Final answer:
The increase of calcium concentration triggers the binding of calcium ions to troponin, causing a conformational change that shifts tropomyosin and exposes myosin-binding sites on actin. Myosin heads then bind to actin, causing contraction.
Step-by-step explanation:
Increasing the concentration of calcium in the area of the myofilaments directly leads to actin-myosin binding and, consequently, muscle contraction. In striated muscles such as skeletal and cardiac muscle, this process involves calcium binding to troponin, which holds tropomyosin in place and covers myosin-binding sites on the actin filaments. When calcium ions bind to the troponin, tropomyosin moves, exposing the myosin-binding sites. Subsequently, the myosin heads can bind to these exposed sites on the actin filaments, forming cross-bridges. This is followed by the power stroke, where the myosin heads pull the actin filaments towards the center of the sarcomere, which is powered by ATP. As the muscle relaxes, calcium is removed, and the actin-binding sites are re-shielded by tropomyosin, breaking the cross-bridges and allowing the muscles to return to their original state.