An erection of the penis, clitoris or a nipple is its enlarged and firm state. It is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal.
The maintenance of the erect penis is required for ejaculation, an important aspect of reproduction, and many forms of life could not reproduce in a natural way without this ability.
Penile erection
A 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. After a male has ejaculated during a sexual encounter or masturbation, his erection usually ends, but this may take time depending on the length and thickness of the penis.
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.
An erection results in swelling, hardening and enlargement of the penis, enabling sexual intercourse. The scrotum may also become tightened during an erection. Erection is not required for all sexual activities.
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. 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, although penile enlargement is controversial, and a majority of men were "not satisfied" with the results, according to one study.
Clitoral erectionClitoral erection is a part of sexual arousal in females. The clitoris is the anatomically homologous counterpart of the penis, and the physiological mechanism of its erection is similar.
Swelling and enlargement may also occur during a clitoral erection but because a large proportion of the clitoris lies within the body and because of its smaller size, it is often not as obvious.
Nipple erectionNipple erection may result from three kinds of response. It happens in females during breast feeding. It is also an early part of the sexual response in females and males. Both of these are caused by the release of oxytocin. Nipple erection can also be caused by a tactile response to cold temperature in both males and females. The erection of nipples is not due to erectile tissue, but due to the contraction of smooth muscle under the control of the autonomic nervous system. It is more akin to a hair follicle standing on end than to a sexual erection.
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