844
UNIT SIX
tions in smooth muscles in the walls of the testicular ducts,
epididymides, ductus deferentia, and ejaculatory ducts.
Other sympathetic impulses stimulate rhythmic contractions
of the seminal vesicles and prostate gland.
As the urethra F lls with semen, sensory impulses are
stimulated and pass into the sacral part of the spinal cord.
In response, motor impulses are transmitted from the spinal
cord to certain skeletal muscles at the base of the erectile
columns of the penis, rhythmically contracting them. This
increases the pressure in the erectile tissues and aids in forc-
ing the semen through the urethra to the outside—a process
called
ejaculation
(e-jak
u-la
shun).
The sequence of events during emission and ejaculation
is coordinated so that the fl
uid from the bulbourethral glands
is expelled F
rst. This is followed by the release of fl
uid from
the prostate gland, the passage of the sperm cells, and F nally,
the ejection of fl
uid from the seminal vesicles
(f g. 22.17)
.
Immediately after ejaculation, sympathetic impulses con-
strict the arteries that supply the erectile tissue, reducing the
infl ow of blood. Smooth muscles in the walls of the vascular
PRACTICE
20
Describe the structure of the penis.
21
What is circumcision?
22
How is the penis attached to the perineum?
Erection, Orgasm, and Ejaculation
During sexual stimulation, parasympathetic nerve impulses
from the sacral portion of the spinal cord release the vasodi-
lator nitric oxide, which dilates the arteries leading into the
penis, increasing blood fl
ow into erectile tissues. At the same
time, the increasing pressure of arterial blood entering the
vascular spaces of the erectile tissue compresses the veins
of the penis, reducing fl ow of venous blood away from the
penis. Consequently, blood accumulates in the erectile tis-
sues, and the penis swells and elongates, producing an
erec-
tion
(f g. 22.16)
.
In erectile dysfunction (impotence), the penis cannot become erect or
sustain an erection. Causes of erectile dysfunction include underlying
disease such as diabetes mellitus; paralysis; treatments such as pros-
tate surgery or certain drugs; and excess smoking or drinking alcohol.
Development of drugs to treat erectile dysfunction grew out of under-
standing the physiology of erection. The F
rst drug, Viagra (sildenaF
l),
blocks the enzyme that breaks down cyclic monophosphate, which is
necessary for an erection to persist.
The culmination of sexual stimulation is
orgasm
(or
gazm), a pleasurable feeling of physiological and psycho-
logical release. Orgasm in the male is accompanied by emis-
sion and ejaculation.
Emission
(e-mish
un) is the movement of sperm cells
from the testes and secretions from the prostate gland and
seminal vesicles into the urethra, where they mix to form
semen. Emission is a response to sympathetic nerve impulses
from the spinal cord, which stimulate peristaltic contrac-
(a)
(b)
Skin
Subcutaneous
tissue
Connective tissue
(fascia)
External urethral orifice
Superficial dorsal vein
Deep dorsal vein
Dorsal nerve
Dorsal artery
Deep artery
Corpora cavernosa
Tunica albuginea
Urethra
Corpus spongiosum
Prepuce
Glans penis
FIGURE 22.15
Structure of the penis. (
a
) Interior and (
b
) cross section of the penis.
Sexual stimulation
Blood accumulates in the vascular
spaces within erectile tissues of penis
Penis swells and becomes erect
Veins are
compressed,
reducing blood
flow away from
penis
Parasympathetic
neurons release
nitric oxide, causing
dilation of small
arteries to penis
FIGURE 22.16
Mechanism of penile erection in the male.
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