Although smooth muscles do not form regular arrays of thick and thin filaments like the sarcomeres of striated muscles, contraction is still due to the same sliding filament mechanism controlled by myosin crossbridges interacting with actin filaments. The thin filament of smooth muscle is made of actin, tropomyosin, caldesmon, and calmodulin. Within this type of muscle, caldesmon and calmodulin control the tropomyosin-mediated transition between on and off activity states. Caldesmon binds to actin, tropomyosin, calmodulin, and myosin, of which its interactions with actin are most important. The binding of caldesmon is strongly influenced by tropomyosin. Caldesmon is an inhibitor of actinomyosin ATPase and motility, and both actin binding and caldesmon inhibition are greatly enhanced in the presence of tropomyosin.