How does erection occur?

Erection begins with sexual stimulation. Sexual stimulation can be tactile (for example, by touching the penis), or mental (for example, by having sexual fantasies). Sexual stimulation generates electrical impulses along the nerves going to the penis and causes the nerves to release nitric oxide, which in turn increases the production of cyclic GMP (cGMP) in the smooth muscle cells of the corpora cavernosa. The cGMP causes the smooth muscles of the corpora cavernosa to relax, and allow rapid blood flow into the penis. The in-coming blood fills the corpora cavernosa, making the penis expand