Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT), also known as 'morning wood'. Erection subsides when parasympathetic activity reduces to baseline.Īs an autonomic nervous system response, an erection may result from a variety of stimuli, including sexual stimulation and sexual arousal, and is therefore not entirely under conscious control. The arteries dilate causing the corpora cavernosa of the penis (and to a lesser extent the corpus spongiosum) to fill with blood simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and circulation of this blood.
Physiologically, erection is triggered by the parasympathetic division of the autonomic nervous system, causing the levels of nitric oxide (a vasodilator) to rise in the trabecular arteries and smooth muscle of the penis.
The shape, angle, and direction of an erection varies considerably between humans.
Penile erection is the result of a complex interaction of psychological, neural, vascular, and endocrine factors, and is often associated with sexual arousal or sexual attraction, although erections can also be spontaneous. An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged.