During which stage of muscle contraction does actin bind to myosin?
a. Coupling
b. Relaxation
c. Discharging
d. Excitation
ANS: A
During coupling, depolarization of the transverse tubules occurs. This triggers the release of cal-cium ions from the sarcoplasmic reticulum, exposing binding sites on the actin molecule. In the presence of calcium, actin is free to bind with myosin. Relaxation is the last step. It begins as cal-cium ions are actively transported back into the sarcoplasmic reticulum, removing ions from in-teraction with troponin. Discharging is not a step in the process.
Excitation is the first step of muscle contraction and it begins with the spread of an action poten-tial from the nerve terminal to the neuromuscular junction.
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