The Hypothalamus
The hypothalamus is an important integration center for many visceral, endocrine, and behavioral functions, all of which work to keep the animal alive. These functions include the control of biologic rhythms, appetite, water balance, body temperature, cardiovascular performance, sexual behavior and activity, sleep, and emotion.
The hypothalamus must receive and coordinate information from most other parts of the nervous system, including visceral information as well as ostensibly somatic information. Information on the somatic activities is relayed via the thalamic nuclei to which the somatic afferent pathways lead. Information concerning visceral function is received from mesencephalic nuclei and the medullary reticular formation. For example, the nucleus of the solitary tract in the medulla is the principal visceral sensory nucleus that receives topographically organized input from major organ systems by way of the glossopharyngeus (IX) and vagus (X) nerves. This nucleus is the region of initial processing of visceral, cardiovascular and respiratory, and gustatory information, which it then projects to the hypothalamus. A further very important contribution to the hypothalamus comes from the telencephalon, specifically the prefrontal cortex, and especially from the hippocampus, via the fornix. This arrangement enables emotional inputs to be related to and coordinated with the visceral and somatic information. Hypothalamic input from peripheral organ systems is also possible by way of blood-borne signals.The hypothalamus regulates activity through both neural and endocrine mechanisms, sometimes in combination. Axonal pathways from the hypothalamus extend to the brainstem and spinal cord by direct routes or by multisynaptic pathways within the reticular formation, in which final integration takes place. Hypothalamic regions that exert control over visceral efferent neurons— namely, preganglionic sympathetic and parasympathetic neurons (see later)—are anatomically distinct, in that caudal hypothalamic regions control sympathetic functions whereas more cranial regions exert control over parasympathetic functions. Other hypothalamic projections provide a feedback to the forebrain routed through rostral thalamic nuclei.
The endocrine pathways operate through neurosecretory cells whose products either may enter the bloodstream directly for general distribution or may be conveyed specifically to the hypophysis by means of a system of portal vessels (see Fig. 6.3).
Anatomically, the hypothalamus is largely concealed, and only the caudal parts—namely, the tuber cinereum and mammillary bodies—are exposed on the ventral surface of the brain (Figs. 8.19/9 and 11 and 8.21/20 and 23).