Sliding Filament Theory

• structure of myosin


  hinged, globular heads with binding sites for actin and ATP; filament formed by tails of several hundred myosin molecules
• structure of actin


  molecules wrapped around each other, myosin binding sites blocked by tropomyosin, so filaments cannot slide without impulse
• what is the structure of a myofibril


  actin molecules wrapped around each other, with a molecule of tropomyosin, which is held in place by troponin
• 1 - Ca2+ ions move in from sarcoplasmic reticulum, due to electrical impulse, and bind to troponin molecules
• 2 - troponin changes shape, which displaces the tropomyosin and exposes the myosin binding sites
• 3 - heads of myosin attach to the newly exposed binding sites on actin, called crossbridging
• 4 - myosin heads change position, causes the actin filament to slide, ADP released
• 5 - ATP binds to myosin heads, causing them to detach from the actin
• 6 - ATP then hydrolysed into ADP and a phosphate molecule, allowing the heads to resume resting position
• creatine phosphate is a reserve supply of phosphate to bind with ADP, which is quickly used up
• why is glucose needed for skeletal muscle contraction

  • glucose respired to form ATP
  • ATP needed to break cross-bridges between actin and myosin
  • ATP hydrolysed into ADP and Pi - to reset myosin heads
  • ATP for active transport of Ca 2+ ions back into sarcoplasmic reticulum