Question

1. the first step of the sliding filament theory is initiated whenever........ calcium is released, causing thick myosin filaments to form cross - bridges with actin filaments. the cross - bridge changes shape and pulls on filaments. the cross - bridge detaches from the actin filaments when atp bonds to the myosin head. the myosin head repeats the process of bonding to the myosin head to repeat the cycle.

Explanation:

The Sliding Filament Theory explains muscle contraction. The first step is calcium release. Calcium binds to troponin, causing a conformational change that exposes the binding sites on actin. This allows myosin (thick filaments) to form cross - bridges with actin (thin filaments). The other options describe subsequent steps in the cycle (e.g., cross - bridge movement, detachment due to ATP, and repetition are later events in the contraction process).

Answer:

calcium is released, causing thick myosin filaments to form cross - bridges with actin filaments.