When couples talk about having "chemistry" together, who knew it was such an accurate description?
The brain sends signals to NANC cells in the artery. The NANC cells release nitric oxide (NO). Nitric oxide acts as a signaling molecule and stimulates an enzyme called guanylate cyclase in nearby cells. The guanylate cyclase converts a chemical called GTP into another chemical called cGMP. cGMP causes muscles in the walls of the arteries to relax. This relaxation increases blood flow. Meanwhile, PDE is decomposing the cGMP and turning it back into GTP. There is a cycle -- guanylate cyclase turns GTP into cGMP, and PDE turns cGMP into GTP. Nitric oxide turns the cycle on.
cGMP is produced as long as the brain is sending messages down the nerve fibers in the artery, which generate nitric oxide and keep the cycle going. When the brain stops sending the signal, all of the cGMP goes away because PDE is deactivating it. This way, the brain can turn valves on and off whenever it wants to.
So how does this relate to an erection?
When the brain gets aroused, it sends a signal to the penis. Nerve cells in the penis' corpora cavernosa start producing nitric oxide, which leads to the creation of cGMP. The cGMP causes arteries in the corpora cavernosa to dilate, causing lots of blood to flow into the penis. The extra blood flowing in causes the penis to inflate like a balloon. An erection occurs.
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When a man has erectile dysfunction, there can be many reasons for the problem. But one of the most common reasons, especially in older men, is that the arteries in the penis aren't dilating enough when the brain sends the signal. The man is aroused and the nerves in the penis are producing nitric oxide, but the amount of cGMP produced isn't enough to maintain an erection.
The way that Viagra goes about solving this problem is quite ingenious.