Describe the physiology of penile erection.

1 . Stimulation of the parasympathetic neurons on sexual arousal causes the release of the neurotransmitter nitric oxide.
2 . Nitric oxide diffuses into the smooth muscles, causing the release of cGMP that causes relaxation and dilatation of the smooth muscles of the penile arteries and trabeculae within the corpora cavernosa.
3 . Blood under pressure fills the cavernous bodies.
4 . The increased arterial blood flow and pressure compress the draining veins, preventing the outflow of blood and causing the penis to become rigid and erect.
5 . cGMP is broken down by the phosphodiesterase enzyme.