16. Skeleton and Movements - part 02- Structure of myosin and actin filaments and Mechanism of muscle contraction,relaxation

 

16. Skeleton and Movements - part 02- Structure of myosin and actin filaments and Mechanism of muscle contraction,relaxation


Structure of myosin and actin filaments :

Myosin filament :


  • Each myosin filament is a polymerized protein. 
  • Many monomeric proteins called meromyosins constitute one thick filament.
  • Myosin molecule consists of two heavy chains (heavy meromyosin/HMM) coiled around each other forming double helix.
  • One end of each of these chains is projected outwardly. It is known as cross bridge. This end is folded into a globular protein mass called myosin head.
  • Two light chains are associated with each head (Total 4 light chains/light meromyosin/LMM).
  • Myosin head has a special ATPase activity. It can split ATP to produce energy. 
  • Myosin contributes 55% of muscle proteins.


Actin filament :


  • It is also a complex type of contractile protein. 
  • It consists of three different components.
           i. F actin : Forms backbone of actin filament.
                                       -double stranded protein. 
                                       -Each strand is composed of polymerized G actin molecules.
                                       -One ADP molecule is attached to each G actin molecule.

          ii. Tropomyosin : The actin filament contains two additional protein strands that are
                                       polymers of tropomyosin molecules.
                                       -Each strand is loosely attached to an F actin. 
                                       -In the resting stage,tropomyosin physically covers the active binding sites
                                       for myosin of the actin strand.

           iii. Troponin : It is a complex of three globular proteins, which are attached approximately 2/3                                    rd distance along each tropomyosin molecule.
                                  -It has affinity for actin, tropomyosin and calcium ions.
                                  -The troponin  complex is believed to attach the tropomyosin to the actin.
                                  -The strong affinity of troponin for calcium ions is believed to initiate the                                                contraction process.


Mechanism of muscle contraction :


  • Sliding filament theory was putforth by H.E. Huxley and A.F. Huxley. It is also called walk along theory or Ratchet theory.
  • Interaction between actin and myosin is the basic cause of muscular contraction. 
  • Actin filaments are interdigitated with myosin filaments. (like the crossing of fingers of two hands)
  • The head of the myosin is joined to the actin backbone by a cross bridge forming a hinge joint. 
  • From this joint, head can not tilt in forward and backward directions. This movement is an active process which requires use of ATP.
  • Myosin head contains ATPase activity.It can derive energy by the breakdown of ATP molecule. 
  • This energy can be used for the movement of myosin heads.
  • During contraction process, the myosin heads gets attached to the active site of actin filaments and pull them inwardly, so that actin filaments slide over the myosin filaments. This results in the contraction of muscle fibre.


Physiology of muscle relaxation :

  •  When the muscle is relaxed, the active sites remain covered with tropomyosin and troponin complex. 
  • Due to this, myosin cannot interact with active site of actin and therefore contraction cannot occur.
  • When an action potential (impulse) comes to muscle through motor end plate, it spreads throughout the sarcolemma of the myofibril.
  • The transverse tubules of sarcoplasmic reticulum releases large number of calcium ions into sarcoplasm. 
  • These calcium ions interact with troponin molecules. This interaction inactivates troponin-tropomyosin complex. This leads to change in the structure of tropomyosin.
  • As a result, it gets detached from the active site of actin (F actin) filament. Thus active site becomes uncovered. 
  • head of the myosin cleaves the ATP and derives energy.
  • Using this energy, myosin gets attached to the uncovered active site of actin and results in the formation of actomyosin complex.
  • The myosin heads are now tilted backwards and pull the attached actin filament inwardly. This results in contraction of the muscle fibres

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