Figure 1.

Pathways of penile erection: Dilation of cavernosal arteries and smooth muscle cells (SMCs) of cavernosal spaces is mediated by the nitric oxide (NO)–cyclic guanosine monophosphate (cGMP) pathway. NO is derived from nonadrenergic, noncholinergic nerves (nNOS, NOS type I), and the endothelium (eNOS or NOS type III) of cavernosal sinuses (both calcium²⁺/calmodulin-dependent isoforms). NO crosses plasma membranes to enter vascular smooth muscle cells (VSMCs) and promotes the synthesis and accumulation of cGMP, which induces a cascade reaction, leading to smooth muscle relaxation by reducing intracellular calcium level. NNOS = neuronal nitric oxide. ENOS = endothelial nitric oxide. See the text for details.