The hypothalamus plays three roles in the actions of the nervous system. The first, as the "head ganglion" of the autonomic nervous system, was described in the preceding chapter; the second, as the circadian and seasonal clock for behavioral and sleep–wake functions, was considered in Chap. 19 on sleep; and the third, as the neural center of the endocrine system, is the subject of this chapter. In the hypothalamus, these systems are integrated with one another as well as with neocortical, limbic, and spinal influences. Together, they maintain homeostasis and participate in the substructure of emotion and affective behavior.
The expansion of knowledge of neuroendocrinology during the past few decades stands as one of the significant achievements in neurobiology. It has been learned 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 activating or inhibiting endocrine, renal, and vascular cells at distant sites.
The concept of neurosecretion probably had its origins in the observations of Speidel, in 1919, who noted that some of the hypothalamic neurons had the morphologic characteristics of glandular cells. Their suggestion that such cells might secrete hormones into the bloodstream was so novel, however, that it was rejected by most biologists at the time. This seems surprising now that neurosecretion is viewed as a fundamental part of the science of endocrinology.
Following these early observations, it was found that peptides secreted by neurons in the central and peripheral nervous systems were also contained in glandular cells of the pancreas, intestines, and heart. This seminal observation was made in 1931 by Euler and Gaddum, who 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: TRH, and somatostatin 1973, GnRH, CRH, and GHRH. In the background was always dopamine that acted as an inhibitor of pituitary hormone secretion. Subsequently, a number of other neuropeptides including enkephalin, neuropeptide Y, orexin as discussed in Chap. 19.
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.