A complex interaction of neural, vascular, and biomolecular activity results in normal erectile function. Two chambers in the penis, the corpus cavernosa, contain sinusoids, which fill with blood and expand to become rigid when smooth muscle is relaxed. Blood flow out of the penis is temporarily stopped (veno-occlusion) to maintain the erection. Neural control is complex but, in general, the sympathetic nervous system inhibits, and the parasympathetic system excites erectile function. The central nervous system can induce erections without peripheral stimuli, e.g. in nocturnal erections. In general, there are 5 recognized phases in erectile response: latent, tumescent, full erection, rigid erection, and detumescent. The key biochemical event in erectile function is an increase in levels of cyclic guanosine monophosphate, which causes smooth-muscle relaxation and allows an erection to occur. This process is mediated by nitric oxide, produced by neurons and elsewhere, which acts as a gaseous messenger molecule.