Physiology & Pathophysiology
How Erection Works
Penile erection occurs when the penis becomes engorged with blood. The corpora cavernosa and the corpus spongiosum are capable of being distended with seven times their normal amount of blood. When this occurs in association with relaxation of the penile muscles, erection results.
The sequence of events resulting in penile erection is complex. It is usually initiated by sexual arousal stimuli arising in the brain as a result of visual, auditory or olfactory sensations or erotic thoughts. Tactile sensations of the penis acting through the spinal cord play a similar role. Sexual arousal results in the release of nitric oxide (NO) from specialized cells. NO causes the formation of cGMP (cyclic glutamine monophosphate) that is responsible for dilating the blood vessels of the penis and relaxing its muscles, thus allowing for an increase in blood flow and resultant penile erection.
Compression of the dilated blood vessels against the firm outer lining of the penis prevents the blood from escaping and perpetuates the erection. PDE-5 (phosphodiesterase 5) causes the breakdown of cGMP and, with the help of nerves from the sympathetic nervous system, allows the penis to return to its flaccid relaxed state.
