Arrival of calcium ions at myofibrils leads to binding to troponin (in thin filaments).
Binding of calcium to troponin causes actin and troponin to change shape, exposing binding sites on the thin filaments.
Cross bridges from thick filaments bind to the exposed sites on the thin filaments.
Calcium ions activate the breakdown of ATP that is bound to myosin
Myosin acts as an enzyme to catalyze the reaction ATPà ADP + PO42- + energy
Part of the energy released is used to move cross bridges and the rest is released as heat
Cross bridges shift, moving the attached thin filaments toward the sarcomere center=power stoke
Another ATP molecule binds to myosin, providing the energy needed to cause the cross bridge to break its connection to the thin filament.
The cross bridge returns to its upright position.
If calcium is still available, the cross bridge will attach to another thin filament binding site, and the cycle begins again.
The cycle of cross bridge attachment, shift, release, and reattachment to a new site repeats as long as ATP and calcium ions are available, shifting the thin filaments toward the sarcomere center.
As a result, the Z lines of the sarcomere approach each other and the sarcomere shortens in length.
The simultaneous shortening of all sarcomeres forming a myofibril cause the myofibril to shorten in length.