4.1.3

Explain how skeletal muscle contracts by the sliding filament theory.

An action potential causes the release of calcium ions form the sarcoplasmic reticulum. The calcium ions bind to troponin causing the troponin and tropomyosin to move and uncover binding sites on the actin for the myosin to attach to. The myosin head then attaches to the actin myofilament forming a cross bridge. With the release of a phosphate group, the power stroke takes place, sliding the actin towards the M line. The ADP molecule is then released. As a new ATP molecule attaches to the myosin head, the link between the actin and myosin weakens and the cross bridge detaches. As the ATP is split into ADP and a phosphate group, the myosin head is energized and again attaches to the actin, forming a cross bridge as the cycle repeats.