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The process starts with the brain
registering a sexual stimulus. The brain then orchestrates
a cascade of nerve signals that run through the spinal cord
to the penis. Here, the cavernosal nerves release nitric oxide,
a gas that quickly penetrates the smooth muscle cells
that form the penis' spongy tissue. The nitric oxide causes
the cells to produce a substance called the cyclic guanine
mono phosphate (cGMP), which has an expansive, relaxing
effect.
The arteries feeding the
penis expand and the smooth muscle relaxes, opening up tiny
sacs in the erectile tissue that then swell with blood.
The two main chambers of the penis, the
corpus cavernosum, fill and the organ becomes so engorged that the
veins that usually drain away blood are pressed shut against
the tough outer membrane of the erectile chambers. The result
is erection.
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