When does cross bridge cycling end?

Cross bridge cycling is a fundamental process in muscle contraction where myosin heads bind to actin filaments to produce force. This cycle is highly dependent on the presence of calcium ions and ATP. The correct answer highlights that cross bridge cycling ends when sufficient calcium is actively transported back into the sarcoplasmic reticulum.

When a muscle fiber is stimulated, calcium is released from the sarcoplasmic reticulum into the cytoplasm, allowing myosin heads to engage with actin. For muscle relaxation to occur and for cross bridge cycling to stop, calcium must be pumped back into the sarcoplasmic reticulum. This action reduces the calcium concentration in the cytoplasm, leading to the detachment of myosin from actin and effectively stopping the contraction process.

The restoration of calcium to the sarcoplasmic reticulum is critical because without calcium, the binding sites on actin are not exposed, preventing further cross bridge formation and thus halting the muscle contraction cycle. This process is essential for muscle relaxation and is a fundamental aspect of muscle physiology.