of functions that vary across the seasons, such as reproductive activity, the immune response (which is bolstered in the winter; Nelson & Drazen, 1999), and perhaps the duration and the organization of sleep. Melatonin also supports the functions associated with darkness, such as sleep and reduced body temperature.
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Goldman, B.D. (1999). The circadian timing system and reproduction in mammals. Steroids, 64(4), 679-685.
Ibata, Y., Okamura, H., Tanaka, M., Tamada,Y., Hayashi, S., Iijima, N., Matsuda, T., Munekawa, K., Takamatsu, T., Hisa, Y., Shigeyoshi, Y. & Amaya, F. (1999). Functional morphology of the suprachiasmatic nucleus. Frontiers of Neuroendocrinology, 20(3), 241-268.
Nelson, R.J. & Drazen, D.L. (1999). Melatonin mediates seasonal adjustments in immune function. Reproductive and Nuritional Development, 39(3), 383-398.
Parent, A. (1996). Carpenter's human neuroanatomy (9th ed.). London: Williams & Wilkins.
Wittkowski, W., Bockman, J., Kruetz, M.R. & Bockers, T.M. (1999). Cell and molecular biology of the pars tuberalis of the pituitary. International Review of Cytology, 185, 157-194.