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.