Excitation-contraction coupling is the process linking the electrical stimulation to the mechanical contraction of the muscle fiber. When an action potential propagates along the muscle sarcolemma, it travels down invaginated folds termed T-tubules and induces a conformational change in the voltage-gated calcium channels. This change further triggers the opening of the calcium-release channels in the flanking terminal cisternae of the sarcoplasmic reticulum. As a result, calcium ions are rapidly released from the sarcoplasmic reticulum into the sarcoplasm and, eventually, diffuse into the muscle myofibrils. Within the myofibrils, the calcium ions bind to the regulatory protein, troponin, on the actin filaments, causing it to move tropomyosin away from myosin-binding sites. After binding sites are exposed, the myosin heads use ATP hydrolysis to form cross-bridges with the actin. Next, the myosin heads pull the actin filaments past the myosin filaments toward the center of the sarcomere. As the filaments interact, the sarcomeres of the myofibril shorten, pulling the ends of the muscle fiber closer and causing muscle contraction.