According to the sliding filament theory, a muscle shortens because
Correct answer: A. actin filaments slide inwards over the myosin filaments
- A. actin filaments slide inwards over the myosin filaments
- B. both actin and myosin filaments themselves shorten
- C. the sarcomere is stretched by the load
- D. the Z lines dissolve and reform further apart
Explanation
Myosin heads bind actin, swivel, and drag the thin filaments towards the centre of the sarcomere, then detach and repeat, so the filaments overlap more without any of them changing length. The idea that the filaments contract is the older, wrong picture the theory replaced, and the unchanged A band disproves it. Each cycle of attachment and release needs one molecule of ATP.
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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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