Why thick filaments are unable to bind with thin filament in a relaxed muscle fiber?
Correct answer: B. Tropomyosin blocks the myosin binding site
- A. Actin blocks the myosin binding site
- B. Tropomyosin blocks the myosin binding site
- C. Troponin blocks the myosin binding site
- D. Tropomyosin blocks the troponin binding site
Explanation
In a relaxed fibre the long tropomyosin strand lies across the myosin binding sites on actin, so no cross bridge can form. Troponin is the tempting answer, but its job is to hold tropomyosin in that position and then pull it aside once calcium binds to it. Actin carries the binding sites rather than blocking them.
This question appeared on the UHS MDCAT 2025 paper, which you can sit online with every answer explained.
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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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