Asked in Premeth Test 13 — Reproduction, Support and Movement, Immunity (15 March 2025) 2025Moderate

Choose the correct, for muscle in relaxed position:

Correct answer: B. Broad H-zone in sarcomere

  • A. Exposed binding sites on actin
  • B. Broad H-zone in sarcomere
  • C. Formation of cross bridges
  • D. Ca²⁺ released in cytosol

Explanation

In the relaxed state of a muscle:Exposed binding sites on actin: This is incorrect. In the relaxed muscle, the binding sites on actin (for myosin) are typically blocked by tropomyosin, preventing cross-bridge formation.Broad H-zone in sarcomere: This is correct. In a relaxed muscle, the H-zone (the region where there is no overlap between actin and myosin filaments) is broad because the actin and myosin filaments are not overlapping much.Formation of cross bridges: This is incorrect in the relaxed state. Cross-bridges are formed when the muscle is stimulated, and the binding sites on actin are exposed due to the action of calcium ions (Ca²⁺).Ca²⁺ released in cytosol: This is incorrect in the relaxed state. In the relaxed muscle, Ca²⁺ is stored in the sarcoplasmic reticulum and not released into the cytosol. Calcium is released only when the muscle is stimulated to contract.

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About Muscle Contraction

Muscle contraction is explained by the sliding filament model, in which myosin heads pull actin filaments within sarcomeres using ATP. The topic covers calcium, troponin, tropomyosin, the neuromuscular junction and relaxation, with attention to how filament length stays almost constant while the sarcomere shortens.

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