The Contraction Cycle
During the onset of the contraction cycle, the SR releases Ca++ into the cytosol. The Ca++ binds to troponin then moves tropomyosin away from the myosin-binding sites on actin. Once the sites are free, the contraction cycle begins consisting of four steps:
Steps
1. ATP Hydrolysis
The myosin head includes: an ATP-binding site, and ATPase (enzyme that hydrolyzes ATP into ADP and a phosphate group). This reaction reorients and energizes the myosin head. ADP and the phosphate group are still attached to the myosin head.
2. Crossbridge formation – The Attachment of Myosin to Actin
The myosin head binds to an available myosin-binding site on actin to form a crossbridge and releases the phosphate group produced by ATP hydrolysis.
3. Power stroke
During the power stroke, the site on the crossbridge where ADP is still bound opens as the myosin head (crossbridge) rotates and releases the ADP molecule. The myosin head (crossbridge) rotates toward the centre of the sarcomere, sliding the thin filament past the thick filament toward the M-line.
4. Detachment
Following the power stroke, the crossbridge remains firmly attached to actin until it binds another molecule of ATP. The crossbridge detaches when ATP binds to the ATP -binding site on the myosin head.
The process continues…..
The contraction cycle repeats as the myosin ATPase hydrolyzes the newly bound ATP molecule. This cycle continues as long as ATP is available and the Ca++ levels around the thin filaments are sufficiently high. The cross bridges continue to rotate back and forth with the crossbridges in one thick filament attaching and detaching about 5X per second. Movement of the crossbridges applies the force that draws Z-discs toward each other, and the sarcomere shortens. During contraction, the Z-discs pull on neighbouring sarcomeres and the whole muscle fibre shortens. Just prior to shortening, elastic components in the surrounding fascia stretch, pulling tendons taut and exerting tension on bone, creating movement at a joint. The contraction cycle continues as muscle contraction continues, and more myosin heads bind to actin filaments as required to sustain contraction.
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