Penile blood flow is controlled by the autonomic erection centre, which provides parasympathetic (S2–S4) and sympathetic (T12–L2) input to the pelvic plexus,8 including the cavernous nerves that innervate the cavernosal arteries and trabecular smooth muscle. These nerves are responsible for the delivery of high local concentrations of nitric oxide to the trabecular smooth muscle, which results in relaxation (Fig. 2).9,10 Nitric oxide diffuses across the smooth-muscle membrane and activates guanylate cyclase to produce cyclic guanosine monophosphate (cGMP); the biochemical cascade that ensues results in altered potassium and calcium ion channel permeability; ultimately, the decrease in cytosolic calcium concentration causes smooth-muscle relaxation and increased regional blood flow.11,12 Phosphodiesterase enzymes (PDEs) regulate this pathway by inactivating cGMP, which results in elevated cytosolic calcium concentrations and smooth-muscle contraction. PDE type 5 is the most important isoenzyme in the corpora cavernosa.13,14,15 ERECTILE DYSFUNCTION Call your doctor or seek emergency treatment immediately if you experience any of the following symptoms: How to get viagra if you haven't prescription? canadian pharmacy viagra for sale free samples viagra canadian pharmacy viagra What is tadalafil (Cialis)? Waves of sound are beamed through the skin, and although they pass harmlessly through body tissue, they are at just the right pressure to smash up kidney stones into tiny sand-like particles that are then passed out of the body in urine. Licensed Pharmacy Viagra real viagra online without prescription