Physiology of Sexual Function

Summary

The parasympathetic nervous system provides the primary pro-erectile innervation of the penis. Originating from the sacral nerve roots (S2–S4) the nerves provide vasodilating innervation to the cavernosal tissue. The parasympathetic nerves release a cocktail of pro-erectile neurotransmitters, of which the most important is nitric oxide (NO), which acts on the smooth muscle cell via a second messenger system involving cyclic GMP. The sympathetic innervation mediates detumescence and originates within the thoraco-lumbar cord (T11-L2) via release of noradrenaline. Our knowledge of the pharmacology of erection has recently provided us with a number of therapeutic approaches to the treatment of erectile dysfunction. The pudendal nerve represents the somatic innervation of the penis carrying both afferent impulses from the genitalia and motor fibres to the muscles of the pelvic floor.

The erection itself is a vascular event during which the degree of erection depends upon the balance between the arterial inflow and venous outflow of the penis. Parasympathetic stimulation (with an accompanying reduction in sympathetic stimulation) results in smooth muscle relaxation in the penile arteries, relaxation of the cavernosal (trabecular) smooth muscle and closure of the venous outflow from the penis.

Ejaculation has two phases, emission and ejection, the latter usually being accompanied by orgasm. Emission involves the sequential contraction of the epididymis, vas deferens, seminal vesicles and prostate, with ejaculatory fluid being “emitted” into the posterior urethra. Ejection is accompanied by tight closure of the bladder neck, with contraction of the prostatic musculature together with a sequence of variably coordinated contractions of the bulbocavernosus, ischiocavernosus and other pelvic floor muscles. The ejaculate is propelled into the anterior urethra and beyond.

The foreskin or the prepuce is a specialized, junctional mucocutaneous tissue that marks the boundary between mucosa and skin. It may have a number of functions including one as a sensory erogenous area and there may be others, but little is actually known.