There are three types of erection. The first is psychogenic, these occur as a result of visual or auditory stimuli, or as a result of fantasy. Reflexogenic erections occur as a result of tactile stimulation of the penis, and are important in maintaining the erection during sexual activity. Finally, nocturnal erections occur during REM sleep. Although the exact mechanism is unknown, it is thought that low androgens levels is one of the components.
The physiological mechanism by which an erection occurs begins with an increase in blood flow to the penis, filling the sinusoids of the corpora cavernosum. The cavernosal smooth muscle relaxes to allow for expansion to facilitate the extra blood volume. This expansion presses on the venous plexus, which creates an outflow obstruction, thereby increasing the pressure within the penis and aiding rigidity. The relaxation of the smooth muscle of the penis relies on the parasympathetic nervous system.
The sympathetic nervous system controls ejaculation, and also causes constriction of smooth muscle. This constricts the blood containing lacunar spaces of the penis and empties them of blood, therefore aiding detumescence (subsiding from sexual arousal).
The maintenance of an erection and the tone of the cavernosal smooth muscle is determined by an integrated response to neural stimulation and paracrine or autocrine systems.