THE HYPOTHALAMUS
An important integration center is the hypothalamus, of which the rostral part is concealed and the caudal part—exemplified by the tuber cinereum and mamillary bodies—is exposed on the surface of the brain (Figures 8-19 and 8-22).
The hypothalamus includes many areas of specialized function and responsibility. A brief summation of its functions must include the control of biological rhythms, appetite, water balance, body temperature, cardiovascular performance, sexual behavior and activity, sleep, muscle tension (orexin), and emotion. Deficiencies in the orexinZhypocretin system may cause severe cataplexy and narcolepsy (also in the dog). Several hypothalamic cell groups implicated in reproductive function are sexually dimorphic. Because almost every body function has visceral implications, the hypothalamus must receive (and coordinate) information from most other parts of the nervous system, including those of ostensibly somatic function. Information on the somatic activities is projected via the basal nuclei and relays on the extrapyramidal motor pathways via the thalamic nuclei to which the somatic afferent pathways lead. Information concerning visceral function is received from mesencephalic nuclei and the reticular formation. The nucleus of the solitary tract 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. As such it is the region of initial processing of visceral, cardiovascular and respiratory, and gustatory information. A further very important contribution comes from the telencephalon (from the prefrontal cortex) and especially from the hippocampus, via the fornix. This enables emotional inputs to be related to and coordinated with the rest. Hypothalamic input from peripheral organ systems is also possible by way of blood-borne signals.The hypothalamus regulates activity through both nervous and humoral mechanisms, sometimes in combination. The nervous pathways extend to the brainstem and spinal cord by direct routes or by multisynaptic pathways within the reticular formation, in which final integration takes place. Other projections provide a feedback to the forebrain routed through rostral thalamic nuclei.
The humoral pathway operates through neurosecretory cells whose products may enter the bloodstream directly for general distribution or whose products may be conveyed specifically to the hypophysis by means of a system of portal vessels (see Figure 6-3).