What causes muscle fibers to stop contracting?

• A. ATP or Ca2+
• B. Excess oxygen
• C. Continuous nerve stimulation
• D. ATP depletion energy stores

Answer: (A)

Contraction needs two key players to work in concert: calcium and ATP. Calcium release exposes the binding sites on actin by moving the troponin-tropomyosin complex away from the myosin heads, allowing cross-bridge formation and force generation. But for contraction to end, calcium must be removed or the system must lose ATP. If calcium is withdrawn, the binding sites are blocked again and cross-bridges can’t form, so the fiber relaxes. If ATP is unavailable, the myosin heads can’t detach after a power stroke and the pumps can’t resequester calcium into the sarcoplasmic reticulum, so the cycling stops and the muscle can’t maintain contraction. In short, without either calcium signaling or ATP, muscle fibers stop contracting. Excess oxygen won’t trigger stopping, and continuous nerve stimulation would tend to keep them contracting rather than stop them.