Neuroanatomy of penile erection: its relevance to iatrogenic impotence

The neuroanatomy of erection in men is not well defined. Recently, we isolated successfully the cavernous nerves for acute and chronic neurostimulation to induce penile erection in dogs and monkeys. We then investigated the anatomy of these nerves in humans by cadaveric dissection and serial histologic sectioning. Our experience in tracing the spinal nuclei responsible for vesical and urethral function by transportation of horseradish peroxidase enabled us to explore the location and organization of the spinal center for erection. Thus, systemic knowledge of the neuroanatomy of erection was accumulated. The spinal nuclei for control of erection are located in the intermediolateral gray matter at the S1 to S3 and T12 to L3 levels in dogs, and the S2 to S4 and T10 to L2 levels in humans. From these sacral nuclei axons issue ventrally and join the axons of the nuclei for the bladder and rectum to form the sacral visceral efferent fibers. These fibers emerge from the anterior root of S2 to S4, and join the sympathetic fibers to form the pelvic plexus, which then branches out to innervate the bladder, rectum and penis. The fibers innervating the penis (cavernous nerves) travel along the posterolateral aspect of the seminal vesicle and prostate, and then accompany the membranous urethra through the genitourinary diaphragm. These fibers are located on the lateral aspect of the membranous urethra and ascend gradually to the 1 and 11 o'clock positions in the proximal bulbous urethra. Some of the fibers penetrate the tunica albuginea of the corpus spongiosum, while others spread to the trifurcation of the terminal internal pudendal artery and innervate the dorsal, deep and urethral arteries. Shortly before the 2 corpora cavernosa merge the cavernous nerves penetrate the tunica albuginea along with the deep artery and cavernous vein. The terminal branches of these nerves innervate the helicine arteries and the erectile tissue within the corpora cavernosa. Because of the intimate relationship of the cavernous nerves to the rectum, prostate and urethra, they can be damaged easily during urological and pelvic procedures. This systemic knowledge of the human cavernous nerves from the spinal center to the erectile tissue should permit a better understanding of erection and impotence. Furthermore, with the aid of intraoperative neurostimulation, the cavernous nerves may be identified and preserved, thereby preventing iatrogenic impotence.