Actin provides binding sites for myosin, myosin generates the force that pulls actin, troponin detects calcium ions, and tropomyosin regulates access to the myosin-binding sites. Together, they produce contraction through the sliding filament mechanism.
| Protein | Role in muscle contraction |
|---|---|
| Actin | Forms the thin filaments and contains binding sites to which myosin heads attach. It is pulled toward the centre of the sarcomere during contraction. |
| Myosin | Forms the thick filaments. Its projecting heads bind to actin, form cross-bridges, hydrolyse ATP, and perform power strokes that pull the thin filaments inward. |
| Troponin | Binds calcium ions released from the sarcoplasmic reticulum. This causes troponin to change shape and move tropomyosin. |
| Tropomyosin | Covers the myosin-binding sites on actin when the muscle is relaxed. It moves away from these sites when troponin binds calcium ions. |
When a muscle fibre is stimulated, calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm. Calcium binds to troponin, changing the troponin complex's shape. This shifts tropomyosin away from the binding sites on actin.
Energized myosin heads can then attach to the exposed sites, forming cross-bridges. The release of inorganic phosphate initiates the power stroke, during which the myosin head pivots and pulls actin toward the centre of the sarcomere. ADP is then released.
A new ATP molecule binds to myosin, causing it to detach from actin. ATP is hydrolysed, re-energizing and repositioning the myosin head. This cross-bridge cycle repeats while calcium ions and ATP remain available, shortening the sarcomere without shortening the individual actin or myosin filaments.
A common misconception is that actin itself actively contracts. It does not: actin slides past myosin as myosin heads generate force.
Exam technique: For an HL explanation, state each protein's role and present the sequence from calcium binding to cross-bridge formation, power stroke, ATP-dependent detachment, and head reactivation. Link the mechanism explicitly to sarcomere shortening.
