Calcium ions start muscle contraction by binding to
Correct answer: C. troponin, which moves tropomyosin off the binding sites on actin
- A. the myosin heads
- B. tropomyosin directly
- C. troponin, which moves tropomyosin off the binding sites on actin
- D. ATP, activating it
Explanation
In a relaxed fibre tropomyosin lies across the myosin binding sites of actin. Calcium released from the sarcoplasmic reticulum binds troponin, troponin changes shape and pulls tropomyosin aside, and only then can cross bridges form. Calcium therefore acts as the switch rather than as the energy source, and pumping it back into the reticulum is what causes relaxation.
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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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