![]() ![]() The ischiocavernosus and bulbospongiosus muscles also compress the veins of the corpora cavernosa, limiting the venous drainage of blood. The arteries dilate, filling the corpus spongiosum and corpora cavernosa with blood. Nitric oxide diffuses to the smooth muscle of the arteries (called trabecular smooth muscle ), acting as a vasodilating agent. 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 the release of nitric oxide from endothelial cells in the trabecular arteries. 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. In some cases, the scrotum becomes tightened during an erection, and in most uncircumcised males, the foreskin automatically and gradually retracts throughout the various stages of erection, exposing the glans, though some individuals have to manually retract their foreskin. ![]() This may also become slightly engorged with blood, but less so than the corpora cavernosa. ![]() ![]() 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 result from any of various physiological stimuli, also known as sexual stimulation and sexual arousal. Īn erection occurs when two tubular structures, called the corpora cavernosa, that run the length of the penis, become engorged with venous blood. After reaching puberty, erections occur much more frequently. Side views and comparison of the stages of an uncircumcised (top and bottom) and a circumcised (middle) human penis erection.Īn erection is necessary for natural insemination as well as for the harvesting of sperm for artificial insemination, and is common for children, and infants. 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. Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes of erectile dysfunction and impotence. Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT), also known as "morning wood". Erection subsides when parasympathetic activity reduces to baseline.Īs 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. 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. Physiologically, an erection is required for a male to effect penetration or sexual intercourse and is triggered by the parasympathetic division of the autonomic nervous system, causing the levels of nitric oxide (a vasodilator) to rise in the trabecular arteries and smooth muscle of the penis. The shape, angle, and direction of an erection vary considerably between humans. Penile erection is the result of a complex interaction of psychological, neural, vascular, and endocrine factors, and is often associated with sexual arousal, sexual attraction or libido, although erections can also be spontaneous. An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. ![]()
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