Penile anatomy and the mechanism of erection
The penile shaft, or body of the penis, contains a pair of cylinders called the corpora cavernosa, which are surrounded by tough fibrous tissue known as the tunica albuginea. Inside the corpora cavernosa the erectile tissue contains a network of vascular spaces which inflate with blood when the penis becomes erect.
Most of this network of erectile tissue consists of vascular spaces, otherwise known as sinusoids, with smooth muscle in their walls.
Underneath the corpora cavernosa, each of which is known individually as a corpus cavernosum, lies another body of erectile tissue called the corpus spongiosum.
The urethra runs through the centre of the corpus spongiosum along the lower surface of the penis.
The pressure during an erection is lower within the corpus spongiosum than it is within the corpora cavernosa, so that the urethra is not compressed. This allows ejaculation to take place without restriction.
It follows from that the corpora cavernosa are most directly responsible for the erection of the penis. Where they meet at the base of the penis the two corpora cavernosa diverge and are attached firmly to the pelvic bones by their crura.
Towards the end of the penis near the body, these internal chambers have additional layers of muscle, the bulbospongiosus and the ischiocavernosus muscles: these are the muscles which contract during ejaculation and eject semen from the end of the urethra.
It is not known whether they also have a role in the development of penile erection.
At the other end of the penis, the corpus spongiosum widens out to form the glans of the penis, which in an uncircumcised man is covered by a loose fold of skin called the prepuce or foreskin.
Underneath the penis towards the end of the shaft, the foreskin is attached to the underside of the glans by the frenum (or frenulum), a longitudinal fold of skin stretching to the bottom edge of the glans penis.
On erection, the foreskin will partially retract because the extension of the penile shaft increases the tension of the penile skin, and the tip of the glans will be exposed, showing the urethral meatus.
As a man thrusts during sexual intercourse, his foreskin moves backwards and forwards along the shaft of his penis as it thrusts against the vaginal walls, a mechanism which intermittently exposes and covers the glans.
In some circumstances, such as when the man has a phimosis, a tight opening at the end of the foreskin, the mobility of the foreskin along the shaft and glans is restricted.
This can cause difficulty and discomfort during intercourse. It may only be on a man's first attempt at intercourse that the tightness of the foreskin is revealed, since he experiences pain when it cannot retract.
Small tears in the frenulum may happen during excessively vigorous masturbation or sex, and these can be very painful.
Any restriction of mobility of the preputial skin should be attended to, often with surgery, to ensure that both masturbation and intercourse can take place without pain.
A torn frenulum also needs specialist attention, usually a frenuloplasty, a surgical procedure needed to prevent recurrent tearing.
Any injury to the foreskin or glans, including abrasion of the glans due to lack of lubrication, may promote infection or soreness, including yeast infections.
Although possibly less common in men than women, they can be just as unpleasant, and need attention to prevent soreness and itching, followed by flaking skin and discharge.
Beyond the part of the urethra where it passes into the body but before its junction with the bladder, the corpus spongiosum expands to form the urethral bulb. This is the part of the urethra into which semen is initially released in the emission phase of sexual arousal before it is reflexively expelled from the body during ejaculation.
Beyond the urethral bulb is the firm fibromuscular prostate gland, which contains within its body numerous branching glands whose purpose is to add additional fluid to the ejaculate produced in the seminal vesicles.
If the prostate enlarges it can block the urethra and it may be necessary to surgically remove either the enlarged tissue or the whole prostate gland.
Care is needed to spare the nerves during prostate surgery so that ejaculation can take place, but any operation on the prostate gland may produce retrograde ejaculation where the semen passes backwards into the bladder rather than upwards into and out of the urethra.
The testes lie in a pouch of skin and muscle known as the scrotum.
Although there is a widespread belief that a boy's testicles descend at puberty (as in "his balls dropped at 14"), the testes do in fact migrate down into the scrotum towards the end of the fetal development in uterus.
They are positioned outside the body because they require a lower temperature than that of the body environment for normal sperm production to take place.
If a boy's testes do not descend, they can be damaged both in their capacity to produce sperm and their endocrinological functions (mostly the production of testosterone), although the operation to bring the testicles into the scrotum is commonplace.
Within the scrotum the testes freely move up and down under the influence of two muscles: the dartos muscle, which is the muscle that can corrugate and contract the scrotal wall, and the cremaster muscle, which forms a support around the spermatic cord and the testicle inside the scrotum.
Sperm are produced in the interstitial cells of the testicle, and gradually mature as they pass along the epididymis to the storage area within the seminal vesicles.
These are two sacs which lie behind the bladder and prostate gland, and which produce large amounts of fluid in which the sperms are discharged.
Other sexual secretions originate in the bulbo-urethral or Cowper's glands, which are located one on each side of the urethra near the urethral bulb, and the urethral glands which occur along the inside of the penis.
The Cowper's glands produce a fluid which is discharged from the penis before ejaculation takes place, at an early stage of a man's sexual arousal. The fluid produced by these glands is clear, transparent and viscous, and the amount produced varies hugely from one man to another.
About 22% of men in Kinsey's pioneering research reported that they produced no pre-ejaculatory fluid, while about 18% of men experienced it dripping from the penis, in some cases in large quantities which might be the source of some embarrassment.
The mechanism of erection
The mechanism by which the penis becomes erect has been a source of considerable puzzlement for a number of years, although the mechanism is now more or less completely understood.
There have been a number of particularly important stages in the development of knowledge about the mechanism of erection, each of which has produced a considerable advance in the understanding of how the nervous system and muscles within the body interact to produce a state of erection.
An erection is not just simply the result of an increased blood flow into the penis; it also requires increased retention of blood within the penis. This is achieved by the compression of the veins of the penis by the inelastic wall of the erectile chambers.
On a cellular level, the first major development in understanding how this happens was the recognition that the smooth muscle within the corporate cavernosa, mostly within the walls of the sinusoids, were responsive to both cholinergic and adrenergic compounds.
The adrenergic receptors are mostly present in smooth muscle walls of the sinusoids, and occur 10 times more frequently than the cholinergic receptors. The second important piece of evidence about the mechanism of erection was produced by the injection of various compounds into the corpora cavernosa of male volunteers.
All of the compounds which produced an erection had in common a capacity for relaxing smooth muscle, so this was clearly a fundamental aspect of the mechanism by which the penis becomes erect.
In the late 1980s it was understood that the relaxation of the smooth muscles in the penis allowed the lacunae within the corpora cavernosa and corpus spongiosum to fill with blood, and that a passive compressive force on the veins and venules caused by the tunica albuginea impeded venous outflow and increased sinusoidal filling.
It was also thought that additional hardness of erection might be caused by the temporary contraction of the ischiocavernosus and bulbcavernosus muscles.
These muscles contract as a reflex response to the squeezing of the glans of the penis, and it's possible that the entry of the penis into the opening of the vagina elicits this reflex response, so as to cause further stiffening and therefore easier entry for intercourse.
But the final element of the understanding of the mechanism of erection came in the 1990s where it was discovered that nitrous oxide and cyclic guanesine monophosphate (cGMP) were responsible for relaxation of the smooth muscle in the corpus cavernosum.
Phosphodiesterase is an enzyme which occurs in several forms in the human body and is normally responsible for breaking down cyclic GMP.
It follows therefore that a phosphodiesterase inhibitor might have an influence on the mechanism of erection, and so it proved when Pfizer were testing such a product as a possible treatment for angina.
Although it didn't work very effectively as a heart medication, it certainly proved to have an effect on men's capacity for erection.
This led to a much clearer understanding of the biochemical reactions involved in sexual arousal and erection in men. See www.viagra.com for more details.
Basically, state of tone of the smooth muscle in the corpus cavernosum determines whether the penis is erect or flaccid.
To maintain a flaccid condition, the muscles must be in a state of tone or contraction. This is maintained by high levels of calcium, released by stimulation of cells by the sympathetic nerve supply.
The inhibition of this nerve supply therefore reduces calcium and is one aspect of the erectile mechanism.
But erections can develop when there is both parasympathetic (atropine) and sympathetic blockers, so this led to a research focus on the so-called non-adrenergic -- non-cholinergic neuroeffector system.
Essentially, this has a role to play in the reduction of intracellular calcium through an increase in levels of cGMP and cAMP.
Nitric oxide is produced in response to sexual stimulation and released from the nerve cells in the penis as well as the vascular endothelium of the erectile tissues.
It increases the availability of cGMP, which is then broken down by the predominant form of phosphodiesterase in penile tissues.
Compounds such as Viagra, which are phosphodiesterase inhibitors, act to slow down the destruction of cGMP, which means that the muscle relaxing effect of cGMP is prolonged.
There are other compounds involved in the mechanism erection, but this is certainly one of the major ones.
But no matter how effective a man's neurophysiology may be, it's also absolutely crucial that the veins leaving the penis are subject to compressive forces for the full development of intracavernosal pressure, and a rigid erection.
The pressure of the corpora cavernosa against the tunica albuginea effectively blocks the veins and venules which would otherwise drain blood from the penis.
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Updated November 11, 2016