L5.3.2.2: Schematic Representation of Contraction Mechanism

• In a relaxed muscle fiber, the regulatory proteins forming part of the actin myofilaments block and prevent myosin binding. When an action potential (AP) sweeps along its sarcolemma and a muscle fiber is excited, calcium ions (Ca2+) are released from intracellular storage areas (the sacs of the sarcoplasmic reticulum).

  
• The flood of calcium acts as the final trigger for contraction, because as calcium binds to the regulatory proteins on the actin filaments, the proteins undergo a change in both their shape and their position on the thin filaments. This action exposes myosin-binding sites on the actin, to which the myosin heads can attach, and the myosin heads immediately begin seeking out binding sites. The binding of myosin to actin constitutes cross bridge formation.

  
• Using energy from ATP, free myosin heads are “cocked,” much like an oar ready to be pulled on for rowing. Myosin attachment to actin causes the myosin heads to snap (pivot) toward the center of the sarcomere in a rowing motion. When this happens, the thin filaments are pulled slightly toward the center of the sarcomere. ATP provides the energy needed to release and recock each myosin head so that it is ready to attach to a binding site farther along the thin filament