There are three mechanisms of erection in humans genital stimulation (contact or reflex), central stimulation (non-contact or psychogenic) and central origin (nocturnal). Genital-stimulated erections are induced by tactile stimulation of the genital area; they are shorter in duration and less subjectively controlled, but can be preserved in higher spinal cord lesions. Centrally stimulated erections are more complex and are induced by memory, fantasy, or audiovisual stimulation. Central origin erections can occur spontaneously in the absence of stimulation or during sleep. Most sleep erections occur during rapid eye movement (REM) sleep. The mechanism that triggers REM sleep is located in the pontine reticular formation. The number and duration of nocturnal erections are significantly reduced in individuals with hypogonadism or on anti-androgen therapy. Activation of the autonomic nerve leads to full erection, i.e., infusion and storage of blood into the corpus cavernosum. After full erection, the sciatic cavernous muscle contracts (somatic nerve activation) and squeezes the proximal penile corpus cavernosum, causing intracavernosal pressure to exceed systolic blood pressure and tonic erection to occur in the penis. The tonic erection phase occurs naturally during masturbation or sexual intercourse, but can also occur with mild penile curvature that does not require muscle contraction. The process of penile erection can be divided into six periods, as detailed in the table below. The hemodynamic mechanism of the penile head differs from that of the penile corpus cavernosum in that it is not surrounded by a white membrane and thus functions as an arteriovenous fistula during the fully erect phase. However, during the tonic erection phase, most of the venous channels are temporarily extruded and the head of the penis is further enlarged. Staging of the penile erectile process Weak phase Only a small amount of arterial and venous blood flow; blood gas values are equivalent to venous blood gas values. Blood flow rate: 2.5-8 ml/100g/min. Pre-filling (perfusion) phase The blood flow in the internal pubic artery increases in both systole and diastole. Decrease in intrapubic artery pressure; no change in intracavernosal pressure. Penile length increases. Filling phase Intracavernosal pressure increases until full erection is achieved. The penis is thicker, longer, and pulsating. Along with the increase in intracavernous pressure, the blood flow rate decreases, and when the intracavernous pressure reaches diastolic levels, blood flows in only during the systolic phase. During full erection intracavernosal pressure rises to 80-90% of systolic pressure. The intrapubic arterial pressure increases, but is slightly lower than the body circulation pressure. Arterial blood flow is significantly less than during the perfusion phase, but still more than during the weak phase. Venous blood flow is still more than in the weak phase, despite the fact that most of the venous channels are compressed. Blood gas values approach those of arterial blood. Bony or tonic erection phase Due to contraction of the sciatic cavernous muscles, intracavernosal pressure rises above the level of systolic blood pressure, resulting in tonic erection. There is little blood flow through the cavernous arteries during this period; however, because of the short duration, tissue ischemia and injury do not occur. The waning phase After ejaculation or termination of sexual stimulation, the sympathetic nerves resume releasing transmitters that cause smooth muscle contraction in the cavernous sinuses and small arteries. The contraction of the smooth muscle reduces the arterial blood flow to the level of the weak phase, draining a large amount of blood from the cavernous sinuses and reopening the venous channels. The penis regains its original length and circumference in weakness.