336
Chapter 11
Long-loop feedback does not exist for prolactin because
this is one anterior pituitary hormone that does not have
major control over another endocrine gland—that is, it does
not participate in a three-hormone sequence. Nonetheless,
there is negative feedback in the prolactin system, for this
hormone itself acts upon the hypothalamus to
stimulate
the
secretion of dopamine, which then
inhibits
the secretion of
prolactin. The infl uence of an anterior pituitary hormone on
the hypothalamus is known as a
short-loop negative feed-
back
(see Figure 11–20). Like prolactin, several other anterior
pituitary hormones, including growth hormone, also exert
such feedback on the hypothalamus.
The Role of “Nonsequence” Hormones on
the Hypothalamus and Anterior Pituitary
Keep in mind that there are many stimulatory and inhibitory
hormonal infl
uences on the hypothalamus and/or anterior
pituitary other than those that fi t the feedback patterns just
described. In other words, a hormone that is not itself in a
particular hormonal sequence may nevertheless exert impor-
tant infl uences on the secretion of the hypophysiotropic or
anterior pituitary hormones in that sequence. For example,
estradiol markedly enhances the secretion of prolactin by the
anterior pituitary, even though estradiol secretion is not nor-
mally controlled by prolactin. Thus, the sequences we have
been describing should not be viewed as isolated units.
SECTION B SUMMARY
Control Systems Involving the Hypothalamus
and Pituitary
I. The pituitary gland, comprising the anterior pituitary and
the posterior pituitary, is connected to the hypothalamus by a
stalk containing nerve axons and blood vessels.
II. Specifi c axons, whose cell bodies are in the hypothalamus,
terminate in the posterior pituitary and release oxytocin and
vasopressin.
III. The anterior pituitary secretes growth hormone (GH),
thyroid-stimulating hormone (TSH), adrenocorticotropic
hormone (ACTH), prolactin, and two gonadotropic
hormones—follicle-stimulating hormone (FSH) and
luteinizing hormone (LH). The functions of these hormones
are summarized in Figure 11–15.
IV. Secretion of the anterior pituitary hormones is controlled
mainly by hypophysiotropic hormones secreted into
capillaries in the median eminence of the hypothalamus
and reaching the anterior pituitary via the portal vessels
connecting the hypothalamus and anterior pituitary. The
actions of the hypophysiotropic hormones on the anterior
pituitary are summarized in Figure 11–17.
adrenocorticotropic hormone
(ACTH)
331
anterior pituitary
330
antidiuretic hormone
(ADH)
331
beta-endorphin
331
beta-lipotropin
331
corticotropin-releasing hormone
(CRH)
334
dopamine (DA)
334
follicle-stimulating hormone
(FSH)
331
gonadotropic hormone
331
gonadotropin-releasing
hormone (GnRH)
334
growth hormone (GH)
331
growth hormone–releasing
hormone (GHRH)
334
hypophysiotropic
hormone
331
hypothalamo-pituitary portal
vessel
331
hypothalamus
330
infundibulum
330
insulin-like growth factor 1
(IGF-1)
332
long-loop negative feedback
335
luteinizing hormone (LH)
331
median eminence
331
oxytocin
331
pituitary gland
330
posterior pituitary
330
prolactin
331
short-loop negative
feedback
336
somatostatin (SS)
334
thyroid-stimulating hormone
(TSH)
331
thyrotropin-releasing hormone
(TRH)
334
vasopressin
331
V. The secretion of each hypophysiotropic hormone is controlled
by neuronal and hormonal input to the hypothalamic neurons
producing it.
a. In each of the three-hormone sequences beginning
with a hypophysiotropic hormone, the third hormone
exerts negative feedback effects on the secretion of the
hypothalamic and/or anterior pituitary hormone.
b. The anterior pituitary hormone may exert a short-loop
negative feedback inhibition of the hypothalamic releasing
hormone(s) controlling it.
c. Hormones not in a particular sequence can also infl uence
secretion of the hypothalamic and/or anterior pituitary
hormones in that sequence.
SECTION B KEY TERMS
SECTION B REVIEW QUESTIONS
1. Describe the anatomical relationships between the
hypothalamus and the pituitary.
2. Name the two posterior pituitary hormones and describe their
site of synthesis and mechanism of release.
3. List all six well-established anterior pituitary hormones and
their major functions.
4. List the major hypophysiotropic hormones and the hormone
whose release each controls.
5. What kinds of inputs control secretion of the hypophysiotropic
hormones?
6. Diagram the CRH-ACTH-cortisol system.
7. What is the difference between long-loop and short-loop
negative feedback in the hypothalamo-anterior pituitary system?
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