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The penis is one of the places in the body where the brain needs to be able to turn the blood flow on and off with a valve. To understand how the brain controls this particular valve, let's start with the basic concept at work: How does the brain control blood flow to different parts of the body?
Turning Valves On and Off
In the human body, the "valves" open and close using muscles in the walls of arteries. When these muscles relax, the arteries open up and blood flow increases. The valves respond to chemical messages that the brain can control.
The mechanism that the body uses to "open a valve" in any part of the body involves four steps:
The brain sends a signal down a particular nerve fiber. This nerve fiber ends in an NANC nerve cell in an artery, somewhere near the point where blood flow needs to change. NANC stands for nonadrenergic-noncholinergic, and what it means is that the NANC nerve cell is able to create nitric oxide.
The NANC nerve endings inject nitric oxide into the blood and surrounding cells.
The nitric oxide stimulates an enzyme called guanylate cyclase in nearby cells, and this enzyme starts producing a chemical called cyclic guanosine monophosphate (cGMP).
cGMP tells smooth muscles that line an artery to relax. When they relax, blood flow increases.
This mechanism is a simple little chemical machine, and the brain uses it to increase blood flow in several different parts of the body. But there is one final part to this chemical machine: Another enzyme called phosphodiesterase (PDE) deactivates the cGMP.