The Endocrine System 8
Prof. Dr. Sa'ad Merza Alaraji
University Of Babylon ,College Of Medicine, Dep. Of Medical Physiology
The Endocrine System 8
Posterior Pituitary
Although the posterior pituitary or neurohypophysis does not synthesize
hormones, it does store and release two hormones. It consists of axons and axon
terminals of more than 10,000 hypothalamic neurosecretory cells. The cell
bodies of the neurosecretory cells are in the paraventricular and supraoptic nuclei
of the hypothalamus; their axons form the hypothalamohypophyseal tract (hı¯
_-po¯ -thal_-a-mo¯-hı¯-po¯-FIZ-e¯ -al). This tract begins in the hypothalamus
and ends near blood capillaries in the posterior pituitary (Figure 13).
Figure 13: Axons of hypothalamic neurosecretory cells form the hypothalamohypophyseal tract,
which extends from the paraventricular and supraoptic nuclei to the posterior pituitary.
Hormone molecules synthesized in the cell body of a neurosecretory cell are packaged into secretory
vesicles that move down to the axon terminals. Nerve impulses trigger exocytosis of the vesicles,
thereby releasing the hormone.
Oxytocin and antidiuretic hormone are synthesized in the hypothalamus and released into the capillary
plexus of the infundibular process in the posterior pituitary.
The paraventricular nucleus synthesizes the hormone oxytocin (OT; ok_-se¯TO¯ -sin; okytoc = quick birth) and the supraoptic nucleus produces antidiuretic
hormone (ADH; an-tı¯-dı¯-yoo-RET-ik; anti- = against; -diuretic= increased
urine production), also called vasopressin (va¯-so¯- PRES-in; vaso- = blood; pressus = to press). The axon terminals in the posterior pituitary are associated
with specialized neuroglia called pituicytes (pi-TOO-i-sı¯tz). These cells have a
supporting role similar to that of astrocytes. After their production in the cell
bodies of neurosecretory cells, oxytocin and antidiuretic hormone are packaged
into secretory vesicles, which move by fast axonal transport to the axon
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The Endocrine System 8
Prof. Dr. Sa'ad Merza Alaraji
University Of Babylon ,College Of Medicine, Dep. Of Medical Physiology
terminals in the posterior pituitary, where they are stored until nerve impulses
trigger exocytosis and release of the hormone.
Blood is supplied to the posterior pituitary by the inferior hypophyseal arteries
(see Figure 8), which branch from the internal carotid arteries. In the posterior
pituitary, the inferior hypophyseal arteries drain into the capillary plexus of the
infundibular process, a capillary network that receives secreted oxytocin and
antidiuretic hormone (see Figure 8). From this plexus, hormones pass into the
posterior hypophyseal veins for distribution to target cells in other tissues.
Oxytocin
During and after delivery of a baby, oxytocin affects two target tissues:
1. The mother’s uterus and
2. Breasts.
During delivery, oxytocin enhances contraction of smooth muscle cells in the
wall of the uterus; after delivery, it stimulates milk ejection (“letdown”) from the
mammary glands in response to the mechanical stimulus provided by a suckling
infant. The function of oxytocin in males and in nonpregnant females is not
clear. Experiments with animals have suggested that it has actions within the
brain that foster parental caretaking behavior toward young offspring.
It may also be responsible, in part, for the feelings of sexual pleasure during
and after intercourse.
Clinical Connection: Oxytocin and Childbirth
Years before oxytocin was discovered, it was common practice in midwifery to
let a first-born twin nurse at the mother’s breast to speed the birth of the second
child. Now we know why this practice is helpful—it stimulates the release of
oxytocin. Even after a single birth, nursing promotes expulsion of the placenta
(afterbirth) and helps the uterus regain its smaller size. Synthetic OT (Pitocin)
often is given to induce labor or to increase uterine tone and control
hemorrhage just after giving birth.
Antidiuretic Hormone
As its name implies, an antidiuretic (an-tı¯-dı¯-yoo-RET-ik; -anti- = against;
dia = throughout; ouresis = urination) is a substance that decreases urine
production. ADH causes the kidneys to return more water to the blood, thus
decreasing urine volume. In the absence of ADH, urine output increases more
than tenfold, from the normal 1 to 2 liters to about 20 liters a day. Drinking
alcohol often causes frequent and copious urination because alcohol inhibits
secretion of ADH. ADH also decreases the water lost through sweating and
causes constriction of arterioles, which increases blood pressure. This hormone’s
other name, vasopressin, reflects this effect on blood pressure.
The amount of ADH secreted varies with blood osmotic pressure and blood
volume. Figure 14 shows regulation of ADH secretion and the actions of ADH.
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The Endocrine System 8
Prof. Dr. Sa'ad Merza Alaraji
University Of Babylon ,College Of Medicine, Dep. Of Medical Physiology
Figure 14: Regulation of secretion and actions of antidiuretic hormone (ADH).
ADH acts to retain body water and increase blood pressure.
1. High blood osmotic pressure—due to dehydration or a decline in blood
volume because of hemorrhage, diarrhea, or excessive sweating—stimulates
osmoreceptors, neurons in the hypothalamus that monitor blood osmotic
pressure.
Elevated blood osmotic pressure activates the osmoreceptors directly; they also
receive excitatory input from other brain areas when blood volume decreases.
2. Osmoreceptors activate the hypothalamic neurosecretory cells that synthesize
and release ADH.
3. When neurosecretory cells receive excitatory input from the osmoreceptors,
they generate nerve impulses that cause exocytosis of ADH-containing vesicles
from their axon terminals in the posterior pituitary. This liberates ADH, which
diffuses into blood capillaries of the posterior pituitary.
4. The blood carries ADH to three target tissues: the kidneys, sudoriferous
(sweat) glands, and smooth muscle in blood vessel walls. The kidneys respond
by retaining more water, which decreases urine output. Secretory activity of
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The Endocrine System 8
Prof. Dr. Sa'ad Merza Alaraji
University Of Babylon ,College Of Medicine, Dep. Of Medical Physiology
sweat glands decreases, which lowers the rate of water loss by perspiration from
the skin. Smooth muscle in the walls of arterioles (small arteries) contracts in
response to high levels of ADH, which constricts (narrows) the lumen of these
blood vessels and increases blood pressure.
5. Low osmotic pressure of blood (or increased blood volume) inhibits the
osmoreceptors.
6. Inhibition of osmoreceptors reduces or stops ADH secretion. The kidneys
then retain less water by forming a larger volume of urine, secretory activity of
sweat glands increases, and arterioles dilate. The blood volume and osmotic
pressure of body fluids return to normal.
Secretion of ADH can also be altered in other ways. Pain, stress, trauma, anxiety,
acetylcholine, nicotine, and drugs such as morphine, tranquilizers, and some
anesthetics stimulateADH secretion. The dehydrating effect of alcohol, which
has already been mentioned, may cause both the thirst and the headache typical
of a hangover. Hyposecretion of ADH or nonfunctional ADH receptors causes
diabetes insipidus .
Table 5 lists the posterior pituitary hormones, control of their secretion, and their
principal actions.
Table 5: Summary of Posterior Pituitary Hormones
HORMONE
TARGET TISSUES
CONTROL OF
SECRETION
Neurosecretory cells of
hypothalamus secrete
OT in response to
uterine distention and
stimulation of nipples.
Oxytocin
(OT)
Neurosecretory cells of
hypothalamus secrete
ADH in response to
elevated blood osmotic
pressure, dehydration,
loss of blood volume,
pain, or stress; low
blood osmotic
pressure, high blood
volume, and alcohol
inhibit
ADH secretion.
Antidiuretic
hormone
(ADH) or
vasopressin
PRINCIPAL
ACTIONS
Stimulates
contraction of
smooth muscle
cells of the uterus
during childbirth;
stimulates
contraction of
myoepithelial cells
in the mammary
glands to cause
milk ejection
Conserves body
water by
decreasing urine
volume; decreases
water loss through
perspiration; raises
blood pressure by
constricting
arterioles
Thank You
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