What molecular event terminates muscle contraction in skeletal muscle?

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Question: What molecular event terminates muscle contraction in skeletal muscle?

Option 1: ATP hydrolysis by myosin. This step powers the cross-bridge cycle and is essential for contraction to proceed, not for its termination. Once ATP binds and myosin detaches from actin, another cycle could begin if calcium remains high, so this is not the terminating event.

Option 2: Decrease in cytosolic calcium due to active uptake into the sarcoplasmic reticulum. This is the key terminating event. As calcium is pumped back into the sarcoplasmic reticulum by SERCA pumps, calcium levels fall, troponin returns to its inhibitory state on actin, and the actin-myosin interaction is halted, leading to relaxation and cessation of contraction.

Option 3: Detachment of acetylcholine from its receptor. This occurs at the neuromuscular junction and stops the neural signal that would otherwise trigger muscle contraction, but it does not directly terminate contraction once calcium is already elevated inside the muscle fiber. It prevents initiation of new contractions rather than terminating a current one.

Option 4: Breakdown of troponin by proteases. While troponin is involved in regulating contraction, its proteolytic breakdown is not the normal mechanism by which contraction ends. Troponin must remain functional to regulate calcium sensitivity; proteolysis is not the standard termination pathway for contraction.

In summary, the primary molecular mechanism terminating skeletal muscle contraction is the reduction of cytosolic calcium via active reuptake into the sarcoplasmic reticulum, which reverses the calcium-activated state that allows cross-bridge cycling.

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