Molecular basis of Sliding filament theory

Molecular basis of sliding filament theory :

The contraction of skeletal muscle generates the force necessary to move the Skeleton. A contraction is triggered by a series of molecular events known as cross Bridge cycle. In a skeletal muscle fibre the function of unit contraction is called sarcomere. A sarcomere shortens when myosin head in thick myofilament forms cross bridges with actin molecules.

1. initiation :

the formation of a cross Bridge is initiated when calcium ions, released from the sarcoplasmic reticulum binds to troponin. This binding causes troponin to change shape. Tropomyosin moves away from the myosin binding site on actin allowing myosin head to bind and form a cross Bridge.

2. activation of myosin head :

Activation of myosin head occurs when ATP binds to myosin head and is hydrolysed to ADP and inorganic phosphate. The energy liberated activate the myosin head forcing it into the curved position.

3. Cross Bridge formation :

The activated myosin head binds to actin forming a cross Bridge. Now inorganic phosphate is released. And the bond between myosin and actin becomes stronger.

4. The power stroke :

ATP is released and the activated myosin head moves sliding the thin filament towards the centre of sarcomere.

5. Crossbridge detachment :

When another ATP binds to the myosin head a link between the myosin head and actin weekens and the myosin head detaches.

6. Reactivation of the myosin head :

ATP is hydrolysed to ADP and inorganic phosphate. The energy released during hydrolysis reactivate the myosin head returning it into the curved position.

As long as the binding sites on actin remains exposed the crossbridge cycle will repeat and as a cycle repeats the thin filaments are pulled towards each other and the sarcomere shortens. The shortening causes the whole muscle to contract. When the calcium ions are actively transported back into the sarcoplasmic reticulum and returns to its original shape allowing tropomyosin to glide over and cover the myosin binding site on acting.

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