Penile erection is a hemodynamic event in the smooth muscle of corpus cavernous. PDE5 is the main cGMP hydrolysing enzyme found in penile corpus cavernous.  Erection is triggered by release of the neurotransmitter nitric oxide (NO) from non-adrenergic and non-cholinergic neurons from nerve ending in the penis as well as from endothelial cells. NO activates soluble guanylyl cyclase in smooth muscle cells in the penis which results in increased production of 3-5-cyclic guanosine monophosphate from guanosine-5'-triphosphate (GTP). Cyclic GMP binds to the cGMP-dependent protein kinase (PKG1) which phosphorylates several proteins that results in decreased intracellular calcium. Lower intracellular calcium leads to smooth muscle relaxation and ultimately penile erection. This pathway is demonstrated in .