Final answer:
In muscle contraction, Ca ions bind to troponin, causing tropomyosin to slide away from the actin-binding sites, allowing myosin heads to bind and form cross-bridges.
Step-by-step explanation:
To initiate muscle contraction, tropomyosin has to expose the myosin-binding site on an actin filament to allow cross-bridge formation between the actin and myosin microfilaments. The first step in the process of contraction is for Ca to bind to troponin so that tropomyosin can slide away from the binding sites on the actin strands. This allows the myosin heads to bind to these exposed binding sites and form cross-bridges.