Innervation of Genitalia I
Genitourinary organs receive a blend of
autonomic and somatic nervous innervation. Autonomic nerves provide afferent
and efferent innervation to organs, blood vessels, and smooth muscle and are
characterized by the presence of peripheral synapses. Somatic nerves supply
afferent and efferent innervation to skeletal muscle. Although these two nerve
types leave the spinal cord within shared nerves, their course and function
diverge widely.
The
autonomic system is further divided into sympathetic and parasympathetic fibers.
Sympathetic preganglionic fibers are found in the thoracic and lumbar spine, and
parasympathetic preganglionic fibers originate in the cranial and sacral spinal
cord. The pelvic organs receive a blend of these two autonomic nerve types
through several pelvic ganglia. This autonomic innervation is demonstrated
diagrammatically here, with a complete description of the anatomic and functional
connections found elsewhere in this Collection. The parasympathetic fibers leave
the spinal cord with the anterior spinal nerve roots from the sacral cord
segments S2 through S4. After passing through the sacral foramen, they (nervi
erigentes) enter the pelvic nerve plexus (inferior hypogastric) and follow
blood vessels to visceral organs, including the descending and sigmoid colon,
rectum, bladder, penis, and external genitalia (see table).
The
sympathetic fibers are derived from the thoracolumbar spinal cord segments
(T10-L2). They descend through the preaortic plexus and abdominal chains to the
presacral area and form a distinct midline nerve plexus usually located below
the aortic bifurcation called the superior hypogastric plexus. Below this
point, various ramifications of these nerves form the inferior hypogastric nerve
plexus and branches from these two plexuses pass on to the pelvic organs. These
adrenergic nerves terminate as postganglionic fibers and innervate the bladder
neck, prostate, vasa deferentia, and seminal vesicles. They are primarily responsible
for seminal emission (see table). Resection of this plexus or division of the
abdominal sympathetic chain generally results in smooth muscle paralysis in
these organs that is clinically manifest as either retrograde ejaculation or
complete anejaculation, depending on the degree of injury.
The nerve
supply of the penis is derived from the somatic pudendal nerve (S2-S4) and from
the pelvic autonomic plexus. The pudendal nerve traverses the pelvis adjacent
to the internal pudendal artery (see Plate 2-6) and is distributed to the same
organs as the vessel supplies. The perineal branch supplies somatic motor
function to the bulbospongiosus and ischiocavernosus muscles and also to the
muscles of the urogenital diaphragm, including the sphincter urethrae (external
sphincter). These muscles are important for somatic nervous system control of
expulsion of the ejaculate that occurs with ejaculation. Sensory branches of
this nerve are distributed to the skin of the penis (dorsal nerve of the
penis), perineum, and posterior scrotum (see Plate 2-11).
Nerves
emanating from the pelvic autonomic plex- uses also distribute to the penis and
through the cavernous nerve innervate the smooth muscle of the paired cavernous
spaces and the central cavernous artery and arterioles within the penis. Thus, through the
parasympathetic nervous system, they control cavernous vascular filling and
erections (see table).
Somatic
nerves to the pelvic organs are derived from the lumbosacral plexus containing
input from T12 to S4. They are demonstrated on the left side of the diagram.
The iliohypogastric and ilioinguinal nerves are both derived from L1 and supply
multiple motor branches to the abdominal wall muscles and sensory innervation to the
skin of the lower abdomen and genitalia. The lateral femoral cutaneous nerve
and the genitofemoral nerve are derived from L1-L3 and provide sensory input to
the skin of the upper thigh and lateral genitalia. The genital branch of the
genitofemoral nerve supplies the cremaster and dartos layers of the scrotum and
is responsible for the cremasteric reflex that can be compromised with swelling
of the spermatic cord
as a consequence of testis torsion.
