11. In resting skeletal muscle, contraction does not occur because
a. there is very little ATP in the cytoplasm
b. there is very little myosin in the cell
c. there is very little calcium in the cytoplasm
d. myosin is inactivated
Why Skeletal Muscle Does Not Contract at Rest: CSIR NET Life Sciences Explained
In resting skeletal muscle, contraction is prevented primarily due to low cytoplasmic calcium levels, which keep tropomyosin blocking myosin-binding sites on actin.
Correct Answer
The right choice is c. there is very little calcium in the cytoplasm. At rest, calcium ions stay sequestered in the sarcoplasmic reticulum, ensuring tropomyosin covers actin’s binding sites and blocks cross-bridge formation between actin and myosin. This maintains muscle relaxation until a nerve impulse triggers calcium release for contraction.
Option Analysis
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a. there is very little ATP in the cytoplasm: Incorrect, as skeletal muscle maintains sufficient ATP at rest for immediate use in contraction and relaxation processes. ATP powers myosin detachment and calcium pumps, with low levels only causing fatigue during prolonged activity.
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b. there is very little myosin in the cell: Incorrect, since skeletal muscle fibers contain abundant myosin thick filaments arranged in sarcomeres for force generation.
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c. there is very little calcium in the cytoplasm: Correct, as cytoplasmic Ca²⁺ concentration remains below 10⁻⁷ M at rest, preventing troponin binding that would expose actin’s sites.
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d. myosin is inactivated: Incorrect, because myosin remains active and ATP-ready; inhibition stems from blocked actin access, not myosin itself.
