Ganong's Review of Medical Physiology, 23rd Edition

406 SECTION IVEndocrine & Reproductive Physiology

Normally, erection is terminated by sympathetic vasocon-
strictor 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 sympa-
thetic 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 mus-
cle, 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 pros-
tate-specific antigen (PSA). The gene for PSA has two andro-
gen 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 un-
known. 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.

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 pro-

duction 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 ster-

oid 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. An-

drostenedione is also formed via progesterone and 17-hydrox-

yprogesterone, but this pathway is less prominent in humans.

Dehydroepiandrosterone and androstenedione are then con-

verted to testosterone.

The secretion of testosterone is under the control of LH, and

the mechanism by which LH stimulates the Leydig cells involves

FIGURE 25–15 Biosynthesis of testosterone. The formulas of the precursor steroids are shown in Figure 22–7. Although the main secre-
tory product of the Leydig cells is testosterone, some of the precursors also enter the circulation.

Cholesterol

Pregnenolone

17 α-Hydroxypregnenolone

Dehydroepiandrosterone

Progesterone

17 α-Hydroxyprogesterone

Androstenedione

O O

OH

Testosterone

OH

Dihydrotestosterone

H

In some

target tissues

5 α-reductase,

type 1 or type 2