THE PERIPHERAL NERVOUS SYSTEM
Nonadrenergic, noncholinergic (NANC) neurons are widely distributed in peripheral tissues, especially the gastrointestinal and reproductive tracts . Considerable evidence implicates NO as a mediator of certain NANC actions, and some NANC neurons appear to release NO. Penile erection is thought to be caused by the release of NO from NANC neurons; NO promotes relaxation of the smooth muscle in the corpora cavernosa—the initiating factor in penile erection—and inhibi-tors of NOS have been shown to prevent erection caused by pelvic nerve stimulation in the rat. An established approach in treating erectile dysfunction is to enhance the effect of NO sig-naling by inhibiting the breakdown of cGMP by the phosphodi-esterase (PDE isoform 5) present in the smooth muscle of the corpora cavernosa with drugs such as sildenafil, tadalafil, and vardenafil .