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.