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Cross Bridge Cycle

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The five organic molecules and the calcium ions function together in a coordinated manner to cause the thin filament to slide past the thick filament. We will first show an animation of a single cross bridge cycle and then describe this process step-by-step. [Music] As you’ve just seen, cross bridge cycling is a continuous event. For the purposes of easier understanding, however, let’s break it down into six separate steps. 1. The influx of calcium, triggering the exposure of binding sites on actin. 2. The binding of myosin to actin. 3. The power stroke of the myosin head that causes the sliding of the thin filaments. 4. The binding of ATP to the myosin head, which results in the myosin head disconnecting from actin. 5. The hydrolysis of ATP, which leads to the re-energizing and repositioning of the myosin head. 6. The transport of calcium ions back into the sarcoplasmic reticulum. The first step is exposing actins binding sites. When a muscle cell is stimulated, the action potential brings about the release of calcium ions from the terminal cisternae of the sarcoplasmic reticulum. The calcium ions flood into the cytosol and bind to the troponin. This causes tropomyosin to shift, exposing the binding sites on actin. When a binding site on actin is exposed, an energized myosin head can bind to it, forming a cross-bridge. The binding of myosin to actin brings about a change in the conformation of the myosin head, resulting in the release of ADP and inorganic phosphate. At the same time, the myosin head flexes, pulling the thin filament inward toward the center of the sarcomere. This movement is called the power stroke. The chemical energy of ATP has been transformed into the mechanical energy of a muscle cell contraction. In order to disconnect the myosin head from actin, an ATP molecule must bind to its site on the myosin head. The release of the myosin head from actin triggers the hydrolysis of the ATP molecule into ADP and inorganic phosphate. Energy is transferred from ATP to the myosin head, which returns to its high-energy conformation. In the final step, calcium is actively transported from the cytosol back to the sarcoplasmic reticulum by ion pumps. As the calcium is removed, the tropomyosin again covers the binding sites on actin.