Free Muscle Contraction MCQs with Answers

11. Rigor mortis occurs due to

• A. Excess ATP
• B. Lack of ATP
• C. Excess calcium
• D. Lack of calcium

Explanation: Without ATP, myosin heads cannot detach from actin, causing persistent muscle contraction (rigor mortis).

12. The A-band contains

• A. Only actin
• B. Only myosin
• C. Both actin and myosin
• D. Neither

Explanation: The A-band (dark) contains the entire length of myosin filaments and overlapping actin filaments.

13. Why thick filaments are unable to bind with thin filament in a relaxed muscle fiber?

• 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.

14. Once an action potential reaches the membrane of a skeletal muscle fiber, what is the first event that occurs in the contraction process?

• 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.