Answer:
Calcium ions
Explanation:
For muscle contraction to occur, Troponin, a regulatory protein which regulates the tropomyosin, is activated by calcium ions, and is kept at extremely low concentrations in the sarcoplasm. If these calcium ions are released from the sarcoplasm by signals from neurotransmitters, calcium ions bind to troponin, causing conformational changes in troponin that allows tropomyosin to uncover the myosin-binding sites on actin. Tropomyosin a regulatory protein on the myofibril (composed of actin and myosin filaments) must change its conformation and uncover the myosin-binding site (it hides the binding site) on an actin molecule, thereby allowing cross-bridge formation between the actin and the myosin head. Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering muscle contraction.
