CHAPTER
Endocrine Metabolism II:
Hypothalamus and Pituitary
31.1
Hypothalamus
The hypothalamus is a small region of the brain in the ven-
tral aspect of the diencephalon. In the adult human, it is
about 2.5 cm in length and weighs about 4 g. Ventromedi-
ally, it surrounds the third ventricle and is continuous with
the infundibular stalk of the pituitary (hypophysis). This
cone-shaped region of the hypothalamus, the
median emi-
nence,
consists mainly of axonal fibers from hypothalamic
neurons, which either terminate in the median eminence
or continue down into the posterior lobe of the pituitary,
and it is perfused by a capillary network (primary plexus)
derived from the carotid arteries. Blood from the primary
plexus is transported by portal vessels (hypophyseal portal
vessels) to another capillary network (secondary plexus)
in the anterior lobe of the pituitary (adenohypophysis)
(Figure 31-1).
The hypothalamus contains a high density of nerve cell
bodies clustered into nuclei or areas. Neurons in each
of these nuclei tend to send their axons to the same re-
gions in the form of tracts. These nuclei innervate the
median eminence, other hypothalamic nuclei, the pos-
terior pituitary, and various structures in the extrahy-
pothalamic central nervous system. All of the hypotha-
lamic neurons are presumably monoaminergic (i.e., they
A list of expanded acronyms appears in Appendix VIII.
synthesize and release the neurotransmitter amines nore-
pinephrine, serotonin, or dopamine); however, many are
also peptidergic (i.e., they synthesize and release neu-
ropeptides). At least 12 hypothalamic neuropeptides have
been identified, and it is expected that many more will be
discovered.
The hypothalamus is an important integrating area in the
brain. It receives afferent signals from virtually all parts
of the central nervous system (CNS) and sends efferent
fibers to the median eminence, the posterior pituitary, and
certain areas of the central nervous system. In the me-
dian eminence, a number of peptidergic fibers terminate
in close proximity to the primary plexus, which trans-
ports their neuropeptide secretions via the portal blood
flow to the anterior pituitary. Since these neuropeptides
affect the function of the anterior pituitary cells, they are
called “
hypophysiotropic
Two hypothalamic nuclei, the
paraventricular nucleus (PVN) and the supraoptic nucleus
(SON), consist of two populations of neurons that dif-
fer in size: the parvicellular (small-celled) neurons and
the magnocellular (large-celled) neurons. The parvicellu-
lar neurons of the paraventricular nucleus produce a pep-
tide (CRH; see below) that is transported by axoplasmic
flow to the median eminence and then released into the hy-
pophyseal portal blood. The magnocellular neurons of the
paraventricular and supraoptic nuclei send their long ax-
ons directly into the posterior pituitary (neurohypophysis),
729
