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 discharge 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 certain
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 somatostatin
by Brazeau and colleagues in 1973 and the endogenous opioids (enkephalin)
by Hughes and coworkers in 1975. Since then, a series of hypothalamic-releasing factors
that act on the pituitary gland have been isolated.
The hypothalamus lies on each side of the third ventricle and
is continuous across its floor. 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
Nauta and Haymaker have subdivided the hypothalamus sagittally.
The lateral part lies lateral to the fornix; it
is sparsely cellular ...