The Sliding Filament Model

The Sacromere
1. Striped appearance of voluntary muscle under the microscope is different when muscles are relaxed and when they are contracted.
2. The span of one Z line to the next.
3. In a relaxed state, 2.5µm in length
4. Z lines closer together during contraction
Two Types of Protein Filament in Contraction
1. Actin
  1. Thin filaments are two strands made mostly of actin (F Actin), coiled around each other (twisted)
  2. Each strand composed of G Actin subunits
  3. Tropomyosin molecules coil around the F Actin to reinforce it.
  4. A troponin complex is attached to each Tropomyosin molecule.
  5. Each troponin complex consists of three polypeptides
    1. One binds to actin, one to Tropomyosin and one to calcium ions
2. Myosin
  1. Thick filaments are bundles of the protein myosin
  2. Each myosin molecule consists of a tail and two protruding heads
  3. Each thick filament consists of many myosin molecules whose heads stick out from opposite ends of the filament
The Power Stroke (Muscle Contraction)
1. 1) Myosin head groups attach to the surrounding actin filaments forming a cross-bridge.
  1. 2) The head group then bends, causing the thin filament to be pulled along and overlap more with the thick filament (Power Stroke). ADP and Pi are released.
    1. 3) The cross bridge is then broken as new ATP attaches to the myosin head.
      1. 4) The head group moves backwards as the ATP is hydrolysed to ADP and Pi. It can then form a cross bridge with the thin filament further along and bend again.

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The Sliding Filament Model

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