Sliding mechanism of contraction

SLIDING MECHANISM OF MUSCLE CONTRACTION

"Walk- along theory" of contraction (Sliding mechanism of contraction)

  • In the relaxed state, actin filaments lie adjacent and parallel to the myosin filaments. In contracted state, the actin filaments extending from the z-discs are also pulled inward by head portion of the free ends of myosin filaments.
  • Thus, muscle contraction occurs by sliding action actin over myosin filament.
  • In the relaxed muscle, the troponin -tropomyosin complex covers active binding sites “G” actin filaments and prevents its attachment with myosin heads to cause contraction.

  • In the presence of large amounts of Ca++, troponin-C readily binds with Ca++, the troponin complex undergoes a conformational change and moves the tropomyosin molecules deeper into the groove between two stands of actin strands.
  • This effect “uncovers” the active site “G” actin and the myosin head binds with the active sites and initiates muscle contraction.

Interaction between actin and myosin filaments

  • As soon as the actin filament becomes activated by the Ca++, the heads of the cross bridge of the myosin filaments are immediately attracted to the active sites, “G”actin. This causes sliding movement of actin filament over the myosin filament, which is known as “Walk along” theory (“ratchet” theory) of contraction.
  • ATP molecule binds with heads of the cross-bridges of myosin molecule. ATPase activity in the myosin head causes a partial hydrolysis of ATP to ADP and the inorganic phosphate (Pi). The head extends perpendicular to the actin filament and shows high affinity to actin filament
  • The troponin - tropomyosin complex binds with Ca++. Active sites the “G”actin are uncovered.
  • The bond between the head of the cross-bridges and “G” actin cause a conformational change in the head causes complete hydrolysis of ATP and release of ADP and Pi. The head shows tiling towards the arm of the cross-bridge. This known as the “power-stroke” which pulls the actin filaments in wards. The actin filaments slide over the myosin filament.
  • Once the head of the cross-bridge is tilted, ADP and Pi are released. Now a new molecule of ATP binds with the head of cross-bridge detaches the myosin head from the “G” actin.
  • The new ATP is cleaved to begin the next cycle leading to power-stroke, i.e. the energy ‘cocks’ the head back to its perpendicular position ready to begin another power-stroke cycle again.
  • Similar repeated back and fourth movements of the cross bridge makes the heads walk along the actin filament step by step thus pull the actin filament towards the centre of the sarcomere, until the z-membrane reaches the ends of the myosin filaments.

The role of Ca ++ in muscle relaxation

  • The Ca++ does not remain in the myofibrils for more than few milli seconds. As the action potential is over, the `L' tubules almost immediately reabsorb the Ca++ from the sarcoplasm, and stored it in the cisternae.
  • When the Ca++ concentration drops in the sarcoplasm, the strong affinity of Ca++ for troponin `C' is lost. This causes the movement of tropomyosin molecule to its original position to cover the active binding site, the “G”actin results in relaxation of the muscle.
Last modified: Saturday, 3 December 2011, 5:19 AM