For contraction to occur, myosin must bind with actin, but the tropomyosin in the structure of the actin filaments blocks myosin's binding sites on the actin proteins during muscle rest. When stimulated by a nerve impulse, however, the troponin in the structure of the actin filaments shifts the location of tropomyosin so that myosin can bind with actin. This change in the structure of the actin filaments, that allows actomyosin to form and, thus, contraction to occur, is regulated by the presence of ions. When a motor nerve triggers an action potential in a muscle cell by releasing a stimulatory transmitter substance, the voltage differential between the exterior and interior surfaces of the muscle cell reverses itself. The reversed polarization is instantly transmitted throughout the entire muscle fiber by the cell's T tubules, deep invaginations of the muscle fiber's cell membrane that penetrate the cell's interior reaching every sarcomere. One T tubule lies over each Z line. On either side of each T tubule are the terminal cisternae of the sarcoplasmic reticulum, a second but entirely separate system of tubules that also penetrates the muscle cell reaching every sarcomere. The sarcoplasmic reticulum is extremely sensitive to changes in polarization. When polarization is reversed, the terminal cisternae become permeable to ions and release their large stores of these ions into the sarcomeres. The ions bind to troponin enabling the troponin to bind with tropomyosin and shift it from myosin's binding sites on the actin proteins. Once bound to actin, myosin can break down ATP and release the energy needed for its
