Human Physiology, 14th edition (2016)

Endocrine Glands 333

The release of ADH and oxytocin from the posterior pitu-

itary is controlled by neuroendocrine reflexes. In nursing

mothers, for example, the mechanical stimulus of suckling acts,

via sensory nerve impulses to the hypothalamus, to stimulate

the reflex secretion of oxytocin (chapter 20, section 20.6). The

secretion of ADH is stimulated by osmo receptor neurons in

the hypothalamus in response to a rise in the plasma osmolality

(chapter 6, section 6.2). An increased osmolality (and osmotic

pressure) stimulates an increased frequency of action potentials

in the neurons that produce ADH. This causes a greater open-

ing of voltage-gated Ca^2 1 channels at the axon terminals, which

produces a greater release of ADH by exocytosis. This is similar

to the way axon terminals release neurotransmitter (chapter 7;

see fig. 7.23), but in this case ADH is secreted as a hormone

from the posterior pituitary gland into the blood. Conversely,

ADH secretion can be inhibited by sensory input from stretch

receptors in the left atrium of the heart, which are stimulated

when there is a rise in blood volume (chapter 14, section 14.2).

Hypothalamic Control

of the Anterior Pituitary

At one time the anterior pituitary was called the “master gland”

because it secretes hormones that regulate some other endo-

crine glands ( fig.  11.14 and table  11.6 ). Adrenocorticotropic

hormone (ACTH), thyroid-stimulating hormone (TSH), and the

concentrations of MSH are insignificant. Some cells of the adeno-
hypophysis derived from the fetal pars intermedia produce a large
polypeptide called pro-opiomelanocortin (POMC). POMC is a
prohormone whose major products are beta- endorphin (chapter 7,
section 7.6), MSH, and ACTH. Because part of the ACTH mol-
ecule contains the amino acid sequence of MSH, elevated secre-
tions of ACTH (as in Addison’s disease; see section 11.4) cause
a marked darkening of the skin.
The posterior pituitary, or pars nervosa, stores and
releases two hormones, both of which are produced in the
hypothalamus.

1. Antidiuretic hormone (ADH). The human form of this hor-
   mone is also chemically known as arginine vasopressin
   (AVP), but the “pressor” effect (a rise in blood pressure due
   to vasoconstriction) is of secondary significance in humans.
   The “antidiuretic” effect of this hormone—its stimulation of
   water retention by the kidneys, so that less water is excreted
   in the urine—is far more significant. Because of this, the hor-
   mone will be termed antidiuretic hormone (ADH) in this text.


2. Oxytocin. In females, oxytocin stimulates contractions
   of the uterus during labor and for this reason is needed
   for parturition (childbirth). Oxytocin also stimulates con-
   tractions of the mammary gland alveoli and ducts, which
   result in the milk-ejection reflex in a lactating woman. In
   men, a rise in oxytocin secretion at the time of ejaculation
   has been measured, but the physiological significance of
   this hormone in males remains to be demonstrated.

CLINICAL APPLICATION

Pitocin, a brand name for oxytocin injection, may be given

to a pregnant woman to hasten a vaginal delivery. Oxytocin

receptors become more plentiful in the myometrium (smooth

muscle of the uterus) during pregnancy to promote more

forceful contractions. Injected oxytocin may be necessary,

for example, if the fetal membranes have prematurely rup-

tured, if there is Rh incompatibility (chapter 13, section 13.2),

or if there is preeclampsia (pregnancy-induced hypertension;

chapter 14, section 14.7). Oxytocin may also be injected

into the mother postpartum (after delivery) to minimize

hemorrhage.

Hypothalamic Control

of the Posterior Pituitary

Both of the posterior pituitary hormones—antidiuretic hormone
(ADH) and oxytocin—are actually produced in neuron cell bod-
ies of the supraoptic and paraventricular nuclei in the hypo-
thalamus. The ADH and oxytocin hormones produced in the
hypothalamus are transported along axons of the hypothalamo-
hypophyseal tract ( fig.  11.13 ) to the posterior pituitary, where
they are stored and later released in response to appropriate stim-
ulation. The posterior pituitary is thus more a storage organ than
a true gland.

Figure 11.13 Hypothalamic control of the posterior

pituitary. The posterior pituitary, or neurohypophysis, stores and

releases hormones—vasopressin and oxytocin—that are actually

produced in neurons within the supraoptic and paraventricular

nuclei of the hypothalamus. These hormones are transported to

the posterior pituitary by axons in the hypothalamo-hypophyseal

tract.

Posterior pituitary

Infundibulum

Anterior

pituitary

Optic chiasma

Hypothalamus

Supraoptic

nucleus

Paraventricular

nucleus ADH and oxytocin

produced here

ADH and

oxytocin

released

Hypothalamo-

hypophyseal

tract