The erection builder

Thanks to its relaxing effect on smooth muscle cells, NO is now widely recognized as the major, if not the sole neurochemical responsible for causing penile erections. Although acetylcholine (ACh) in the parasympathetic nerves that serve the penis has long been thought to be the major neurotransmitter involved in causing the penis to become erect, the most recent findings do not support this conclusion. ACh is certainly important for erections, but it is not the final step in the process. In laboratory experiments, blocking the cholinergic activity of these nerves does not necessarily affect the ability to have an erection. However, if you disrupt NO synthesis by applying chemicals that interfere with NOS activity, you completely prevent erections. NO synthesis can be restored after blocking NOS by supplying additional L-arginine [Burnett, 1992]. Dr. Arthur Burnett, a urologist at The Johns Hopkins University, and leading NO researcher, says that nitric oxide acts as an essential chemical trigger in producing erections. "It's like turning on a light switch." Noting that all you need to light up a room are wires, lamps and light bulbs, he adds, "It is the switch that starts the process." To light up your sexual life, it seems you need to flick the NO switch first [Associated Press, 1992]. To understand the role of NO in erections, it helps to understand how erections happen in the first place. Men get erections when sexual thoughts originating in the brain initiate a flow of nerve signals down the spinal cord to the arteries and smooth muscle in the penis. When they receive these signals, the arteries that supply the penis dilate and the muscles that control the two rods of sponge-like tissue filling the core of the penis-the corpora cavernosa and the corpus spongiosum-relax. As they relax, they allow the increased flow of blood through the penile arteries to fill the spongy space with blood. The increasing pressure in the penis compresses the veins that drain blood from the penis, preventing outflow. The more blood that fills the penis, the larger and harder the erection will be, because as long as blood is flowing in through the arteries, the outflow remains severely restricted. The penis returns to its flaccid state when the penile arteries constrict, relaxing pressure on the veins and allowing the blood to drain out. "We used to think that impotence was caused by poor inflow of blood to the penis," said Dr. Jacob Rajfer, a UCLA urologist. "We now know that most impotent men have a problem holding blood in the penis."

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