Contraction cycle of a sarcomere

• A single nervous signal releases Ca2+ ions into the sarcoplasm and initiates the contraction cycle. step 1. ATP hydrolysis • ATP provides the to move myosin molecules back into the energized configuration necessary to perform the power stroke. Step 2. Crossbridge attachment • Myosin temporarily binds to actin. Step 3. Power stroke • When the phosphate is released, the myosin filament moves the actin filament towards the center Of the sarcomere and ADP is released. Step 4. Crossbridge detachment Binding of ATP to myosin crossbridge detaches myosin. • Contraction cycles continue if: • Electrical impulses continue to promote release of more Ca2+ ions. • ATP molecules are regenerated in adequate concentrations. • Once the action potentials Stop, then calcium concentration will drop and the contraction cycle ends. Contraction and movement • Movement by myosin filaments pulls actin filaments, sliding them toward the center of the sarcomere. • The actin filaments are connected to the sarcolemma through dystrophin proteins. When the actin filaments slide towards the center of the sarcomere, causes the sarcolemma to move. • When the sarcolemma shortens then the entire muscle cell undergoes contraction. • When more muscle cells contract at Once, the entire muscle contracts more forcefully. • The force of the contracting muscle is transmitted through the connective tissues linking the muscle cells to the muscle attachment site on the bone or body part. Contraction and movement In summary, the movement of an actin filament sliding along a myosin filament is transmitted through a series of structures to move bones and body parts. • This movement results in the most moveable attachment site of the muscle (the insertion) moving towards the least moveable attachment site of the muscle (the origin).

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