Calcium Explanation: Calcium is the key that alters troponin’s shape and moves tropomyosin away from the actin binding site, allowing the myosin head to form a cross-bridge with the actin filament. Myosin is the thick filament that attaches to the actin binding site creating a cross-bridge. Tropomyosin is the cover on the actin binding site preventing cross-bridge formation. Troponin can be considered the lock that holds tropomyosin on top of the actin binding site.