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.

igneous rocks are formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.  

the composition depends on the source magma. there are almost a thousand types of composition found till now.  

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