A. Physiology of Penile Erection
ED is a disorder of the blood supply to the penis. Nitric oxide (NO) bioavailability is at the core of normal and abnormal erectile function. Nitric acid is synthesized from the amino acid L-arginine in response to sexual stimula¬tion. Physical or emotional stimulation causes brain or neuronal NO synthase (bNOS) located in nonadrenergic, noncholinergic cavernous nerves of the penis to release NO. Nitric oxide is a powerful dilator of blood vessels and is formed during the conversion of L-arginine to L-citrulline by the enzyme NO synthase. It is a small unsta¬ble molecule that mediates many of the normal functions of the endothelial lining of blood vessels.
The endothelium is the inner lining of blood vessels that is in contact with circulating blood. NO is responsible for neurally induced vasomotor changes at the level of the corpora cavernosa which generates penile erection. Also, cholinergic nerves release acetylcholine, which acts on the surface receptors of endothelial cells leading to activation of endothelial NO synthase (eNOS) and in healthy endothelium it releases significant levels of NO. The bio-active NO diffuses to the trabecular smooth muscle cells surrounding the sinusoidal spaces and acts on guanylyl cyclase, which generates cyclic guanosine monophosphate (cGMP). This has a vasodilatory effect and appears to cause relaxation of the cavernosal smooth muscle, allowing a marked inflow of blood into the sinusoidal spaces and engorgement of the penis. As the penis fills with blood, a veno-occlusive process comes into play blocking the outflow of blood. This maintains the erection (see Fig. 1).

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