power-stroke that pulls the actin filaments toward the center of the sarcomere. These power-strokes will continue and, thus, muscle contraction will continue as long as there is free in the sarcomeres. When nervous stimulation stops, however, the membrane of the terminal cisternae rapidly pumps the remaining free back into the cisternae. Without the presence of ions, tropomin is no longer able to bind to tropomyosin, tropomyosin again blocks myosin's binding sites on the actin proteins, and contraction ceases.
