7&8 FA Structure Skeletal Muscle & Sliding Filament Theory

• Skeletal muscle


  Smallest contractile unit of the muscle which is responsible for the muscle shortening. Contains actin and myosin.
• Parts of skeletal muscle structure


  • Epimysium
  • Fascicle
  • Perimysium
  • Muscle fibre
  • Endomysium
  • Myofibril
• Muscle contraction


  1. Neurochemical stimulation releases calcium ions into the muscle fibre

  2. Myosin crossbridges attach and reattach at different times, pulling on the actin filaments to create movement and maintain tension

  3. Actin moves into the centre of the sarcomere, shortening the myofibril and causing the actin + myosin filaments to almost fully overlap

  4. As each sarcomere shortens, so does the total length of each muscle fibre

  5. When the contraction finishes, the myosin and actin filaments return to a relaxed position
• Myofibril


  Small, thread like strands that run the length of each muscle fibre, made up of sarcomeres
• Sarcomere


  The smallest unit of skeletal muscle that can contract
• Sarcoplasmic reticulum


  Network of tubules and channels surrounding the myofibril, in which calcium is stored - important in muscle contraction
• Actin


  Thin myofilament
• Myosin


  Thick myofilament, shaped like a golf club with a head (crossbridge) that projects outwards and attaches to actin
• Crossbridge


  Projection on the myosin filament that attaches to actin
• Sliding filament theory


  Theory used by biomechanists to explain the shortening of the sarcomere and resulting muscle contraction
• Sliding filament theory


  1. Neurochemical stimulation releases calcium ions into the muscle fibre

  2. Myosin crossbridges attach and reattach at different times, pulling on the actin filaments to create movement and maintain tension (requires ATP breakdown)

  3. Actin moves into the centre of the sarcomere, shortening the myofibril and causing the actin + myosin filaments to almost fully overlap

  4. As each sarcomere shortens, so does the total length of each muscle fibre

  5. When the contraction finishes, the myosin and actin filaments return to a relaxed position
• Concentric contraction


  H zone gets smaller, Z lines come closer together, actin filaments overlap
• Eccentric contraction


  H zone gets bigger, Z lines move further apart, actin filaments move further apart
• Z line


  • Found at either end of the sarcomere
  • Move closer together during concentric contraction
  • Move further apart during eccentric contraction
• H zone


  • Decreases in length during concentric contraction
  • Increases in length during eccentric contraction