Final answer:
Calcium ions released during muscle contraction interact with troponin, leading to the exposure of actin binding sites for myosin heads, thus initiating contraction.
Step-by-step explanation:
During muscle contraction, the calcium concentration in myofibrils increases. Calcium interacts with troponin to initiate the contraction. This interaction is part of the excitation-contraction coupling mechanism in muscle fibers. When an action potential reaches a muscle fiber, it results in the release of calcium ions (Ca++) from the sarcoplasmic reticulum (SR). Calcium binds to troponin, causing a conformational change in the troponin-tropomyosin complex, which in turn exposes the binding sites on actin filaments. This allows myosin heads to attach to these sites and perform a power stroke, which is fueled by adenosine triphosphate (ATP), thereby sliding the thin filaments over the thick filaments and leading to muscle fiber shortening.