Triggering muscle contraction

• What is muscle contraction triggered by?

  an influx of calcium ions
• 1)What happens when an AP from a MOTOR neurone stimulates a MUSCLE CELL?


  It DEPOLARISES the SARCOLEMMA

  • depolarisation spreads down the T-tubules to the sarcoplasmic reticulum
• 2) what does the depolarisation spreading down the T-Tubules cause?


  The sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm
• 3) what do the calcium ions do?


  Bind to a protein ATTACHED TO TROPOMYOSIN, causing the protein to CHANGE SHAPE

  • This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament
• 5) what is the name of the bond formed when a myosin head binds to an actin filament?


  actin-myosin cross bridge
• 6) what do calcuim ions do?

  also activate the enzyme ATP hydrolase which hydrolyses ATP to provide the energy needed for muscle contraction
• 7) what does the energy released from ATP cause ?


  cause the myosin head to bend, which pulls the actin filament along (in a sort of rowing action)
• 8) what does another ATP molecule do?


  Provides the energy to break the actin-myosin cross bridge, so the myosin head detaches from the actin filament after it is moved
• 9) what does the myosin head then do?


  Reattaches to a different binding site further along the actin filsment

  • a new actin-myosin cross bridge formed and cycle is repeated
• 10) what form?

  many cross bridges form and break very rapidly, pulling the actin filament along - which shortens the sarcomere, causing the muscle to contract
• How long will the cycle continue?


  The cycle will continue as long as calcium ions are present
• Calcium ions- their role in triggering muscle contraction:

  • Moves tropomyosin/troponin
  • to reveal binding sites on actin
  • allows myosin heads to bind with actin
  • activates ATPase
• Decrease of conc of calcium ions can result in decrease in force of muscle contraction. How? AQA Past exam question

  • Less/no tropomyosin moved from binding site
  • Fewer actinomyosin bridges formed
  • Myosin head doesn't move , myosin head does not pull actin filaments. Less ATP hydrolase activation.
• How ATP produced continually by anaerobic respiration by converting pyruvate to lactate? AQA Exam Past Paper question

  • Oxidises reduced NAD
  • So glycolysis continues
• Role of glycogen granules in skeletal muscle? AQA Past exam question

  • As a store of glucose / to be hydrolysed to glucose
  • For respiration/ to provide ATP
• Roles of calcium ions & ATP in the contraction of a myofibril. AQA Past exam question.

  1. calcium ions diffuse into myofibrils from sarcoplasmic reticulum
  2. calcium ions cause movement of tropomyosin on actin
  3. this movement causes exposure of the binding sites on actin
  4. myosin heads attach to binding sites on actin
  5. hydrolysis of ATP on myosin heads causes myosin heads to bend
  6. the bending pulls actin molecules.
  7. A new ATP molecule attached to each myosin head- this causes myosin heads to detach from actin sites.