The Endocrine System
335
Hormonal Feedback Control of the
Hypothalamus and Anterior Pituitary
A prominent feature of each of the hormonal sequences ini-
tiated
by
a
hypophys
iotrop
ic
hormone
is
negative
feedback
exerted upon the hypothalamo-pituitary system by one or more
of the hormones in its sequence. For example, in the CRH-
ACTH-cortisol sequence (see Figure 11–19), the fi nal hor-
mone, cortisol, acts upon the hypothalamus to reduce synthesis
and secretion of CRH. In addition, cortisol acts directly on the
anterior pituitary to reduce the response of the ACTH-secret-
ing cells to CRH. Thus, by a double-barreled action, cortisol
exerts a negative feedback control over its own secretion.
Such a system is effective in dampening hormonal
responses—that is, in limiting the extremes of hormone secre-
tory rates. For example, when a painful stimulus elicits
increased secretion, in turn, of CRH, ACTH, and cortisol, the
resulting elevation in plasma cortisol concentration feeds back
to inhibit the hypothalamus and anterior pituitary. Therefore,
cortisol secretion does not rise as much as it would without
negative feedback.
The situation described for cortisol, in which the hormone
secreted by the third endocrine gland in a sequence exerts a
negative feedback effect over the anterior pituitary and/or hypo-
thalamus, is known as a
long-loop negative feedback
(
Figure
11–20
). This type of feedback exists for each of the three-
hormone sequences initiated by a hypophysiotropic hormone.
Target cells for cortisol
Respond to increased cortisol
Plasma cortisol
Plasma ACTH
Plasma CRH
(in hypothalamo-pituitary portal vessels)
Neural inputs
Adrenal cortex
Cortisol secretion
Anterior pituitary
ACTH secretion
Hypothalamus
CRH secretion
Begin
Figure 11–19
CRH-ACTH-cortisol sequence. Neural inputs include those related to
stressful stimuli and nonstress inputs like circadian rhythms. Cortisol
exerts a negative feedback (
E
symbols) control over the system
by acting on (1) the hypothalamus to inhibit CRH synthesis and
secretion and (2) the anterior pituitary to inhibit ACTH production.
Figure 11–19
physiological
inquiry
What hormonal changes would be expected if a patient developed
a benign tumor of the left adrenal cortex that secreted large
amounts of cortisol in the absence of external stimulation? What
would happen to the right adrenal gland?
Answer can be found at end of chapter.
Figure 11–20
Short-loop and long-loop feedbacks. Long-loop feedback is exerted
on the hypothalamus and/or anterior pituitary by the third
hormone in the sequence. Short-loop feedback is exerted by the
anterior pituitary hormone on the hypothalamus.
Target cells for hormone 3
Respond to hormone 3
Plasma hormone 3
Plasma hormone 2
Plasma hormone 1
(in hypothalamo-pituitary portal vessels)
Stimulus
Third endocrine gland
Hormone 3 secretion
Anterior pituitary
Hormone 2 secretion
S
hort-loop feedb
a
ck
Hypothalamus
Hormone 1 secretion
Long-loop feedb
a
ck
Begin
previous page 363 Vander's Human Physiology The Mechanisms of Body Function read online next page 365 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off