(in male may
A combination of Figures 11–15 and 11–17 summarizes the hypothalamic–anterior pituitary system. The
symbols indicate stimulatory
and inhibitory actions, respectively.
and the text of this chapter summarize only those hypophy-
siotropic hormones that have clearly documented physiologi-
cal roles in humans.
Several of the hypophysiotropic hormones are named
for the anterior pituitary hormone whose secretion they con-
trol. Thus, secretion of ACTH (corticotropin) is stimulated
corticotropin-releasing hormone (CRH),
growth hormone is stimulated by
secretion of thyroid-stimulating hor-
mone (thyrotropin) is stimulated by
and secretion of both luteinizing hormone
and follicle-stimulating hormone (the gonadotropins) is stim-
gonadotropin-releasing hormone (GnRH).
Note, however, in Figure 11–17, that two of the hypophy-
siotropic hormones do not
the release of an anterior
pituitary hormone but rather
its release. One of them,
inhibits the secretion of growth hormone.
inhibits the secretion of prolactin.
As Figure 11–17 shows, growth hormone is controlled by
hypophysiotropic hormones—somatostatin, which inhib-
its its release, and growth hormone–releasing hormone, which
stimulates it. The rate of growth hormone secretion depends,
therefore, upon the relative amounts of the opposing hormones
released by the hypothalamic neurons, as well as upon the rela-
tive sensitivities of the anterior pituitary to them. Such dual con-
trols may also exist for the other anterior pituitary hormones,
but the importance of such control, if it exists, is uncertain.
summarizes the information presented
in Figures 11–15 and 11–17 to illustrate the full sequence of
hypothalamic control of endocrine function.
Given that the hypophysiotropic hormones control ante-
rior pituitary function, we must now ask: What controls secre-
tion of the hypophysiotropic hormones? Some of the neurons
that secrete hypophysiotropic hormones may possess sponta-
neous activity, but the ﬁ
ring of most of them requires neural
and hormonal input.
Neural Control of Hypophysiotropic Hormones
Neurons of the hypothalamus receive stimulatory and inhib-
itory synaptic input from virtually all areas of the central
nervous system, and speciﬁ c neural pathways inﬂ uence the
secretion of the individual hypophysiotropic hormones. A
large number of neurotransmitters, such as the catechol-
amines and serotonin, are released at the synapses on the
hormone-secreting hypothalamic neurons. Not surprisingly,
therefore, drugs that inﬂ
uence these neurotransmitters can
alter the secretion of the hypophysiotropic hormones.
illustrates one example of the role of
neural input to the hypothalamus. Corticotropin-releas-
ing hormone (CRH) from the hypothalamus stimulates the
anterior pituitary to secrete ACTH, which in turn stimu-
lates the adrenal cortex to secrete cortisol. A wide variety of
sensory stimuli resulting from physical or emotional stress
act via neural pathways to the hypothalamus to increase
CRH secretion and, therefore, ACTH and cortisol secre-
tion. Even in the absence of stressful stimuli, however, cor-
tisol secretion varies in a regular manner during a 24-hour
period because neural rhythms within the central nervous
system also impinge upon the hypothalamic neurons that