Once an action potential reaches the membrane of a skeletal muscle fiber, what is the first event that occurs in the contraction process?
Correct answer: C. Calcium is released from the sarcoplasmic reticulum
- A. Actin and myosin form a cross-bridge
- B. Calcium is pumped back into the sarcoplasmic reticulum
- C. Calcium is released from the sarcoplasmic reticulum
- D. ATP is hydrolyzed by troponin
Explanation
The action potential travels down the T tubules and triggers the sarcoplasmic reticulum to release calcium, which then binds troponin and moves tropomyosin off the binding sites so cross bridges can form. Cross bridge formation therefore comes second, not first, which is the trap in the first option. Pumping calcium back is what ends the contraction.
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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