muscle contraction

• Contraction stimulation:

  Depolarisation spreads down the T-tubule deep into the fibre through the sarcoplasm and to the sarcoplasmic reticulum. 

  Sarcoplasmic reticulum releases stored calcium ions into the sarcoplasm - triggering a muscle contraction. 

   
• Action of contraction:

  • Action potential at neuromuscular junction causes a wave of depolarisation down the t tubule membrane. 
  • This causes calcium ion channels to open on the sarcoplasmic reticulum releasing calcium ions.
  • Calcium ions bind to the troponin and cause the tropomyosin to move its position alongside the actin filament
  • Exposing myosin binding site 
  • The myosin heads binds to the actin filament and forms cross bridges 

   

   
• Contraction 2:

  • The myosin head moves in a power stroke pulling the actin filaments towards the centre of the sarcomere. 
  • This causes the sarcomere to shorten 
  • ADP and Pi which were attached to the head are released 
  • ATP binds to the myosin head and the cross bridge is broken 
  • The ATP is hydrolysed which causes the head to reset and can rebind again to the actin filament further back - calcium ions activate ATP hydrolase 
  • The head changes its position in another power stroke and pulls the actin filament over the myosin towards the centre of the sarcomere 
  • This is the sliding filament theory 
• Final stage of contraction:

  • As action potentials stop arriving at the muscle, calcium ions are actively transported back into the sarcoplasmic reticulum 
  • Tropomyosin recovers the myosin binding sites in the actin filament 

   
• Evidence for sliding theory:

  • The width of the I band becomes shorter 
  • The sarcomere width becomes shorter 
  • The A band doesn’t change (not the myosin fibres contracting) 
• Energy for contraction:

  ATP is used to break the actin-myosin cross bridge. Hydrolysis of ATP provides energy for the movement of myosin heads and the active transport of calcium ions back into tubules. 

   
• Energy for contraction:

  • Resting muscles only contain enough ATP for 3 - 4 seconds  of intensive energy 
  • Mitochondria generate more ATP through respiration of glucose but full aerobic respiration is slow 
  • Anaerobic respiration is also slow 
• Energy for contraction (anaerobic):

  Muscles fibres contain phosphocreatine which rapidly regenerates ATP from ADP by transferring a phosphate ion to ADP in anaerobic conditions. Catalysed by an enzyme creatine phosphokinase. 

   
• Energy for muscle contraction (anaerobic):

  •  limited supply of phosphocreatine 
  • Using phosphocreatine in anaerobic respiration means intense activity for a length of time