The mechanism of erection

As a sexual tool, the penis is remarkably complex and elegantly effective. If it's to enlarge and stiffen sufficiently to permit penetration, three different systems must play a part.

The nervous system must receive and send signals, the vascular system must supply sufficient blood to fill the penile chambers, and the muscular system in those lower regions of the body must exert pressures allowing the erection to be created and maintained.

Without all of these systems working together, a normal, functioning erection is not on the cards. There are, of course, similar co-operative requirements for reaching orgasm and ejaculation, and to enable sex to work satisfactorily for both the man and the woman, all these elements must be in harmony.

Erections begin as a result of the stimulation of the nervous system. This can occur in many different ways, but all of the communications are filtered through the brain.

Even the direct sensory stimulation of the genital organs will send messages along the pudendal nerve and up the spine to the central nervous system message centers. Everything else gets there, too.

Sexual feelings, verbal or visual communications with the loved one, touching and kissing, or, if a man is by himself, erotic fantasy, sexual imagery, anticipation, memory, or any other excitation of desire will each in its -turn stimulate various regions of the brain.

All these regions combine their messages in the hypothalamus, from whence the good news is sent southwards in a cascade of neurotransmitter signals. Certain parts of the message go directly down the spinal cord to the penis.

But the autonomic nervous system, that which controls unconscious functions of the body such as breathing and digesting, also receives the message and transmits it through chemical signals that the entire body responds to.

So an extraordinary variety of incoming stimuli - everything from the words, "I love you," to the gentle touch of a loving hand - combine to send a variety of neuronal signals towards the groin, all with an essentially similar message: "Attention, please!"

The penis is, of course, primed to respond to the signals the body is so eager to send it. A man's sexual organ is a pretty unusual piece of equipment, basically an inflatable, multi-chambered filling tank with a slender tube running down its center.

No other part of either the male or the female body carries out at short notice such an extraordinary transformation as the penis.

Indeed, arguably no other part of the body is designed to give its owner such pleasure, provided of course that all works well. The complexity of the system is such that it is no surprise that sexual issues are a major problem for both men and women.

These issues include sexual problems caused primarily by social and psychological factors - such as premature and delayed ejaculation - and those caused by physical problems such as organic erectile dysfunction.

Triggered by stimuli from the central nervous system and/or the peripheral nervous system, the muscular walls of the arteries dilate allowing more blood to flow into the genital region.

Anatomy of the penis

The penis is a specialized vascular organ composed Of complex vascular tissue responsive to neurological impulses that create penile rigidity. The penis is composed Of two paired corpora cavernosa and a third cylindrical corpus spongiosum that contains the urethra and is contiguous with the glans penis.

The corpora cavernosa or penile erectile bodies are surrounded by a thick fibrous sheath (tunica albuginea) that is relatively non-distensible, composed of elastic fibers and collagen that support and maintain the rigidity of erectile function. Surrounding the tunica albuginea is a second gossamer layer of fascia called Buck's fascia.

Within these functional structures courses a complex vascular sinusoidal network of spongy tissue that activates erection.

While the corpora cavernosa are generally considered to be separate cylinders, there is no clear septum between the two corpora and the distal portion of the penis and free vascular nication occurs.

The corpora themselves are composed of sinusoids made up of smooth muscle tissue and lined by endothelial cells. The corpora spongiosum is composed of similar spongy tissue but is surrounded by a less rigid thinner tunica albuginea resulting in less rigidity on activation.

The blood supply of the corpora cavernosa originates from the internal iliac arteries and courses to the internal pudendal arteries that terminate in the arterioles to the penis.

These small penile arteries include the dorsal artery to the penis outside the tunica albuginea, the bulbo-urethral artery that travels within the corpus spongiosum lateral to the urethra and the central cavernosal arteries that travel in the central portion of each of the paired corpora cavernosa and supply the blood for erection.

A proximal perineal branch of the pudendal artery provides blood supply to the perineal skin and scrotum. The dorsal artery of the penis is responsible for the blood supply of the penile skin and glans penis and contributes little to erectile function.

The cavernosal arteries on the other hand enter the corpora cavernosum at the hilum of each corpus cavernosum and give rise to multiple small helicine arteries that drain directly into the vascular lacunar spaces of the corpus cavernosum. Accessory pudendal arteries may also provide blood supply to the penis.

 These variable arteries may originate from the obturator artery, inferior pudendal artery, iliac trunk, or inferior gluteal artery and frequently lie close to the capsule of the prostate. You can deal with high blood pressure quite easily - check out how to lower your blood pressure for more information on this vital health issue. 

Venous drainage of the penis is important both anatomically and functionally. The lacunar spaces or vascular sinusoids of the corpora cavernosa drain through subtunical veins beneath the tunica albuginea into the emissary veins by way of the deep dorsal vein of the penis.

The deep dorsal vein culminates in the periprostatic venous plexus superior and lateral to the prostate.

The superficial dorsal penile vein lies above Buck's fascia and provides drainage predominantly for the penile skin culminating in the saphenous vein.

The proximal penile shaft and proximal corpus cavernosum drains through veins exiting the crura of the corpora cavernosa termed the crural veins that join to form the internal pudendal vein. As with most venous systems, the venous drainage of the penis is variable and complex and has many interconnections.

Cholesterol can block the arteries of the penis and lead to erectile dysfunction.

A number of other problems can interfere with sexual functioning, either directly, because the organs of the body are affected - cholesterol deposition would be a good example of this - or indirectly because you are in pain and discomfort. Gout and gouty arthritis are typical examples of this. Here's a website which gives you more information on this whole area of home remedies for gout.

Erectile physiology: Sinusoidal smooth muscle is contracted and blood flows from the internal pudendal arteries via the cavernosal arteries and the helicine arteries to the lacunar spaces and out through the open emissary veins; during erection the smooth muscle of the sinusoids relaxes allowing blood to flow into the lacunar spaces.

The resultant pressure compresses the emissary veins beneath the rigid tunica albuginea, reducing venous outflow. To reduce cholesterol, check out how to lower cholesterol naturally fats.

The nerve supply to the penis controls not only sensation, but also vascular supply to the penis. A pair of sympathetic nerves from S2-4 nerve roots primarily control erectile function while the sympathetic nerves from T11-L2 control detumescence and also contribute to ejaculation and emission.

These autonomic nerve fibers form the pelvic plexus of nerves and enter the penis within the cavernous nerves that course lateral and inferior to the prostate. It is these nerves that are preserved during nerve sparing radical prostatectomy.

In addition to the autonomic nervous system, peripheral nerves form sensory and motor elements through a reflex arc in the sacral spinal cord at Onufs nucleus.

Peripheral nerves containing sensory elements are also responsible for erectile function, especially maintenance of erection. Ultimate central nervous system control is likely initiated in the hypothalamus in the medial pre-optic area that integrates psychological and tactile stimuli.

Physiology of erection

Excitatory stimuli from the central nervous system pass through the autonomic nervous system and produce erections through a variety of neurotransmitters. Many neurotransmitters including acetylcholine (ACh) and vasoactive intestinal polypeptide (VIP) contribute to erectile function.

The most important neurotransmitter in the corpora cavernosa is nitric oxide (NO).

NO functions via the cyclic guanosine monophosphate (cGMP) system. It is is produced from the precursor L-arginine by the enzyme nitric oxide synthase (NOS). The secondary transmitter cGMP is ultimately responsible for smooth muscle relaxation in the corpus cavernosum and producing erection.

The relaxation is mediated by calcium efflux from the smooth muscle cell. cGMP is broken down in the corpus cavernosum smooth muscle cell by the enzyme phosphodiesterase (PDE) with subsequent detumescence. The most active PDE in the corpus cavernosum is type 5 (PDE-5). Additional smooth muscle relaxation occurs through a secondary pathway mediated by cyclic adenosine monophosphate (cAMP). cAMP, which can be stimulated by prostaglandin E1 (PGEI), VIP, or papaverine assists the primary eGMP system.

Once a central nervous system or tactile stimulus has occurred, secretion of NO is initiated and smooth muscle sinusoids of the corpus cavernosum begin to relax. Initial stimuli produce dilation and relaxation of the central cavernosal artery and the helicine arterials that produce increased influx of blood to the lacunar spaces of the corpora cavernosa.

As this increased flow and downstream relaxation continues, the lacunar spaces or sinusoids fill with blood, increase in size, and produce decreases in venous outflow by compressing the subtunical venous channels against the tunica albuginea producing a high-pressure rigid erection.

Rigidity is increased to a level beyond that of the abdominal aorta from the contraction of the perineal muscles especially the bulbocavernosus and ischiocavernosus muscles. At the conclusion of sexual activity, psychological and physical stimuli diminish, smooth muscle sympathetic tone increases, vasoconstriction occurs, cGMP is broken down and the erection subsides.