The cross bridge cycle is responsible for the contraction of muscles. The sarcomere is what actually contracts. A muslce is made up of myosin and actin. Actin is the thin filament and myosin is the thick filament. The muscle recieves a stimulus from a nerve cell that results in the release of calcium from an internal storage within the muscle. The increase of calcium concentration within the microfibules is what allows the cross bridge cycle to take place. The cross bridges cannot form without calcium because calcium is what allows the active sites to become exposed and without exposed acctive sites, the bridges cannot form. Calcium binds to troponin and as a result troponin changes it's shape. This shape change alters the positioning of the tropomyosin which exposes the active sites. The cross birdges then form. In the presence of calcium, the myosin binds to the actin. The next step is the powerstroke which is when the myosin head pivots, pulling the actin to the center of the sarcomere. ADP is released in this step. In the next step, ATP binds which triggers detachment. The cross bridge detaches as a result of atp hydrolysis. During the last step, ATP hydrolysis the myosin head is coked. The use of energy from ATP to ADP is used to rebind. This is one full cylce of the cross bridge cycle.
There are two factors that are necessary in cross bridge formation. These two factors are an elevated concentration of calcium because cross bridges cannot form without calcium changing the shape of troponin. An adequate supply of ATP is also necessary becaue this proces requires energy.