Painful erection

An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. 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. The shape, angle, and direction of an erection varies considerably in humans. Physiologically, erection is triggered by the parasympathetic division of the autonomic nervous system, causing nitric oxide (a vasodilator) levels to rise in the trabecular arteries and smooth muscle of the penis. 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. Erection subsides when parasympathetic activity reduces to baseline. As 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. Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT). Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes of erectile dysfunction and impotence. The state of a penis which is partly, but not fully, erect is sometimes known as semi-erection (clinically: partial tumescence); a penis which is not erect is typically referred to as being flaccid, or soft. An erection occurs when two tubular structures, called the corpora cavernosa, that run the length of the penis, 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. The scrotum may, but not always, become tightened during erection. Generally, the foreskin automatically and gradually retracts, exposing the glans, though some men may have to manually retract their foreskin. 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. 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. Nitric oxide diffuses to the smooth muscle of the arteries (called trabecular smooth muscle), acting as a vasodilating agent. The arteries dilate, filling the corpus spongiosum and corpora cavernosa with blood. The ischiocavernosus and bulbospongiosus muscles also compress the veins of the corpora cavernosa, limiting the venous drainage of blood. Erection subsides when parasympathetic stimulation is discontinued; baseline stimulation from the sympathetic division of the autonomic nervous system causes constriction of the penile arteries and cavernosal sinosoids, forcing blood out of the erectile tissue via erection-related veins which include one deep dorsal vein, a pair of cavernosal veins, and two pairs of para-arterial veins between Buck's fascia and the tunica albuginea. Erection rigidity is mechanically controlled by reduction blood flow via theses veins, and thereby building up the pressure of the corpus cavernosum and corpus spongiosum, an integral instructure, the distal ligament, buttresses the glans penis.