Sympathetic stimulation maintains a tonic contractile state of the intimal cushion, a smooth muscle lying in the center of the artery.

This keeps the artery coiled, and little blood flow occurs, instead routing to arteriovenous shunts to the deep dorsal vein.

NO binds to the enzyme guanylate cyclase, which results in increased levels of cyclic guanosine monophosphate (cGMP).

cGMP in turn triggers relaxation of the smooth muscle and results in dilation of blood vessels.

By preventing PDE5 from breaking down cGMP, the effects of NO are amplified and vasodilation occurs, thus resulting in increased penile erection.