The Endocrine System
331
found between the anterior and posterior portions of the pitu-
itary, but this is not the case in humans. The intermediate lobe
in such animals produces a hormone called melanocyte-stimu-
lating hormone that controls coat color.
The axons of two well-defi ned clusters of hypothalamic
neurons (the supraoptic and paraventricular nuclei) pass down
the infundibulum and end within the posterior pituitary in close
proximity to capillaries (the smallest of blood vessels) (
Figure
11–13b
). Thus, these neurons do not form a synapse with other
neurons. Instead, their terminals end directly on capillaries.
In contrast to the neural connections between the hypo-
thalamus and posterior pituitary, there are no important neural
connections between the hypothalamus and anterior pituitary.
There is, however, an unusual blood vessel connection (see
Figure 11–13b). The capillaries at the junction of the hypo-
thalamus and infundibulum (the
median eminence
) recom-
bine to form the
hypothalamo-pituitary portal vessels.
The
term
portal
denotes blood vessels that connect two capillary
beds. The hypothalamo-pituitary portal vessels pass down the
stalk connecting the hypothalamus and pituitary and enter the
anterior pituitary, where they drain into a second capillary bed,
the anterior pituitary capillaries. Thus, the hypothalamo-pitu-
itary portal vessels offer a local route for blood fl ow directly
from the hypothalamus to the cells of the anterior pituitary.
This offers the advantage of a rapid response and minimizes the
amount of hypothalamic hormone that must be synthesized to
reach an effective blood concentration (because the hormone
is not diluted into the general circulation of the body).
Posterior Pituitary Hormones
We emphasized that the posterior pituitary is really a neural
extension of the hypothalamus (see Figure 11–13). The hor-
mones are not synthesized in the posterior pituitary itself but
in the hypothalamus, specifi cally in the cell bodies of the supra-
optic and paraventricular nuclei, whose axons pass down the
infundibulum and end in the posterior pituitary. Enclosed in
small vesicles, the hormone moves down the axons to accumu-
late at the axon terminals in the posterior pituitary. Stimuli such
as neuro transmitters generate action potentials in the neurons;
these action potentials propagate to the axon terminals and
trigger the release of the stored hormone by exocytosis. The
hormone then enters capillaries in the posterior pituitary to be
carried away by the blood returning to the heart. In this way, the
brain can receive stimuli and respond as if it were an endocrine
organ. By releasing its hormones into the general circulation, the
posterior pituitary can modify the function of distant organs.
The two posterior pituitary hormones are
oxytocin
and
vasopressin.
Oxytocin stimulates contraction of smooth muscle
cells in the breasts, which results in milk ejection during lacta-
tion, and stimulates contraction of uterine smooth muscle during
labor. Although oxytocin is also present in males, its functions
in males are uncertain. Vasopressin acts on smooth muscle cells
around blood vessels to cause muscle contraction, which con-
stricts blood vessels and increases blood pressure. Vasopressin
also acts within the kidneys to decrease water excretion in the
urine, thus retaining fl uid in the body and helping to main-
tain blood volume. Because of its kidney function, vasopressin
is also known as
antidiuretic hormone,
or
ADH
(a loss of
excess water in the urine is known as a
diuresis
).
Anterior Pituitary Hormones and the
Hypothalamus
Hypothalamic neurons different from those that produce the
hormones released from the posterior pituitary, secrete hor-
mones that control the secretion of all the anterior pituitary
hormones. For simplicity’s sake, Figure 11–13 depicts these neu-
rons as arising from a single nucleus, but in fact several hypo-
thalamic nuclei send axons whose terminals end in the median
eminence. The hypothalamic hormones that regulate anterior
pituitary function are collectively termed
hypophysiotropic
hormones
(recall that another name for the pituitary is hypoph-
ysis); they are also commonly called hypothalamic releasing or
inhibiting hormones. “Hypophysiotropic hormones” denotes
only those hormones from the
hypothalamus
that infl uence the
anterior pituitary. We will see later that nonhypothalamic hor-
mones can also infl uence the anterior pituitary, but they are not
categorized as hypophysiotropic hormones.
With one exception (dopamine), each of the hypophy-
siotropic hormones is the fi rst in a
three
-hormone sequence:
(1) A hypophysiotropic hormone controls the secretion of
(2) an anterior pituitary hormone, which controls the secre-
tion of (3) a hormone from some other endocrine gland
(
Figure 11–14
). This last hormone then acts on its target
cells. The adaptive value of such sequences is that they permit
a variety of types of important hormonal feedback. They also
allow amplifi cation of a response of a small number of hypo-
thalamic neurons into a large peripheral hormonal signal. We
begin our description of these sequences in the middle—that
is, with the anterior pituitary hormones—because the names
and actions of the hypophysiotropic hormones are mostly
based on the names of the anterior pituitary hormones.
Overview of Anterior Pituitary Hormones
As shown in Table 11–1, the anterior pituitary secretes at
least eight hormones, but only six have well-established func-
tions. All peptides, these six “classical” hormones are
follicle-
stimulating hormone (FSH),
luteinizing hormone (LH),
growth hormone
(
GH,
also known as somatotropin),
thyroid-
stimulating hormone
(
TSH,
also known as thyrotropin),
prolactin,
and
adrenocorticotropic hormone
(
ACTH,
also
known as corticotropin). Each of the last four is secreted by a
distinct cell type in the anterior pituitary, whereas FSH and
LH, collectively termed
gonadotropic hormones
(or gonado-
tropins) because they stimulate the gonads, are usually secreted
by the same cells.
What about the other two peptides—
beta-lipotropin
and
beta-endorphin
—secreted by the anterior pituitary?
Their physiological roles, if any, in humans are unclear. In ani-
mal studies, however, beta-endorphin has been shown to have
potent pain-killing effects, and beta-lipotropin can mobilize
fats in the circulation to provide extra fuel. Both of these
functions may contribute to an animal’s ability to cope with
stressful challenges.
Figure 11–15
summarizes the target organs and major
functions of the six classical anterior pituitary hormones. Note
that the only major function of two of the six is to stimulate
their target cells to secrete other hormones (and to maintain
the growth and function of these cells). Thyroid-stimulating
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