The deep drainage system consists of the cavernosal and crural veins

AN ATLAS OF ERECTILE DYSFUNCTION 5

complex at the urethroprostatic junction. The deep drainage system consists
of the cavernosal and crural veins. Emissaryveins in the proximal third of
the penis join to form one or two cavernosal veins which pass between the bulb
and crus of thepenis to drain into the internal pudendal vein (Figure
11).

LYMPHATIC DRAINAGE

Lymph is drained from the penis by lymphatics which pass to the superficial
and deep inguinal lymph nodes of the femoraltriangle (Figure 12). In turn,
these nodes, which may become secondarily involved in patients who have
carcinoma of thepenis (Figure 13), drain to the external and internal iliac
lymphatic chains. Conditions that obstruct these lymphatic channels,such as
metastatic prostate cancer, may result in gross penile and scrotal edema.

NEUROANATOMY

Three sets of peripheral nerves are involved in penile erection and
subsequent detumescence: parasympathetic nerves from thesecond to fourth
sacral (S2-S4) segments, sympathetic nerves from the tenth thoracic to the
second lumbar (T10-L2)thoracolumbar outflow, and somatic fibers via the
pudendal nerves (Figure 14).

The sympathetic nerves reach the corpora, as well as the prostate and bladder
neck, via the hypogastric nerves, where theyare susceptible to injury in
retroperitoneal lymph node dissection performed for the treatment of metastatic
testicular cancer.Postganglionic noradrenergic fibers pass posterolateral to
the prostate in the so-called nerves of Walsh to enter the corporacavernosa
medially.

Parasympathetic nerves stem from the so-called sacral erection center and
their cell bodies lie in the intermediolateral nucleifrom S2 to S4. Exiting
through the sacral foramina, these nerves pass forward lateral to the rectum as
the nervi erigentes toreach the pelvic plexus. In this location,
preganglionic fibers relay in ganglia, and postganglionic non-adrenergic,
non-cholinergic (NANC) fibers pass in the cavernous nerves to the corpora
cavernosa. These nerves are vulnerable during proceduressuch as
abdominoperineal resection of the rectum and radical prostatectomy (Figure
15).

The pudendal nerves comprise motor efferent and sensory afferent fibers which
innervate the ischiocavernosus andbulbocavernosus muscles as well as the
penile and perineal skin. Pudendal motor neuron cell bodies are located in
Onuf'snucleus of the S2-S4 segments. The pudendal nerve enters the perineum
through the lesser sciatic notch at the posterior borderof the ischiorectal
fossa and runs in Alcock's canal towards the posterior aspect of the perineal
membrane. At this point, it givesoff the perineal nerve with branches to the
scrotum and the rectal nerve supplying the inferior rectal region.

The dorsal nerve of the penis emerges as the last branch of the pudendal
nerve. It then runs distally along the dorsal penileshaft lateral to the
dorsal artery. Multiple fascicles fan out distally, supplying proprioceptive and
sensory nerve terminals tothe dorsum of the tunica albuginea and skin of the
penile shaft and glans penis.

CENTRAL NERVOUS SYSTEM CONNECTIONS

Although reflex spinal erections may occur provided that the sacral reflexes
are intact (for example, after cervical or thoracicspinal injury), central
connections are paramount in engendering the normal male sexual response. These
central pathways,however, are as yet incompletely understood.

A number of areas in the brain are involved in the modulation of erection,
including the thalamic nuclei, rhinencephalon,limbic structures and
paraventricular nucleus. Messages are integrated in the medial preoptic area
where dopaminergicneurons are important. Norepinephrine (noradrenaline) and
serotonin have also been identified as neurotransmitters in thisregion.
Efferent pathways enter the medial forebrain bundle and pass caudally into the
mid-brain tegmental region near thelateral part of the substantia nigra.
Caudal to the mid-brain, the efferent pathway travels in the ventrolateral part
of the ponsand medulla, passing down to the sacral spinal centers via the
lateral funiculus of the spinal cord. Activation of theparasympathetic
neurons, located in the spinal cord, leads to intrapenile release of nitric
oxide, mainly by neural terminations.

Superimposed on this hypothalamo-spinal circuit are higher centers, including
the gyrus rectus, cingulate gyrus andhippocampus; these areas are all
capable of modifying the erectile response, although their exact function has
not yet beenelucidated. Diseases specifically affecting these structures
include Parkinson's disease, multiple system atrophy and stroke,all of which
are often associated with erectile dysfunction.