What is the first step in the cross bridge cycle?

The first step in the cross bridge cycle is the binding of the activated myosin head to actin. This process initiates the cycle of muscle contraction. When the muscle is stimulated, calcium ions are released, which bind to troponin, moving tropomyosin away from actin's binding sites. With the binding sites exposed, the energized myosin heads, which have previously hydrolyzed ATP to ADP and inorganic phosphate (Pi), can attach to actin filaments.

This initial binding of the myosin head to actin is critical because it sets the stage for the subsequent conformational changes in the myosin head, leading to the power stroke that pulls the filaments together, thus causing muscle contraction. This step is essential for the entire cycle of muscle contraction to proceed, making it the first and fundamental action in the cross bridge cycle.