[5] Parasympathetic branches extend from the sacral plexus of the spinal nerves into the arteries supplying the erectile tissue; upon stimulation, these nerve branches release acetylcholine, which in turn causes release of nitric oxide from endothelial cells in the trabecular arteries, that eventually causes tumescence.
Bancroft (2005) hypothesizes that the noradrenergic neurons of the locus ceruleus in the brain are perpetually inhibitory to penile erection, and that the cessation of their discharge that occurs during rapid eye movement sleep may allow testosterone-related excitatory actions to manifest as nocturnal penile tumescence.
[6] Suh et al. (2003) recognizes that in particular the spinal regulation of the cervical cord is critical for nocturnal erectile activity.
In another study of healthy older people published in 1988, frequency and duration of nocturnal penile tumescence decreased progressively with age independent of variations in sleep.
[13] SRPE is a rare parasomnia consisting of nocturnal penile tumescence accompanied by pain that awakens the individual.
Much rarer priapism is secondary to blunt trauma to the perineum or penis, with laceration of the cavernous artery, which can generate an arterial-lacunar fistula resulting in a high blood flow state, hence the tumescence.
[14] The existence and predictability of nocturnal tumescence is used by sexual health practitioners to ascertain whether a given case of erectile dysfunction is psychological or physiological in origin.
[3] Regularly, those who experience erectile dysfunction are given a nocturnal penile tumescence test, usually over a three-day period.