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Tropomyosin must change conformation, uncovering the myosin-binding site on an actin molecule and allowing cross-bridge formation. This can only happen in the presence of calcium, which is kept at extremely low concentrations in the sarcoplasm.
Sliding filament theory
According to the sliding filament theory, muscle proteins slide past one another to produce movement during muscular contractions.
The sliding filament theory postulates that during muscle contraction, the actin (thin filaments) and myosin (thick filaments) of muscle fibers glide past each other while remaining at essentially constant lengths.
The myosin binding sites at the Actin filament are covered with the tropomyosin molecules which undergo conformational changes due to the attachment of the troponin to the calcium ions released from the sarcoplasmic reticulum during contraction and thus contraction of muscles takes place.
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