Locomotion And Movement

In a relaxed muscle fibre the protein tropomyosin masks the active myosin-binding sites on actin, preventing cross-bridge formation. NCERT Class 11, Chapter 20 (Locomotion and Movement) describes how an action potential triggers release of calcium ions from the sarcoplasmic reticulum, and these calcium ions bind to the subunit of troponin. This binding causes a conformational shift that moves tropomyosin away from the binding sites, exposing them so that myosin heads can attach to actin and initiate the cross-bridge cycle. ATP binding to actin is incorrect because ATP binds to the myosin head, not to actin, and its role is in detachment and energising the head rather than unmasking sites. Sodium influx into the sarcomere is wrong; while sodium drives the action potential along the membrane, it is calcium, not sodium, that directly controls the contractile machinery. Phosphorylation of tropomyosin is not the mechanism described in NCERT for skeletal muscle activation. Hence calcium binding to troponin is correct. A regulatory plausibility check supports it: contraction must be switched on by the same calcium signal released upon excitation, linking electrical stimulation to mechanical response.