Human Physiology, 14th edition (2016)

Reproduction 721

Erection is controlled by two portions of the central ner-

vous system—the hypothalamus in the brain and the sacral por-

tion of the spinal cord. Conscious sexual thoughts originating

in the cerebral cortex act via the hypothalamus to control the

sacral region, which in turn increases parasympathetic nerve

activity to promote vasodilation and erection of the penis. Con-

scious thought is not required for erection, however, because

sensory stimulation of the penis can more directly activate the

sacral region of the spinal cord and cause an erection.

Male Fertility

The approximate volume of semen for each ejaculation is 1.5

to 5.0 milliliters. The bulk of this fluid (45% to 80%) is pro-

duced by the seminal vesicles and 15% to 30% is contributed

by the prostate. There are usually between 60 and 150 million

Figure 20.21 Role of nitric oxide in erection of the penis. Nitric oxide is released by parasympathetic axons in the penis,
and is also produced as a paracrine regulator by endothelial cells in blood vessels of the penis. (1) Nitric oxide activates guanylate
cyclase, causing the production of cyclic GMP (cGMP). (2) cGMP causes Ca^2 1 channels to close in the plasma membrane of the
vascular smooth muscle cells, decreasing the cytoplasmic Ca^2 1 concentration. (3) This produces relaxation of the smooth muscle cells,
causing vasodilation in the penis and engorgement of the erectile tissue. Viagra and related drugs inhibit the phosphodiesterase that
catalyzes the breakdown of cGMP, thereby increasing the availability of cGMP to promote an erection.

Parasympathetic axon

Nitric

oxide

Activates

guanylate

cyclase

Inhibits

Viagra

PDE

5' GMP

Ca2+

channels

close

cGMP

GTP

Decreased

cytoplasmic Ca2+

Ca2+

NO

Parasympathetic axon

ACh

Vascular

endothelial

cell

L-Arginine

O 2

Vascular

smooth muscle

cell

Smooth muscle

relaxes

Engorgement

of erectile

tissue

Erection

Stimulates

eNOS

1

3

2

CLINICAL APPLICATION

Erectile dysfunction is the inability to produce or sustain an

erection sufficient for sexual intercourse. This may be due to

physical problems that include heart disease, enlarged pros-

tate, atherosclerosis, and metabolic syndrome, or to such

psychological problems as depression and stress. Erection

requires nitric oxide (NO), which activates guanylate cyclase

in smooth muscle cells to raise cGMP as previously discussed

( fig.  20.21 ). Sildenafil ( Viagra ) and Tadalafil ( Cialis ) inhibit

the enzyme phosphodiesterase ( PDE ), which breaks down

cGMP. Specifically, they block the PDE-5 isoenzyme that pre-

dominates in the penis, raising the cGMP to promote erection

( fig. 20.21 ). These drugs have minimal effect on systemic blood

pressure, but vasodilation elsewhere in the body can produce

such side effects as headache, flushing, and nasal congestion.