Chapter 16
The Endocrine System
617
16
isolated from this minute gland, its only major secretory product
is
melatonin
(mel
0
ah-to
9
nin), an amine hormone derived from
serotonin. Melatonin concentrations in the blood rise and fall
in a diurnal (daily) cycle. Peak levels occur during the night and
make us drowsy, and lowest levels occur around noon. Recent
evidence suggests that melatonin also controls the production of
protective antioxidant and detoxification molecules within cells.
Te pineal gland indirectly receives input from the visual
pathways (retina
S
suprachiasmatic nucleus of hypothalamus
S
superior cervical ganglion
S
pineal gland) concerning the
intensity and duration of daylight. In some animals, mating be-
havior and gonadal size vary with relative lengths of light and
dark periods, and melatonin mediates these effects.
The Pineal Gland
Briefly describe the importance of melatonin.
Te tiny, pine cone–shaped
pineal gland
hangs from the roof
of the third ventricle in the diencephalon (see Figure 16.1 and
Figure 12.10 on p. 440). Its secretory cells, called
pinealocytes
,
are arranged in compact cords and clusters. Lying between pin-
ealocytes in adults are dense particles containing calcium salts
(“brain-sand” or “pineal sand”). Tese salts are radiopaque,
making the pineal gland a handy landmark for determining
brain orientation in X rays.
Te endocrine function of the pineal gland is still somewhat
of a mystery. Although many peptides and amines have been
Short-term stress
Prolonged stress
Stress
Hypothalamus
CRH (corticotropin-
releasing hormone)
Corticotropic cells
of anterior pituitary
To target in blood
Adrenal cortex
(secretes steroid
hormones)
Glucocorticoids
Mineralocorticoids
ACTH
Catecholamines
(epinephrine and
norepinephrine)
Short-term stress response
Heart rate increases
Blood pressure increases
Bronchioles dilate
Liver converts glycogen to glucose and releases
glucose to blood
Blood flow changes, reducing digestive system activity
and urine output
Metabolic rate increases
Long-term stress response
Kidneys retain
sodium and water
Blood volume and
blood pressure
rise
Proteins and fats converted
to glucose or broken down
for energy
Blood glucose increases
Immune system
supressed
Adrenal medulla
(secretes amino acid–
based hormones)
Preganglionic
sympathetic
fibers
Spinal cord
Nerve impulses
Figure 16.17
Stress and the adrenal gland.
Stressful stimuli cause the hypothalamus
to activate the adrenal medulla via sympathetic nerve impulses and the adrenal cortex via
hormonal signals.
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