Erection – the enlarged and hardened PENIS. Erections begin in the male fetus before birth and continue throughout life. An erection is necessary for SEXUAL INTERCOURSE and in an adult man usually occurs as a result of sexual arousal. Erections also occur spontaneously during sleep, usually during rapid eye movement (REM) sleep. A man typically has up to a half dozen nocturnal erections each night. Doctors do not know what causes nocturnal erections though believe they are a consequence of electrical activity in the BRAIN, not of sexual stimulation.
An erection during sexual arousal occurs as a result of interactions between the thoughts and physical stimulation of the penis. The sequence of interactions triggers the release of nitric acid, acetylcholine, EPINEPHRINE, and other neurotransmitters (biochemicals that conduct NERVE impulses). These substances cause the smooth MUSCLE tissue supporting the corpora cavernosa, the two tubelike channels along the top of the penis, to relax. BLOOD flows into the corpora cavernosa. Some of the layers of tissue within the penis contract to help hold the blood within the corpora cavernosa, and valves in the veins that normally carry blood from the penis close. These events cause the size of the penis to increase in length and circumference.
An erection may come to fullness within a few seconds to several minutes, depending on numerous factors such as the man’s age and lasts generally until EJACULATION. Some men can sustain an extended erection during sexual activity. Nearly as quickly as the tissues of the penis release nitric acid, they also release the enzyme phosphodiesterase (PDE), which counters the actions of nitric acid and other neurotransmitters to allow blood to flow out of the penis. This action gains momentum after ejaculation or other culmination of sexual activity, serving to reverse the actions of these neurotransmitters. The penis again goes limp (becomes flaccid).