In the presence of ATP, the heads are unbound from the actin and much less ordered. It is only in the presence of productive interaction with actin filaments that products are released. On productive interation with actin, the head binds, and releases P i . Cardiac Muscle Cardiac muscle cells as striated muscle. Contraction is triggered by calcium release from SR, and is similar to skeletal muscle contraction.
Smooth muscle is rather different from striated muscle contraction.
Additionally smooth muscle has no permanent thick filaments, but assembles them transiently when its myosin is phosphorylated. Smooth muscle is evolutionarily more primitive than striated muscle, and the mechnism of EC coupling in smooth muscle has been valuable in understanding the evolution of signalling mechanisms. This myosin light chain ordinarily blocks the interaction of myosin with actin, but on binding calcium undergoes a conformation change that modes it out of the way, and allows contraction to proceed. In vertebrate smooth muscle, the switch for contraction is again an increase in calcium, coming in from extracellular stores. This calcium does not bind to a component of myosin, but instead to soluble calmodulin.