sliding filament model

• myosin structure:

  • have heads which enable movement
  • one site for binding to actin
  • another site for ATP binding
• actin filament structure:

  • sites for myosin head attachment - actin-myosin binding site
  • at rest binding site blocked by tropomyosin
  • when the muscle contracts, myosin heads form bonds with actin filaments known as cross bridges
• sliding filament theory:

  1. action potential causes depolarisation and the release of calcium ions
  2. this moves tropomyosin away exposing the binding site
  3. Myosin heads attach to the exposed actin, forming cross-bridges.
  4. myosin heads execute a power stroke, pulling the actin filament along
  5. ATP binds to the myosin , causing a release of actin
  6. ATP is hydrolysed and myosin heads return to resting
  7. if calcium ions is returned to sarcoplasmic reticulum, muscles relax
  8. if calcium ions remain available the cycle repeats
• 1. Impulse arrives at neuromuscular junction, calcium ions diffuse in through voltage gates calcium channels
  2. causes vesicle to fuse with presynaptic membrane and releases acetycholine into the gap by exocytosis
  3. acetycholine binds to receptors on the sarcolemma causing depolarisation (sodium ions in)
  4. depolarisation wave travels down T tubule
  5. this causes calcium ions to be released from sarcoplasmic reticulum
  6. calcium ions bind to protein in muscle which leads to the contraction
  7. acetycholinesterase breaks down acetycholine so that contraction only occurs when impulse arrives
• neuromuscular junction

  • unidirectional
  • only excitatory
  • connect motor neurones to muscle
  • end point of action potential
  • acetycholine binds to receptor on muscle fibre membranes