hypothalamus (hI´po-thal´a-mus) [NA]

The ventral and medial region of the diencephalon forming the walls of the ventral half of the third ventricle; it is delineated from the thalamus by the hypothalamic sulcus, lying medial to the internal capsule and subthalamus, continuous with the precommissural septum anteriorly and with the mesencephalic tegmentum and central gray substance posteriorly. Its ventral surface is marked by, from before backward, the optic chiasma, the unpaired infundibulum that extends by way of the infundibular stalk into the posterior lobe of the hypophysis, and the paired mamillary bodies. The nerve cells of the h. are grouped into the supraoptic paraventricular, lateral preoptic, lateral hypothalamic, tuberal, anterior hypothalamic, ventromedial, dorsomedial, arcuate, posterior hypothalamic, and premamillary nuclei and the mamillary body. It has afferent fiber connections with the mesencephalon, limbic system, cerebellum, and efferent fiber connections with the same structures and with the posterior lobe of the hypophysis; its functional connection with the anterior lobe of the hypophysis is established by the hypothalamohypophysial portal system. The h. is prominently involved in the functions of the autonomic nervous system and, through its vascular link with the anterior lobe of the hypophysis, in endocrine mechanisms; it also appears to play a role in neural mechanisms underlying moods and motivational states. See also hypophysis. [hypo- + thalamus]


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