492
UNIT THREE
it secretes glucagon, as covered in the section entitled
“Hormones of the Pancreatic Islets” (see F g. 13.8
c
).
In each of these cases, as hormone levels rise in the blood
and the hormone exerts its effects, negative feedback inhibits
the system and hormone secretion decreases. Then, as hor-
mone levels in the blood decrease and the hormone’s effects
wane, inhibition of the system ceases, and secretion of that
hormone increases again
(f
g. 13.10)
. As a result of negative
feedback, hormone levels in the bloodstream remain rela-
tively stable, fl uctuating slightly around an average value
(f
g. 13.11)
.
PRACTICE
10
How does the nervous system help regulate hormonal secretions?
11
How does a negative feedback system control hormonal
secretion?
13.5
PITUITARY GLAND
The
pituitary
(pı˘-tu
ı˘-ta
¯r
e)
gland
(hypophysis), at the
base of the brain, is about one centimeter in diameter. It
is attached to the hypothalamus by the pituitary stalk, or
infundibulum,
and lies in the sella turcica of the sphenoid
bone, as F
gure 13.9 shows.
The pituitary gland consists of two distinct portions: an
anterior lobe
(adenohypophysis) and a
posterior lobe
(neu-
rohypophysis). The anterior lobe secretes a number of hor-
mones, including growth hormone (GH), thyroid-stimulating
hormone (TSH), adrenocorticotropic hormone (ACTH), fol-
licle-stimulating hormone (±SH), luteinizing hormone (LH),
and prolactin (PRL). Although the cells of the posterior lobe
(pituicytes) do not synthesize any hormones, specialized
Hormones are continually excreted in the urine and
broken down by enzymes, primarily in the liver. Therefore,
increasing or decreasing blood levels of a hormone requires
increased or decreased secretion. Hormone secretion is pre-
cisely regulated.
Control Sources
Control of hormone secretion is essential to maintaining the
internal environment. In a few cases, primarily in the repro-
ductive systems, positive feedback affects this control.
Generally, hormone secretion is controlled in three ways,
all of which employ
negative feedback
(see chapter 1, p. 9).
In each case, an endocrine gland or the system controlling it
senses the concentration of the hormone the gland secretes,
a process the hormone controls, or an action the hormone
has on the internal environment
(f
g. 13.8)
.
1. The hypothalamus controls the anterior pituitary gland’s
release of
tropic hormones,
which stimulate other
endocrine glands to release hormones (F
g. 13.8
a
). The
hypothalamus constantly receives information about
the internal environment from neural connections and
cerebrospinal fl
uid, made possible by its location near
the thalamus and the third ventricle
(f
g. 13.9)
.
2. The nervous system directly stimulates some glands.
The adrenal medulla, for example, secretes its hormones
(epinephrine and norepinephrine) in response to
preganglionic sympathetic nerve impulses. The
secretory cells replace the postganglionic sympathetic
neurons, which would normally secrete norepinephrine
alone as a neurotransmitter (see F
g. 13.8
b
).
3. Another group of glands responds directly to changes
in the composition of the internal environment. ±or
example, when the blood glucose level rises, the pancreas
secretes insulin, and when the blood glucose level falls,
Nervous system
Endocrine
gland
Endocrine
gland
(a)
(b)
(c)
Target cells
Action
Target cells
Action
Changing level
of substance
in plasma
–––
Anterior pituitary gland
Peripheral
endocrine
gland
Target cells
Action
Hypothalamus
FIGURE 13.8
Examples of endocrine system control: (
a
) one way the hypothalamus controls the anterior pituitary, (
b
) the nervous system controls
some glands directly, and (
c
) some glands respond directly to changes in the internal environment.
indicates negative feedback inhibition.
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