The hypothalamus plays three roles in the actions of the nervous system. First, it serves as the “head ganglion” of the autonomic nervous system, as described in the preceding chapter; second, it is a circadian and seasonal clock for behavioral and sleep–wake functions, as considered in Chap. 18 on sleep; third, it provides neural control of the endocrine system, as discussed in this chapter. The hypothalamus integrates these systems with one another as well as with neocortical, limbic, and spinal networks. Ultimately, the hypothalamus maintains complex homeostatic functions and participates in the substructure of emotion and affective behavior.
The concept of neurosecretion probably had its origins in the observations of Speidel, in 1919, who noted that some hypothalamic neurons had the morphologic characteristics of glandular cells. That idea, which is now viewed as a fundamental part of the science of endocrinology, was so novel that it was rejected by most biologists at the time. The expansion of knowledge of neuroendocrinology during the past century stands as one of the significant achievements in neurobiology. It is now well established that neurons, in addition to transmitting electrical impulses, can synthesize and secrete complex molecules locally and into the systemic circulation, and that these molecules are capable of stimulating or inhibiting endocrine, renal, and vascular cells at distant sites.
Following the observations of Spiedel, Euler and Gaddum made the seminal observation in 1931 that peptides secreted by neurons in the central and peripheral nervous systems were also secreted by glandular cells of the pancreas, intestines, and heart. They isolated a substance from the intestines that was capable of acting on smooth muscle and called it “P” (from powder). But it was not until some 35 years later that Leeman and her associates purified an 11-amnio-acid peptide that is now called substance P (see Aronin et al). Then followed the discovery of six hypothalamic mediators of anterior pituitary hormone secretion: thyrotropin-releasing hormone (TRH), somatostatin, gonadotropin-releasing hormone (GnRH), corticotropin-releasing hormone (CRH), and hormone-releasing hormone (GHRH). Throughout this time it was already known that dopamine acted as an inhibitor of pituitary hormone secretion. Subsequently, a number of other neuropeptides including enkephalin, neuropeptide Y, and orexin were discovered, as discussed in Chap. 18.
The hypothalamus lies on each side of the third ventricle and is continuous across the floor of the ventricle. It is bounded posteriorly by the mammillary bodies, anteriorly by the optic chiasm and lamina terminalis, superiorly by the hypothalamic sulci, laterally by the optic tracts, and inferiorly by the hypophysis. It comprises three main nuclear groups: (1) the anterior group, which includes the preoptic, supraoptic, and paraventricular nuclei; (2) the middle group, which includes the tuberal, arcuate, ventromedial, and dorsomedial nuclei; and (3) the posterior group, comprising the mammillary and posterior hypothalamic nuclei.
According to the system proposed ...