Figure 47.9 The Role of Ca2+ in Smooth Muscle Contraction When a smooth muscle cell is depolarized by a neurotransmitter, Ca2+ enters the sarcoplasm and binds to calmodulin, which in turn activates an enzyme that phosphorylates the myosin heads, causing them to bind to actin. As long as the myosin remains phosphorylated, actin and myosin go through cycles of binding and release. Thus in smooth muscle the Ca2+-mediated change is on myosin, whereas in skeletal and cardiac muscle it is on the actin–