Nitric oxide, or NO, is a gaseous signaling molecule and a unique second messenger that mediates processes such as smooth muscle relaxation, neurotransmission, and visual perception. Acetylcholine bound GPCRs stimulate PLC-β to induce IP3 production, which triggers calcium release. Calcium ions bind calmodulin to form a complex that activates NO synthase. The enzyme produces NO in the endothelial cells, which diffuses into the neighboring smooth muscle cells to act on them. NO binds and activates guanylyl cyclase to produce cyclic GMP that activates protein kinase G or PKG. PKG activates a phosphatase, which dephosphorylates myosin light chains. This induces the folding of myosin heads, dissociating them from the actin filaments. PKG also inhibits IP3-gated calcium channels, thereby reducing cytosolic calcium concentration. Low calcium levels inactivate myosin light chain kinases, preventing actin-myosin reassembly and muscle contraction. The smooth muscle now relaxes, allowing the blood vessels to dilate and increase the blood flow.