Endocrine Glands 337Studies in mice show that neurons in at least 26 brain areas
send axons to the GnRH-producing neurons of the hypothala-
mus! Considering this, it isn’t surprising that various emotional
states and stress can influence the menstrual cycle. These stud-
ies also show that neurons in the olfactory epithelium of the
nose send information (relayed from the olfactory bulb and
amygdala) to the GnRH-producing neurons of the hypothala-
mus. Thus, the sense of olfaction (smell) may influence the
secretion of GnRH, and thereby may affect the functioning of
the reproductive system.
Psychological stress is known to activate the pituitary
adrenal axis, as described more fully in section 11.4. Stress-
ors produce an increase in CRH secretion from the hypothala-
mus, which in turn results in elevated ACTH and corticosteroid
secretion. In addition, the influence of higher brain centers
produces circadian (“about a day”) rhythms in the secretion
of many anterior pituitary hormones. The secretion of growth
hormone, for example, is highest during sleep and decreases
during wakefulness, although its secretion is also stimulated
by the absorption of particular amino acids following a meal.
| CHECKPOINT
5a. Describe the embryonic origins of the
adenohypophysis and neurohypophysis, and list the
parts of each. Which of these parts is also called
the anterior pituitary? Which is called the posterior
pituitary?
5b. List the hormones released by the posterior pituitary.
Where do these hormones originate and how are
their secretions regulated?- List the hormones secreted by the anterior pituitary
and explain how the hypothalamus controls the
secretion of each. - Draw a negative feedback loop showing the
control of ACTH secretion. Explain how this system
would be affected by (a) an injection of ACTH,
(b) surgical removal of the pituitary, (c) an injection
of corticosteroids, and (d) surgical removal of the
adrenal glands.
11.4 Adrenal Glands
The adrenal cortex and adrenal medulla are structurally
and functionally different. The adrenal medulla secretes
catecholamine hormones, which complement the sympa-
thetic nervous system in the “fight-or-flight” reaction. The
adrenal cortex secretes steroid hormones that participate
in the regulation of mineral and energy balance.
LEARNING OUTCOMESAfter studying this section, you should be able to:- Identify the hormones of the adrenal medulla, as well
as the categories of corticosteroid hormones and
their specific origin. - Describe the regulation of adrenal gland secretion,
and the role of stress in adrenal secretion and
function.
The adrenal glands are paired organs that cap the superior
borders of the kidneys ( fig. 11.18 ). Each adrenal consists of an
outer cortex and inner medulla that function as separate glands.
The differences in function of the adrenal cortex and medulla
are related to the differences in their embryonic derivation. The
adrenal medulla is derived from embryonic neural crest ecto-
derm (the same tissue that produces the sympathetic ganglia),
whereas the adrenal cortex is derived from a different embry-
onic tissue (mesoderm).
As a consequence of its embryonic derivation, the adrenal
medulla secretes catecholamine hormones (mainly epineph-
rine, with lesser amounts of norepinephrine) into the blood in
response to stimulation by preganglionic sympathetic axons
(chapter 9). The adrenal cortex does not receive neural innerva-
tion, and so must be stimulated hormonally (by ACTH secreted
from the anterior pituitary). The cortex consists of three zones:
an outer zona glomerulosa, a middle zona fasciculata, and an
inner zona reticularis ( fig. 11.18 ). These zones are believed to
have different functions.Functions of the Adrenal Cortex
The adrenal cortex secretes steroid hormones called
corticosteroids, or corticoids. There are three functional cate-
gories of corticosteroids: (1) mineralocorticoids, which regu-
late Na^1 and K^1 balance; (2) glucocorticoids, which regulate
the metabolism of glucose and other organic molecules; and
(3) adrenal androgens, which are weak androgens (includ-
ing dehydroepiandrosterone, or DHEA ) that supplement the
sex steroids secreted by the gonads. These three categories of
steroid hormones are derived from the same precursor (parent
molecule), cholesterol. The biosynthetic pathways from cho-
lesterol diverge in the different zones of the adrenal cortex, so
that a particular category of corticosteroid is produced in a par-
ticular zone of the adrenal cortex ( fig. 11.19 ).
Aldosterone is the most potent mineralocorticoid. The
mineralocorticoids are produced in the zona glomerulosa and
stimulate the kidneys to retain Na^1 and water while excreting
K^1 in the urine. These actions help to increase the blood vol-
ume and pressure (chapter 14, section 14.2), and to regulate
blood electrolyte balance (chapter 17, section 17.5).
The predominant glucocorticoid in humans is cortisol
(hydrocortisone), which is secreted by the zona fascicu-
lata and perhaps also by the zona reticularis. The secretion