Innervation of the Penis

The autonomic spinal erection center is located in the intermediolateral nuclei of the spinal cord at levels S₂-S₄ and T₁₂-L₂. Nerve fibers from the thoracolumbar (sympathetic) and sacral (parasympathetic) spinal segments join to form inferior hypogastric and pelvic plexuses, which sends branches to the pelvic organs. The fibers innervating the penis (the cavernous nerves) travel along the posterolateral aspect of the seminal vesicles and prostate and then accompany the membranous urethra through the genitourinary diaphragm (Walsh and Donker, 1982;

Figure 37-1). Some of these fibers enter the corpora cavernosa and corpus spongiosum with the cavernous and bulbourethral arteries. Others travel further distally with the dorsal nerve and enter the corpus cavernosum and corpus spongiosum in various locations to supply the mid- and distal portions of the penis. The terminal branches of the cavernous nerves innervate the helicine arteries and the trabecular smooth muscle and are responsible for the vascular events during tumescence and detumescence (_{Paick, Donatucci, and Lue, 1993}).

The center for somatic motor nerves is located at the ventral horn of the S₂-S₄ segment. The motor fibers join the pudendal nerve to innervate the bulbocavernosus and ischiocavernosus muscles. The somatic sensory nerves originate at the receptors in the penis to transmit pain, temperature, touch, and vibratory sensations. The pathway and center for erotic feeling from genital stimulation has not been elucidated. The brain has a modulatory effect on the spinal pathways of erection. Various supraspinal areas that have a role in erectile function include the hypothalamus and limbic system, ventral thalamus, tegmentum of mid brain and lateral substantia nigra, and ventrolateral pons and medulla. Specifically, the medial preoptic area (MPOA) and the paraventricular nucleus of the hypothalamus, the periaqueductal gray of the mid brain, and the nucleus paragigantocellularis of the medulla are the centers intimately involved in the control of penile erection (_{McKenna, 1998}).

Three types of erections are noted in humans: genital-stimulated (contact or reflexogenic), central-stimulated (noncontact or psychogenic), and central-originated (nocturnal). Genital-stimulated erection is induced by tactile stimulation of the genital area. This kind of erection can be preserved in upper spinal cord lesions, although erections are usually short in duration and poorly controlled by the individual. Central-stimulated erection is more complex, resulting from memory, fantasy, or visual or auditory stimuli. The central-originated erection can occur spontaneously without stimulation or during sleep. Most of the sleep erections occur during rapid eye movement (REM) sleep. The mechanism that triggers REM sleep is located in the pontine reticular formation. During REM sleep, the cholinergic neurons in the lateral pontine tegmentum are activated while the adrenergic neurons in the locus ceruleus and the serotonergic neurons in the mid brain raphe are silent. This differential activation may be responsible for the nocturnal erections during REM sleep. The number and duration of erections are markedly reduced in hypogonadism and in men receiving antiandrogen therapy.