The cross bridge cycle starts when select the best answer. view available hint(s) select the best answer. atp binds to troponin and is hydrolyzed to adp and pi ca2+ from the sarcoplasmic reticulum binds to troponin acetylcholine diffuses away from the synaptic cleft ca2+ is actively transported into the sarcoplasmic reticulum ca2+ from the sarcoplasmic reticulum binds to tropomyosin

The answer is;  Ca2+ from the sarcoplasmic reticulum binds to tropomyosin

When the ca2+ ions bind to the tropomyosins on the actin filament, the tropomyosins are displaced and this exposed the myosin binding sites on the actin filament. The ADP + Pi (attached to the myosins heads), from the previous cycle, is released allowing the cocking of the myosin heads hence pulling on the actin filament. ATP then binds on the ATP-site on the myosin heads casing a release of the myosin heads from their site in the actin. This also hydrolyzes the ATP.  This is one cycle of the cross-bridge cycle.