Human Physiology, 14th edition (2016)

720 Chapter 20

that results causes the smooth muscles of the corpora cavernosa

to relax and small blood vessels to dilate, so that the corpora cav-

ernosa fill with blood. As the pressure in the corpora cavernosa

rises and the penis becomes turgid, the venous outflow of blood

is occluded, helping to sustain the erection.

The term emission refers to the movement of semen into

the urethra, and ejaculation refers to the forcible expulsion of

semen from the urethra out of the penis. Emission and ejacula-

tion are stimulated by sympathetic nerves, which cause peri-

staltic contractions of the tubular system, contractions of the

seminal vesicles and prostate, and contractions of muscles at

the base of the penis. Sexual function in the male thus requires

the synergistic action (rather than antagonistic action) of the

parasympathetic and sympathetic systems.

structures—the corpora cavernosa —located on the dorsal side
of the penis, and one unpaired corpus spongiosum on the ven-
tral side ( fig.  20.20 ). The urethra runs through the center of
the corpus spongiosum. The erectile tissue forms columns that
extend the length of the penis, although the corpora cavernosa
do not extend all the way to the tip.
Erection is produced by nitric oxide (NO), mostly released
by parasympathetic axon terminals to the corpora cavernosa.
Additional NO is released by the endothelium of the corpora cav-
ernosa and the penile arteries. The NO enters smooth muscle cells
and binds to heme iron (Fe^2 1 ) in a subunit of the enzyme guanyl-
ate cyclase, activating this enzyme. Guanylate cyclase converts
GTP into cyclic GMP (cGMP), which closes Ca^2 1 channels in
the plasma membrane ( fig. 20.21 ). The fall in intracellular Ca^2 1

Figure 20.20 The structure of the penis. The attachment, blood and nerve supply, and arrangement of the erectile tissue
are shown in both longitudinal and cross section.

Ureter

Ductus

(vas) deferens

Urinary bladder

Ampulla

Seminal vesicle

Prostate

Spermatic cord

Cremasteric fascia

Epididymis

Scrotum

Symphysis pubis

Suspensory ligament

of penis

Body of penis

Dorsal vein of penis

Dorsal artery of penis

Corona glandis

Glans penis

Prepuce

Dorsal vein of penis

Deep dorsal vein of penis

Dorsal nerve of penis

Dorsal artery of penis

Deep artery of penis

Corpora cavernosa

Urethra

Corpus spongiosum

Skin