Calcium ions. Ca2+ binds to troponin, which removes tropomyosin from the myosin binding site on actin.
The myofilaments that are involved in the contraction and relaxation of the muscle are actin and myosin. The myosin attachment site is located on the thin filaments of actin, but the site is blocked by the proteins tropomyosin and troponin.
The tropomyosin's shape is altered when calcium ions in the muscle bind to troponin molecules. This alteration reveals the myosin's attachment point, and the myosin subsequently binds troponin to generate Powerstroke and cause a muscle to contract.
Thus, Calcium ions is correct.
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