muscles simplified

• myosin= thick filaments

  actin= thin filaments
• contraction caused by nerve impulse
• nerve impulse triggers action potential in muscle fibre of a neuromuscular junction
• • myosin heads detachd from actin
  • actin has tropomyosin wrapped around it
  • actin has binding sites for myosin heads
• • calcium ions arrive at actin and myosin as a result of action potential inside muscle fibre
  • release of calcium ions by sarcoplasmic reticulum
• • for myosin head to be able to attach to binding site on actin, calcium ions need to arrive
  • this triggers tropomyosin to change shape and move away from the binding sites
• without calcium ions


  • tropomyosin blocks binding sites on actin
  • prevents myosin heads from attaching to actin
• in muscle fibre, calcium ions released from sarcoplasmic reticulum
• myosin attaches to binding site and forms a bridge between actin and myosin
• myosin attaches to actin binding site = actinomyosin bridge
• myosin head has an active site for ATP - myosin head hydrolyses ATP into ADP and pi
• myosin heads are enzymes - myosin heads are examples of ATPases
• sliding mechanism steps

  1. calcium ions trigger tropomyosin to move away from binding sites
  2. myosin head binds to binding site on actin and forms actinomyosin bridge
  3. ADP and Pi are released from myosin head
  4. ATP binds to myosin head and myosin head detaches from actin
  5. myosin head hydrolyses ATP which allows myosin head to return to starting position
• which event triggers the power stroke of the myosin head?

  • ADP and Pi are released
• when ATP is hydrolysed - myosin head returns to starting position