Contraction of skeletal muscle

• Muscle contraction occurs due to the movement of the actin filament over the myosin filament and the formation of actinomyosin bridges
• Contracted sarcomeres

  • smaller I-band
  • smaller H-zone
  • same A-band
  • shorter distance between M-lines
  • shorter distance between Z-lines
• Structure of myosin

  • myosin heads protrude
  • tails wrap around one another to form filaments
• Structure of actin

  • made from long chains of globular proteins which are coiled around one another in a helix
  • troponin - site where calcium ions bind
  • tropomyosin is a long thread wound around the actin covering the myosin-binding sites
• Role of calcium ions

  • changes position of tropomyosin and uncovers myosin-binding site
  • allows myosin heads to bind to actin -> cross-bridge formation
  • activates ATP hydrolase
• Role of ATP

  • release of myosin head from actin binding site
  • releases energy for recocking of myosin head and reabsorption of calcium ions from the sarcoplasm into the sarcoplasmic reticulum
• Role of phosphocreatine

  • used to produce ATP very quickly
  • replaces ATP that has been hydrolysed
  • amount limited, and varies between fibres
  • ADP + phosphocreatine -> ATP + creatine
• What initiates actinomyosin bridge formation?

  Action potential arrives at T-tubules
• What role do calcium ions play in muscle contraction?

  They bind to troponin, changing tropomyosin's shape
• What happens to tropomyosin when calcium ions bind to troponin?

  Tropomyosin changes shape and exposes binding sites
• What forms when myosin heads bind to actin?

  Actinomyosin cross-bridge
• What occurs during the power stroke in muscle contraction?

  Myosin head pulls actin over the myosin
• What breaks the actinomyosin cross-bridge?

  ATP hydrolysis
• What happens to the myosin head after ATP hydrolysis?

  It recocks back to its original shape
• What does the myosin head do after recocking?

  Re-binds to actin further along the filament
• What are the steps involved in actinomyosin bridge formation and muscle contraction?

  1. Action potential triggers calcium release.
  2. Calcium binds to troponin, changing tropomyosin shape.
  3. Tropomyosin exposes myosin-binding sites on actin.
  4. Myosin head binds to actin, forming cross-bridge.
  5. Myosin head pulls actin (power stroke).
  6. ATP hydrolysis breaks cross-bridge, releasing myosin.
  7. Myosin head recocks to original shape.
  8. Myosin re-binds to actin further along.
  9. Calcium ions are removed, tropomyosin returns to original shape.
• What happens to calcium ions when muscle stimulation ceases?

  They are actively removed from the sarcoplasm
• What is the effect of tropomyosin returning to its original shape?

  It blocks the myosin-binding sites on actin