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04-08-2006, 07:48 AM
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Rank: Member
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Join Date: Mar 2006
Posts: 54
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Nitric Oxide (eNOS) and Erections:
Quote:
How Erections Work
The penis is an organ with paired erection chambers (corpora cavernosa), which are filled with spongy erectile tissue (corporal sinusoids) composed predominantly of muscle. Erection and loss of erection are related primarily to blood flow events regulated by the relaxation and contraction, respectively, of the smooth muscle in the penile arteries and the erectile bodies themselves. Erection is a hydraulic event, regulated by hormones and nerves, which allow increased blood flow into and storage of blood within the erectile bodies leading to an increase in pressure and the development of rigidity (hardness). Penile erection is triggered by one of two main mechanisms: direct stimulation of the genitalia or through stimuli coming from the brain (fantasy, smell, etc).
Upon stimulation, chemicals are released in the brain that cause signals to pass down the spinal cord and outward through special nerves (nervi erigentes) into the penis. These nerves release another chemical (Nitric Oxide) that causes the aforementioned smooth muscle to relax and blood rushes into the erectile bodies, causing erection. Anxiety or fear can prevent the brain signals from reaching the level required to induce erection. Medical conditions can block the erection arteries or cause scarring of the spongy erection tissue and prevent proper blood flow or trapping of blood and, therefore, limit the erection. Thus, the erection mechanism is much like a tire; a firm tire is dependent upon a hose that can deliver air in adequate amounts in a speedy fashion and a valve mechanism that holds the air in place. In the penis the hose is represented by the erection arteries, which rapidly carry blood into the erectile bodies and the valve mechanism, while complicated in its structure, ensures that the blood is trapped inside the erectile bodies until ejaculation occurs or the sexual stimulus has passed.
Suggested Reading
1 Krane RJ et al: Impotence. New England Journal Of Medicine J, 321: 1648-1659, 1989
2 Giuliano FA et al.: Neural Control of Penile Erection. Urologic Clinics Of North America, 22: 747-66, 1995
3 Rajfer J et al.: Nitric oxide as a mediator of relaxation of the corpus cavernosum on response to nonadrenergic, noncholinergic neurotransmission. New England Journal Of Medicine, 326: 90-4, 1992
4 Burnett AL et al.: Nitric oxide: A physiologic mediator of penile erection. Science, 257: 401-3, 1992
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Quote:
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Originally Posted by Arthur Burnett, M.D., associate professor of urology at Hopkins
"Once blood starts flowing into the penis, the source of nitric oxide in the blood vessels is continuously activated so that more nitric oxide is released, more tissue relaxes, more blood comes in and a sustained erection is achieved.
This cascade of events begins when erotic thoughts or physical sensations produce nitric oxide release in nerve endings in the penis. Nitric oxide is a relaxant that allows blood vessels to open up or dilate, bringing increased blood flow and swelling of tissues. The flow of blood also creates a minor stress on the blood vessel wall which activates the release of more nitric oxide. This time it is from cells in the wall of the blood vessel - the endothelial cells - rather than from nerves. Endothelial nitric oxide causes more tissue to relax and the process repeats until the penis is fully erect."
A key element in the attainment of erection is the continuous activation of the source of nitric oxide in blood vessel walls. Finding this source, a special form of the enzyme called endothelial nitric oxide synthase (eNOS), plus the discovery that the pressure of flowing blood against a vessel wall could induce it to produce nitric oxide, were critical pieces of the puzzle. These discoveries offer a fuller picture of the complex physiology of erection".
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