Figure 47.6 Release of Ca2+ from the Sarcoplasmic Reticulum Triggers Muscle Contraction When Ca²⁺ binds to troponin, it changes the conformation of troponin, exposing myosin-binding sites on the actin. As long as binding sites and ATP are available, the cycle of actin and myosin interactions continues and the filaments slide past each other. Note that in the relaxed condition, the myosin heads are energized.