This may result from any of various physiological stimuli, also known as sexual stimulation and sexual arousal. Making Sense of Sex: A Forthright Guide to Puberty, VIAGRA Sex and Relationships for People with Asperger's Syndrome. Parasympathetic branches extend from the sacral plexus into the arteries supplying the erectile tissue; upon stimulation, these nerve branches release acetylcholine, which in turn causes the release of nitric oxide from endothelial cells in the trabecular arteries.

After ejaculation or cessation of stimulation, erection usually subsides, but the time taken may vary depending on the length and thickness of the penis. Physiologically, VIAGRA an erection is required for a male to effect penetration or sexual intercourse and is triggered by the parasympathetic division of the autonomic nervous system, causing the levels of nitric oxide (a vasodilator) to rise in the trabecular arteries and smooth muscle of the penis.