An erect penis can also take on a number of different shapes, ranging from a straight tube to a tube with a curvature up or down or to the left or right. This may also become slightly engorged with blood, but less so than the corpora cavernosa. The following table shows how common various erection angles are for a standing male. An increase in penile curvature can be caused by Peyronie's disease.
Autonomic control In the presence of mechanical stimulation, erection is initiated by the parasympathetic division of the autonomic nervous system with minimal input from the central nervous system. The scrotum may, but not always, become tightened during erection. The ischiocavernosus and bulbospongiosus muscles also compress the veins of the corpora cavernosa, limiting the venous drainage of blood. An erect human penis The length of the flaccid penis does not necessarily correspond to the length of the penis when it becomes erect; some smaller flaccid penises grow much longer, while some larger flaccid penises grow comparatively less. Generally, the foreskin automatically and gradually retracts, exposing the glans , though some men may have to manually retract their foreskin. This may also become slightly engorged with blood, but less so than the corpora cavernosa. The following table shows how common various erection angles are for a standing male. An increase in penile curvature can be caused by Peyronie's disease. Erections are common for children and infants, and even occur before birth. This may result from any of various physiological stimuli, also known as sexual stimulation and sexual arousal. Treatments include oral medication such as colchicine or surgery, which is most often reserved as a last resort. An upward pointing angle is most common. An unusual aversion to the erect penis is sometimes referred to as phallophobia. Parasympathetic branches extend from the sacral plexus 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. The cortex may suppress erection, even in the presence of mechanical stimulation, as may other psychological, emotional, and environmental factors. 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. Socially, such erections can be embarrassing if they happen in public or when undesired. This may cause physical and psychological effects for the affected individual, which could include erectile dysfunction or pain during an erection. Nocturnal erection The penis may erect during sleep or be erect on waking up. 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. Such an erection is medically known as nocturnal penile tumescence informally: An erect penis can also take on a number of different shapes, ranging from a straight tube to a tube with a curvature up or down or to the left or right. Physiology Composite image showing the development of a penile erection with Peyronie's disease An erection occurs when two tubular structures, called the corpora cavernosa , that run the length of the penis, become engorged with venous blood.
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