What is the last step in the excitation-contraction coupling process?

The last step in the excitation-contraction coupling process is the formation of cross bridges between actin and myosin. This step occurs after a series of events that begin with an action potential triggering the release of calcium ions from the sarcoplasmic reticulum. When calcium is released, it binds to troponin, which causes a conformational change that moves tropomyosin away from the binding sites on actin filaments.

Once the binding sites are exposed, myosin heads can attach to actin, initiating the power stroke of muscle contraction. This interaction is crucial as it allows the sliding filament mechanism to occur, leading to muscle shortening and contraction. The formation of these cross bridges is therefore critical in the overall process of muscle contraction.

In contrast, re-entry of calcium into the cistern, binding of calcium to tropomyosin, and the end of the action potential are all earlier steps that facilitate contraction but do not represent the culmination of the process as the formation of cross bridges does. Thus, the act of cross bridge formation is indeed the final and essential step that directly leads to muscle contraction.