Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 25The Gonads: Development & Function of the Reproductive System 405

Effect of Temperature

Spermatogenesis requires a temperature considerably lower
than that of the interior of the body. The testes are normally
maintained at a temperature of about 32 °C. They are kept cool
by air circulating around the scrotum and probably by heat ex-
change in a countercurrent fashion between the spermatic ar-
teries and veins. When the testes are retained in the abdomen
or when, in experimental animals, they are held close to the
body by tight cloth binders, degeneration of the tubular walls
and sterility result. Hot baths (43–45 °C for 30 min/d) and in-
sulated athletic supporters reduce the sperm count in humans,
in some cases by 90%. However, the reductions produced in
this manner are not consistent enough to make the procedures
reliable forms of male contraception. In addition, evidence
suggests a seasonal effect in men, with sperm counts being
greater in the winter regardless of the temperature to which
the scrotum is exposed.

Semen

The fluid that is ejaculated at the time of orgasm, the semen,
contains sperms and the secretions of the seminal vesicles,
prostate, Cowper’s glands, and, probably, the urethral glands
(Table 25–4). An average volume per ejaculate is 2.5 to 3.5 mL
after several days of abstinence. The volume of semen and the
sperm count decrease rapidly with repeated ejaculation. Even
though it takes only one sperm to fertilize the ovum, each mil-
liliter of semen normally contains about 100 million sperms.
Fifty percent of men with counts of 20 to 40 million/mL and
essentially all of those with counts under 20 million/mL are
sterile. The presence of many morphologically abnormal or
immotile spermatozoa also correlates with infertility. The
prostaglandins in semen, which actually come from the sem-
inal vesicles, are in high concentration, but the function of
these fatty acid derivatives in semen is unknown.
Human sperms move at a speed of about 3 mm/min
through the female genital tract. Sperms reach the uterine
tubes 30 to 60 min after copulation. In some species, contrac-
tions of the female organs facilitate the transport of the
sperms to the uterine tubes, but it is unknown if such contrac-
tions are important in humans.

Erection

Erection is initiated by dilation of the arterioles of the penis.
As the erectile tissue of the penis fills with blood, the veins are
compressed, blocking outflow and adding to the turgor of the
organ. The integrating centers in the lumbar segments of the
spinal cord are activated by impulses in afferents from the
genitalia and descending tracts that mediate erection in re-
sponse to erotic psychologic stimuli. The efferent parasympa-
thetic fibers are in the pelvic splanchnic nerves (nervi
erigentes). The fibers presumably release acetylcholine and
the vasodilator vasoactive intestinal polypeptide (VIP) as
cotransmitters (see Chapter 7).

Nonadrenergic noncholinergic fibers are also present in the

nervi erigentes, and these contain large amounts of NO syn-

thase, the enzyme that catalyzes the formation of nitric oxide

(NO; see Chapter 33). NO activates guanylyl cyclase, resulting

in increased production of cyclic GMP (cGMP), and cGMP is

a potent vasodilator. Injection of inhibitors of NO synthase

prevents the erection normally produced by stimulation of the

pelvic nerve in experimental animals. Thus, it seems clear that

NO plays a prominent role in the production of erection. The

drugs sildenafil, tadalafil, and vardenafil all inhibit the break-

down of cGMP by phosphodiesterases and have gained world-

wide fame for the treatment of impotence. The multiple

phosphodiesterases (PDEs) in the body have been divided into

seven isoenzyme families, and these drugs are all most active

against PDE V, the type of phosphodiesterase found in the cor-

pora cavernosa. It is worth noting, however, that these drugs

can also produces significant inhibition of PDE VI (and others,

if taken at high doses). Phosphodiesterase VI is found in the

retina, and one of the side effects of these drugs is a transient

loss of the ability to discriminate between blue and green (see

Chapter 12).

TABLE 25–4 Composition of human semen.

Color: White, opalescent

Specific gravity: 1.028

pH: 7.35–7.50

Sperm count: Average about 100 million/mL, with fewer than 20%

abnormal forms

Other components:

From seminal vesicles

(contributes 60% of

total volume)

Fructose (1.5-6.5 mg/mL)

Phosphorylcholine

Ergothioneine

Ascorbic acid

Flavins

Prostaglandins

Spermine

From prostate (contributes

20% of total volume)

Citric acid

Cholesterol, phospholipids

Fibrinolysin, fibrinogenase

Zinc

Acid phosphatase

Phosphate

Buffers

Bicarbonate

Hyaluronidase

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