How do contractile proteins produce muscle contraction?

Contractile proteins such as actin and myosin cause muscles to contract by sliding past one another. This movement is driven by a process called the cross-bridge cycle, where myosin heads attach to actin, pull it, release, and reattach to the next spot. This repeated, ATP-dependent cycle shortens the muscle fiber and produces the force needed for movement.

The Mechanism of Muscle Contraction

1. Calcium ions are released: A nerve impulse triggers the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum into the muscle cell.
2. Tropomyosin moves: The calcium binds to troponin, causing tropomyosin to shift and expose binding sites on the actin filament.
3. Myosin heads form cross-bridges: Myosin heads, carrying ADP and phosphate (Pi) from a previous cycle, attach to the exposed actin sites to form cross-bridges.
4. Power stroke: The myosin heads release ADP and phosphate, pivoting to pull the actin filaments toward the center of the sarcomere. This action — the power stroke — generates the actual force of contraction.
5. Detachment: A new ATP molecule binds to the myosin head, making it detach from the actin filament.
6. Re-cocking: ATP breaks down into ADP and phosphate, using that energy to reset (or “cock”) the myosin head into its high-energy position, ready for the next cycle.
7. Cycle repeats: As long as ATP and Ca²⁺ are available, this cycle continues — pulling the actin filaments closer and shortening the muscle.
8. Relaxation: When the nerve signal stops, Ca²⁺ returns to the sarcoplasmic reticulum. Tropomyosin covers the binding sites again, and the muscle relaxes.