Ganong's Review of Medical Physiology, 23rd Edition

406 SECTION IV Endocrine & Reproductive Physiology
Normally, erection is terminated by sympathetic vasoconstrictor
impulses to the penile arterioles.
Ejaculation
Ejaculation is a two-part spinal reflex that involves emission,
the movement of the semen into the urethra; and ejaculation
proper, the propulsion of the semen out of the urethra at the
time of orgasm. The afferent pathways are mostly fibers from
touch receptors in the glans penis that reach the spinal cord
through the internal pudendal nerves. Emission is a sympathetic
response, integrated in the upper lumbar segments of
the spinal cord and effected by contraction of the smooth
muscle of the vasa deferentia and seminal vesicles in response
to stimuli in the hypogastric nerves. The semen is propelled
out of the urethra by contraction of the bulbocavernosus muscle,
a skeletal muscle. The spinal reflex centers for this part of
the reflex are in the upper sacral and lowest lumbar segments
of the spinal cord, and the motor pathways traverse the first to
third sacral roots and the internal pudendal nerves.
PSA
The prostate produces and secretes into the semen and the
bloodstream a 30 kDa serine protease generally called prostate-specific
antigen (PSA). The gene for PSA has two androgen
response elements. PSA hydrolyzes the sperm motility
inhibitor semenogelin in semen, and it has several substrates
in plasma, but its precise function in the circulation is unknown.
An elevated plasma PSA occurs in prostate cancer and
is widely used as a screening test for this disease, though PSA
is also elevated in benign prostatic hyperplasia and prostatitis.
O
H
Cholesterol
Pregnenolone
17α-Hydroxypregnenolone
Dehydroepiandrosterone
OH
Dihydrotestosterone
In some
target tissues
5α-reductase,
type 1 or type 2
Vasectomy
Bilateral ligation of the vas deferens (vasectomy) has proved to
be a relatively safe and convenient contraceptive procedure.
However, it has proven difficult to restore the patency of the
vas in those wishing to restore fertility, and the current success
rate for such operations, as measured by the subsequent production
of pregnancy, is about 50%. Half of the men who have
been vasectomized develop antibodies against spermatozoa,
and in monkeys, the presence of such antibodies is associated
with a higher incidence of infertility after restoration of the
patency of the vas. However, the anti-sperm antibodies do not
appear to have any other adverse effects.
ENDOCRINE FUNCTION OF THE TESTES
Chemistry & Biosynthesis of Testosterone
Testosterone, the principal hormone of the testes, is a C 19 steroid
with an –OH group in the 17 position (Figure 25–15). It is
synthesized from cholesterol in the Leydig cells and is also
formed from androstenedione secreted by the adrenal cortex.
The biosynthetic pathways in all endocrine organs that form
steroid hormones are similar, the organs differing only in the
enzyme systems they contain. In the Leydig cells, the 11- and
21-hydroxylases found in the adrenal cortex (see Figure 22–7)
are absent, but 17α-hydroxylase is present. Pregnenolone is
therefore hydroxylated in the 17 position and then subjected
to side chain cleavage to form dehydroepiandrosterone. Androstenedione
is also formed via progesterone and 17-hydroxyprogesterone,
but this pathway is less prominent in humans.
Dehydroepiandrosterone and androstenedione are then converted
to testosterone.
The secretion of testosterone is under the control of LH, and
the mechanism by which LH stimulates the Leydig cells involves
Progesterone
17α-Hydroxyprogesterone
Androstenedione
FIGURE 25–15 Biosynthesis of testosterone. The formulas of the precursor steroids are shown in Figure 22–7. Although the main secretory
product of the Leydig cells is testosterone, some of the precursors also enter the circulation.
O
Testosterone
OH