Physiology of male sexual function.

The male sexual response cycle consists of excitement, plateau, orgasm, and resolution. The initial event, penile erection, is produced by arteriolar dilatation and increased blood flow to the erectile tissue of the penis. Erection is a reflex response initiated by visual, olfactory, or imaginative stimuli impinging upon supraspinal centers or by genital stimulation that in turn activates spinal reflex mechanisms. Sacral parasympathetic and thoracolumbar sympathetic nerves provide the efferent vasodilator input to the penis. Parasympathetic nerves also stimulate secretion from the seminal vesicles and prostate and Cowper's glands during the plateau phase. The orgasmic phase is characterized by seminal emission and ejaculation and the accompanying sensations. Emission of semen into the urethra depends on sympathetic nerves that elicit contractions of smooth muscles in the vas deferens, seminal vesicles, and prostate. Rhythmic contractions of striated muscle (bulbocavernosus and ischiocavernosus) generated by efferent pathways in the pudendal nerve eject semen from the urethra.