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Multiple Choice
During which phase of a muscle twitch does calcium bind to troponin, initiating cross-bridge formation?
A
Latent period
B
Contraction phase
C
Relaxation phase
D
Refractory period
Verified step by step guidance
1
Understand the phases of a muscle twitch: A muscle twitch consists of three main phases—latent period, contraction phase, and relaxation phase. The refractory period is a separate concept related to the time during which a muscle fiber cannot respond to a new stimulus.
Review the latent period: This is the initial phase after a stimulus is applied. During this time, the action potential travels along the sarcolemma, and calcium ions are released from the sarcoplasmic reticulum into the cytoplasm. However, cross-bridge formation has not yet begun.
Examine the contraction phase: During this phase, calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the binding sites on actin. This allows myosin heads to attach to actin, forming cross-bridges and initiating muscle contraction.
Understand the relaxation phase: In this phase, calcium ions are actively pumped back into the sarcoplasmic reticulum, troponin returns to its original shape, tropomyosin covers the binding sites on actin, and the muscle fiber relaxes.
Clarify the refractory period: This is the time following an action potential during which the muscle fiber cannot be stimulated again. It is unrelated to calcium binding or cross-bridge formation.