In the absence of sexual stimulation, the vascular sinusoids in the erectile tissue of the penis contain a small amount of blood and the penis is flaccid (flak’-sid), or soft. Sexual stimulation initiates parasympathetic nerve impulses that cause the dilation of the arterioles and constriction of the venules supplying the erectile tissue. These vascular changes cause the erectile tissue to become engorged with blood, which produces erection, a condition in which the penis swells, lengthens, and becomes erect. The parasympathetic nerve impulses also stimulate the secretion from the bulbo-urethral glands.
Continued sexual stimulation of the glans, as in sexual intercourse, culminates in orgasm, which is characterized by ejaculation (e-jak-u-la’-shun) of semen and a feeling of intense pleasure. Just prior to ejaculation, sympathetic nerve impulses stimulate peristaltic contractions of the epididymides, vasa deferentia, and ejaculatory ducts along with contractions of the seminal vesicles and prostate gland. These contractions force semen into the urethra. Then, ejaculation occurs as certain skeletal muscles contract rhythmically from the proximal penis distally, forcing semen through the urethra to the external environment. A feeling of general relaxation follows.
Immediately after ejaculation, the vascular changes that produced erection are reversed. Sympathetic nerve impulses cause the constriction of the arterioles and dilation of the venules supplying the erectile tissue, allowing the accumulated blood to leave the penis. The penis becomes flaccid as blood is carried away. After orgasm, erection is not possible for a time period that varies from less than an hour to several hours.