Pulsatile emission of semen from the penis is generated by the reflex contraction of the bulbospongiosus muscle that forces semen from the base of the penis and out of the external urethral meatus. Bulbospongiosus muscle is innervated by somatic motor fibers carried in the pudendal nerve (S2–S4). Contraction of the internal urethral sphincter and periurethral smooth muscle, innervated by the sympathetic part of the visceral nervous system, prevents retrograde ejaculation into the bladder.