Chapter 16
The Endocrine System
599
16
Hypophyseal branches of the internal carotid arteries deliver
arterial blood to the pituitary. Te veins leaving the pituitary
drain into the dural sinuses.
Pituitary-Hypothalamic Relationships
Te contrasting histology of the two pituitary lobes reflects the
dual origin of this tiny gland. Te posterior lobe is actually part
of the brain. It derives from a downgrowth of hypothalamic tis-
sue and maintains its neural connection with the hypothalamus
via a nerve bundle called the
hypothalamic-hypophyseal tract
,
which runs through the infundibulum (Figure 16.5).
Tis tract arises from neurons in the
paraventricular
and
supraoptic nuclei
of the hypothalamus. Tese neurosecretory
cells synthesize one of two neurohormones and transport them
along their axons to the posterior pituitary. Te paraventricu-
lar neurons primarily make oxytocin (ok
0
sĭ-to
9
sin), and the su-
praoptic neurons mainly produce antidiuretic hormone. When
these hypothalamic neurons fire, they release the stored hor-
mones into a capillary bed in the posterior pituitary for distri-
bution throughout the body.
Te glandular anterior lobe originates from epithelial tissue
as a superior outpocketing of the oral mucosa. A±er touching
the posterior lobe, the anterior lobe adheres to the neurohy-
pophysis and loses its connection with the oral mucosa. Tere
is no direct neural connection between the anterior lobe and
hypothalamus, but there is a vascular connection. Specifically,
the
primary capillary plexus
in the infundibulum communi-
cates inferiorly via the small
hypophyseal portal veins
with a
secondary capillary plexus
in the anterior lobe. Te primary
and secondary capillary plexuses and the intervening hypo-
physeal portal veins make up the
hypophyseal portal system
(Figure 16.5). Note that a
portal system
is an unusual arrange-
ment of blood vessels in which a capillary bed feeds into veins,
which in turn feed into a second capillary bed.
Via the hypophyseal portal system,
releasing
and
inhibiting
hormones
secreted by neurons in the ventral hypothalamus cir-
culate to the anterior pituitary, where they regulate secretion of
its hormones. Te portal system ensures that the minute quan-
tities of hormones released by the hypothalamus arrive rapidly
at the anterior pituitary without being diluted by the systemic
circulation. All these hypothalamic regulatory hormones are
amino acid based, but they vary in size from a single amine to
peptides to proteins.
The Posterior Pituitary
and Hypothalamic Hormones
Te posterior pituitary consists largely of axon terminals of hy-
pothalamic neurons whose cell bodies are located in the su-
praoptic or paraventricular nuclei. Tese neurons synthesize
one of two neurohormones in their cell bodies: Oxytocin or
antidiuretic hormone (ADH). Axon terminals in the posterior
pituitary release these hormones “on demand” in response to
action potentials that travel down the axons of these same hy-
pothalamic neurons.
Oxytocin and ADH, each composed of nine amino acids, are
almost identical. Tey differ in only two amino acids, and yet
they have dramatically different physiological effects, summa-
rized in ²able 16.1 and described next.
Oxytocin
A strong stimulant of uterine contraction,
oxytocin
is released
in significantly higher amounts during childbirth (
oxy
5
rapid;
tocia
5
childbirth) and in nursing women. Te number of oxy-
tocin receptors in the uterus peaks near the end of pregnancy,
and uterine smooth muscle becomes more and more sensitive
to the hormone’s stimulatory effects. Stretching of the uterus
and cervix as birth nears dispatches afferent impulses to the
hypothalamus, which responds by synthesizing oxytocin and
triggering its release from the posterior pituitary. Oxytocin acts
via the PIP
2
-Ca
2
1
second-messenger system to mobilize Ca
2
1
,
allowing stronger contractions. As blood levels of oxytocin rise,
the expulsive contractions of labor gain momentum and finally
end in birth.
Oxytocin also acts as the hormonal trigger for milk ejection
(the “letdown” reflex) in women whose breasts are producing
milk in response to prolactin. Suckling causes a reflex-initiated
release of oxytocin, which targets specialized myoepithelial cells
surrounding the milk-producing glands. Tese cells contract
and force milk from the breast into the infant’s mouth. Both
childbirth and milk ejection result from
positive feedback mech-
anisms,
which Chapter 28 describes in more detail.
Both natural and synthetic oxytocic drugs are used to in-
duce labor or to hasten labor that is progressing slowly. Less
frequently, oxytocic drugs are used to stop postpartum bleeding
(by compressing ruptured blood vessels at the placental site) or
to stimulate the milk ejection reflex.
Oxytocin also acts as a neurotransmitter in the brain. Tere,
it is involved in sexual and affectionate behavior (as the “cuddle
hormone”), and promotes nurturing, couple bonding, and trust.
Antidiuretic Hormone (ADH)
Diuresis
(di
0
u-re
9
sis) is urine production, so an
antidiuretic
is a
substance that inhibits or prevents urine formation.
Antidiu-
retic hormone (ADH)
prevents wide swings in water balance,
helping the body avoid dehydration and water overload.
Hypothalamic neurons called
osmoreceptors
continually mon-
itor the solute concentration (and thus the water concentration)
of the blood. When solutes threaten to become too concentrated
(as might follow excessive perspiration or inadequate fluid in-
take), the osmoreceptors transmit excitatory impulses to the
hypothalamic neurons, which release ADH. Liberated into the
blood by the posterior pituitary, ADH targets the kidney tubules
via cAMP. Te tubule cells respond by reabsorbing more water
from the forming urine and returning it to the bloodstream. As a
result, less urine is produced and the solute concentration of the
blood declines. As solute levels fall, the osmoreceptors stop depo-
larizing, effectively ending ADH release. Other stimuli triggering
ADH release include pain, low blood pressure, and such drugs as
nicotine, morphine, and barbiturates.
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