How does erection occur?
Erection is an effect of sexual stimulation. The stimulation may have a mental character (for instance: fantasies) or tactile character (for example: touching genitals). Sexual stimulation creates an electrical impulse which goes along the nerves which go to the penis. The impulse influences the nerves so that they start to release nitric oxide. Nitric oxide causes the bigger production of cyclic GMP (cGMP) in the corpora cavernosa (in its smooth muscles). The cyclic GMP relaxes the smooth muscles which allows the big blood flow to get into the penis. The blood fills the corpora cavernosa causing an erection.