Sliding filament theory

Structure of myosin and actin

  • myosin heads

    • hinge enabling movement

    • one site for binding to actin

    • another site for ATP binding

      • provides energy

  • actin filaments

    • sites for myosin head attachment

      • actin-myosin binding sites

    • tropomyosin and troponin proteins attached

      • regulatory role

Muscle contraction

  • involves the actin filaments being pulled closer towards each other and towards the M-line in the centre of the sarcomere

  • the I band and H zone in sarcomere shorten due to increased overlap of actin and myosin filaments

  • A bands remain constant in length

relaxed vs contracted sarcomere

Sliding filament theory

  1. Calcium ions bind to troponin - altering its shape

  2. this change moves tropomyosin away from actin’s binding sites - making them available for myosin so myosin binding sites are exposed

  3. myosin heads attach to these exposed actin filaments - forming actin-myosin cross-bridges

  4. the myosin heads execute a power stroke - pulls the actin filament along and releasing ADP

  5. an ATP molecule binds to the myosin head - leading to its detachment from actin

  6. calcium ions activate myosin’s ATPase activity - breaking down ATP to ADP and phosphate thus releasing energy

  7. this energy resets the myosin head to its original position

  8. the myosin head reattaches to a new actin site further along the filament (10nm)

sliding filament theory

Energy sources for muscle contraction

  • requires ATP

    • generated via:

      • aerobic respiration

      • anaerobic respiration

      • ATP-creatine phosphate system

        • emergency usage

        • takes phosphate from creatine phosphate