hard-on

An uncircumcised penis semi-erect with foreskin unretracted (left) and erect with foreskin retracted (right)

Penile erection is a physiological phenomenon where the penis becomes enlarged and firm. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal. Penile erection can also occur due to a full urinary bladder. In some males, erection can occur spontaneously at any time of day, and is known as nocturnal penile tumescence when occurring during REM sleep.

Physiology

Penile erection occurs when two tubular structures that run the length of the penis, the corpora cavernosa, become engorged with venous blood. This may result from any of various physiological stimuli, also known as sexual stimulation and sexual arousal. The corpus spongiosum is a single tubular structure located just below the corpora cavernosa, which contains the urethra, through which urine and semen pass during urination and ejaculation, respectively. This may also become slightly engorged with blood, but less so than the corpora cavernosa.

During sexual activity

A circumcised penis flaccid (left) and erect (right)

The swelling, hardening and enlargement of the penis enables sexual intercourse. The scrotum may also become tightened during an erection. Erection is not required for all sexual activities.

After a male has ejaculated during a sexual encounter or masturbation, the erection usually ends, but this may take time depending on the length and thickness of the penis.^[1]

Autonomic control

In the presence of mechanical stimulation, erection is initiated by the parasympathetic division of the autonomic nervous system (ANS) with minimal input from the central nervous system. Parasympathetic branches extend from the sacral plexus into the arteries supplying the erectile tissue; upon stimulation, these nerve branches initiate the release of nitric oxide, a vasodilating agent, in the target arteries. The arteries dilate, filling the corpora spongiosum and cavernosa with blood. Erection subsides when parasympathetic stimulation is discontinued; baseline stimulation from the sympathetic division of the ANS causes constriction of the penile arteries, forcing blood out of the erectile tissue.^[2] The cerebral cortex can initiate erection in the absence of direct mechanical stimulation (in response to visual, auditory, olfactory, imagined, or tactile stimuli) acting through erectile centers in the lumbar and sacral regions of the spinal cord. The cortex can suppress erection even in the presence of mechanical stimulation, as can other psychological, emotional, and environmental factors. The opposite term is detumescence.

Shape and size

An erect penis can take on a number of different shapes and angles, ranging from a straight tube angled at a 45-90 degree angle, to a curvature to the left or right, up or down. An increase in penile curvature can be caused by Peyronie's disease. This may cause physical and psychological effects for the affected individual, which could include erectile dysfunction or pain during erection. Treatments include oral medication (such as Vitamin E) or surgery, which is most often reserved as a last resort.

Generally, the size of an erect penis is fixed throughout post-pubescent life. Its size may be increased by surgery,^[3] although penile enlargement is controversial, and a majority of men were "not satisfied" with the results, according to one study.^[4]

Erectile dysfunction

Erectile dysfunction (also known as ED or '(male) impotence') is a sexual dysfunction characterized by the inability to develop or maintain an erection.^[5][6] The study of erectile dysfunction within medicine is known as andrology, a sub-field within urology.^[7]

Erectile dysfunction can occur due to both physiological and psychological reasons, most of which are amenable to treatment. Common physiological reasons include cardiovascular leakage and diabetes. Some drugs used to treat other conditions, such as lithium and paroxetine, may cause erectile dysfunction.^[6][8] The maintenance of the erect penis is required for ejaculation,^{[citation needed]} an important aspect of reproduction.

Erectile dysfunction, tied closely as it is to cultural notions of potency, success and masculinity, can have devastating psychological consequences including feelings of shame, loss or inadequacy;^[9] There is a strong culture of silence and inability to discuss the matter. In fact, around 1 in 10 men will experience recurring impotence problems at some point in their lives.^[10]

See also

• Clitoral erection
• Death erection
• Erectores pilorum
• Nocturnal penile tumescence
• Priapism
• Sexual function

References