Contraction of muscle fibers

• Contraction is triggered by calcium ions and during contraction the length of the sarcomere changes.
• Calcium ion concentration in muscle fiber cytosol is regulated by SR
• In a living animal while in resting phase the cytosol is almost without calcium ions around 5x 10 -8M due to
  sequestring of calcium ions SR by calsequestrin binding. The calsequestrin concentration is higher in fast
  contracting muscles.
• During contraction phase the cytosol Calcium ion concentration rises to 5 x 10 -6M .
• There two forms of contraction or could be blend of both
  I) Isotonic contraction – Muscle shortens against constant load.
  II) Isometric contraction – Muscle generates increased tension in an attempt to shorten.
• The necessities for contraction are decided by voluntary or reflex activities from brain or spinal cord
  respectively.
• Action potential is carried from spinal nerve by axon to all its link muscles.
• In the muscles at neuromuscular junction the action potential releases quanta of acetylcholine in the
  synapses located between the axon and muscle fiber .
• The acetylcholine changes the electrical potential of resting muscle fiber membrane and initiates new
  actin potential. This action potential spreads in the muscle through T– tubules.
• T– tubules are touching the SR so it releases the calcium ions .
• Calcium results in movement of troponin and tropomyosin on actin. This in turn enables the myosin heads
  move along thin filament.
• Single twitch occurs when 60 5 of calcium ions are released and takes place few milliseconds after the
  action potential on the muscle fiber.
• The apparent continuos contraction is due to fusion of individual twitches.

Muscle relaxation

• The action potential on the muscle fiber ceases due to destruction of acetylcholine
  ( at neuromuscular junction) by acetylcholinesterase.
• The release of calcium ions stopped by SR and sequestring of calcium ions starts.
• In absence of calcium ions the cofiguration if troponin and tropomyosin changes which blocks the action of
  myosin molecule heads as a result they cannot reach the actin filaments.
• Hence relaxation takes place.
• To remain in relaxed phase muscle should contain high amount of adenosine triphosphate (ATP). It is
  found in form of Mg2+ complex.

                                                                              
 A+M.ADP.Pi A+M.ATP  (Broken actomyosin Complex)

 
   
  


  Where- 

• A= actin
• M=myosin
• ATP= adenosine triphosphate
• ADP = adenosine diphosphate
• Pi = Inorganic phosphorus

Sources of energy

• Single muscle twitch contraction uses 1000 times more energy as compared to that required for reversal
  of membrane potential. There is a definite relationship between force of contraction and ATP hydrolysis.
• The ATP present in muscle is only sufficient to provide few twitches.
• A fast process of resynthesis of ATP is therefore required .
• The sources of energy in a living muscle are shown in the figure.