Although erection of the penis is a vascular event generated by parasympathetic nerves from spinal levels S2–S4, the formation of semen in the urethra is caused by the contraction of smooth muscle of the ducts and glands of the reproductive system that is innervated by the sympathetic part of the visceral nervous system. Ejaculation of semen from the penis is through the action of skeletal muscles innervated by somatic motor nerves.