334
Chapter 11
FSH
and
LH
Growth
hormone
TSH
ACTH
Prolactin
+
+
+
+
Hypothalamus
Anterior pituitary
GHRH
SS
TRH
DA
CRH
GnRH
Liver and
other cells
Secrete
IGF-1
Adrenal
cortex
Secretes
cortisol
Many organs
and tissues
Protein synthesis,
carbohydrate
and lipid
metabolism
Thyroid
Secretes
thyroxine,
triiodothyronine
Breasts
Breast
development and
milk production
(in male may
facilitate
reproductive
function)
Gonads
Secrete hormones
Female
Estradiol,
progesterone
Male
Testos-
terone
Germ cell
development
Female
Ovum
Male
Sperm
Figure 11–18
A combination of Figures 11–15 and 11–17 summarizes the hypothalamic–anterior pituitary system. The
B
and
E
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
by
corticotropin-releasing hormone (CRH),
secretion of
growth hormone is stimulated by
growth hormone-releasing
hormone (GHRH),
secretion of thyroid-stimulating hor-
mone (thyrotropin) is stimulated by
thyrotropin-releasing
hormone (TRH),
and secretion of both luteinizing hormone
and follicle-stimulating hormone (the gonadotropins) is stim-
ulated by
gonadotropin-releasing hormone (GnRH).
Note, however, in Figure 11–17, that two of the hypophy-
siotropic hormones do not
stimulate
the release of an anterior
pituitary hormone but rather
inhibit
its release. One of them,
somatostatin (SS),
inhibits the secretion of growth hormone.
The other,
dopamine (DA),
inhibits the secretion of prolactin.
As Figure 11–17 shows, growth hormone is controlled by
two
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.
Figure 11–18
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.
Figure 11–19
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
secrete CRH.
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