When does the cross bridge cycle start?

The correct answer states that the cross bridge cycle starts when calcium ions (Ca2+) from the sarcoplasmic reticulum bind to troponin. This is a crucial step in muscle contraction. When the muscle fiber is stimulated by a motor neuron, it triggers the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm of the muscle cell. These calcium ions then bind to troponin, a regulatory protein that is part of the thin filaments (actin).

The binding of calcium to troponin causes a conformational change that moves tropomyosin, another regulatory protein, away from the myosin binding sites on actin filaments. This exposure allows the myosin heads to attach to the actin filaments, forming cross bridges. Once the cross bridges are formed, the cycle of muscle contraction can begin, involving the binding of ATP, the power stroke, and the release of the myosin head.

This sequence is essential for understanding how muscle contraction is regulated at the molecular level, underscoring the importance of Ca2+ in the initiation of the cross bridge cycle.