Innervation of Genitalia II and of Perineum
The nerves supplying
the anterior scrotal wall are the ilioinguinal and the external spermatic
branch of the genitofemoral branch of the lumbar nerves. The superficial
perineal branches of the internal pudendal nerve, along with branches from the
posterior cutaneous nerves of the thigh, innervate the posterior scrotal wall.
The unstriated muscle in the dartos fascia is innervated by fine autonomic fibers
that arise from the hypogastric plexus and reach the scrotum along with the
blood vessels. Because of this complex innervation from various sources, the
entire scrotum is difficult to anesthetize with local anesthesia, unlike the
spermatic cord and testicles.
The
nerves supplying the spermatic cord, epididymis, vas deferens, and testis track
along the internal spermatic artery or the vas deferens to reach these organs
(see Plate 2-10). Three nerves converge in the spermatic cord and innervate
these organs: First, the superior spermatic nerve that penetrates to the
interior of the testicle and supplies it and associated structures. It
accompanies the internal spermatic artery, originating from the tenth thoracic
cord level, and passes through the preaortic and renal plexuses. Second, the
middle spermatic nerve takes origin from the superior hypogastric plexus and
joins the vas deferens at the internal inguinal ring and supplies mainly the
vas deferens and epididymis. Third, the inferior spermatic nerve, derived from
the inferior hypogastric nerve plexus and also coursing with the vas deferens,
also supplies the vas deferens and epididymis.
In the
lower figure, note that perineal nerve and dorsal nerve of the penis, both
derived from the pudendal nerve (upper figure), course medial to the
ischial tuberosity on each side of the perineum. Coursing parallel to these
nerves are the perineal artery and the artery of the penis, both derived from
the internal pudendal artery (see Plate 2-7). Recent research indicates that
the pressure on the male perineum when sitting on a standard bicycle saddle is
sevenfold higher than that observed sitting in a chair. It is thought that this
increased pressure compresses either the perineal and dorsal nerves or the
perineal and dorsal arteries, leading to perineal numbness and erectile
dysfunction. A spinal cord reflex termed the “bulbocavernosus reflex” (more
appropriately, the bulbospongiosus reflex) refers to anal sphincter contraction
in response to squeezing the glans penis. This reflex is mediated through the
dorsal nerve of the penis (afferent) via the pudendal nerve to the inferior
hemorrhoidal nerves (efferent) and tests the integrity of spinal cord levels
S2-S4. In cases of spinal cord injury, the absence of this reflex documents
continuation of spinal shock or spinal injury at the level of the reflex arc
itself (cauda equina injury).
Likewise, the return of the bulbospongiosus reflex signals the termination of
spinal shock.
“Referred
pain” to and from the scrotal region is of considerable clinical interest. In
general, stimulation of the testis, epididymis, and tunica vaginalis can cause
pain locally and also pain that projects to the lower abdomen, above the
internal inguinal ring. Scrotal pain is likely perceived by the genital
(external spermatic) branch of the genitofemoral nerve. Pain in the testis proper is referred to
its point of origin in the retroperitoneum by referral through the superior
spermatic nerve. Pain associated with renal stones may be perceived as arising from
the testicle because both the testicle and kidney, including the renal pelvis,
receive autonomic fibers from the same preaortic autonomic plexus near the renal
arteries. Another source of this pain is radiating pain due to irritation of
the genitofemoral nerve often adjacent to the upper ureter.
