Overview
This lecture explains the sliding filament theory, detailing the molecular mechanism of skeletal muscle contraction, focusing on the roles of sarcomeres, thick and thin filaments, and the specific steps of contraction and relaxation.
The Sliding Filament Theory
- Sliding filament theory describes how muscles contract by filaments sliding past each other, shortening the muscle.
- Contraction involves sarcomeres, the individual contractile units within muscle fibers.
Structure of Sarcomeres
- Each sarcomere contains thick filaments (mainly myosin) and thin filaments (mainly actin, along with tropomyosin and troponin).
- Thick filaments are bundles of 200–300 myosin proteins, with heads that bind to actin and ATP.
- Thin filaments consist of actin (provides myosin binding sites), tropomyosin (covers binding sites), and troponin (controls access to binding sites).
Mechanism of Contraction
- At rest, tropomyosin blocks myosin-binding sites on actin; troponin is attached to tropomyosin.
- Muscle contraction starts when calcium is released from the sarcoplasmic reticulum and binds to troponin.
- Calcium binding changes troponin's shape, moving tropomyosin off actin binding sites.
- Myosin heads bind to exposed actin sites, forming a cross bridge.
- Power stroke: myosin head pivots, pulling actin toward sarcomere center as ADP and phosphate are released.
- ATP binds to myosin, breaking the cross bridge; myosin head resets as ATP is hydrolyzed.
- The cycle repeats as long as calcium and ATP are available.
Muscle Relaxation
- Relaxation occurs when calcium is pumped back into the sarcoplasmic reticulum.
- Calcium removal causes troponin to restore tropomyosin's blockage of actin binding sites.
- No cross-bridges form, and the muscle relaxes with sarcomere returning to original length.
Key Terms & Definitions
- Sarcomere — The basic unit of muscle contraction, composed of thick and thin filaments.
- Myosin — Protein making up the thick filament; forms heads that bind actin and ATP.
- Actin — Main protein of the thin filament, contains binding sites for myosin.
- Tropomyosin — Protein that blocks myosin binding sites on actin.
- Troponin — Protein that binds calcium and moves tropomyosin off actin binding sites.
- Power stroke — Movement of myosin head that pulls actin filament inward.
- Cross bridge — Connection formed between myosin head and actin during contraction.
- Sarcoplasmic reticulum — Organelle that stores and releases calcium in muscle fibers.
Action Items / Next Steps
- Review book diagram 10.11 for a summary of contraction steps.
- Watch posted or recommended videos on the sliding filament theory for visual understanding.