Mechanism of Muscular Contraction :

The Sliding Filament Theory

Mechanism of Muscular Contraction :

The Sliding Filament Theory


Muscular contraction involves the two contractile proteins, actin and myosin, in a mechanical series of events called the sliding filament theory of contraction. Each myosin filament is surrounded by six actin filaments. The myosin filaments contain cross-bridges, which are tiny extensions that reach toward the actin filaments.

When an impulse from the motor nerve reaches the muscle cell, it stimulates the entire fiber, creating chemical changes that allow the actin filaments to join with myosin cross-bridges. The binding of myosin to actin via cross-bridges results in a release of energy that causes the cross-bridges to swivel, pulling or sliding the myosin filament over the actin filament. This sliding motion causes the muscle to shorten (contract), producing force. Once the stimulation ceases, the actin and myosin filaments separate, lengthening the muscle to its resting length and the contraction ceases. This cross-bridge activity explains why the muscular force generated depends upon the initial length of the muscle prior to contraction. The optimal length for muscular contraction is resting length (or slightly greater), because all of the cross-bridges can connect with the actin filaments, slowing maximal tension development.

The contractile force decreases when the length of the muscle prior to contraction is significantly shorter than the resting length (i.e. already partially contracted). This is due to the fact that in an already shortened muscle the actin and myosin filaments overlap to a great degree, which leaves fewer cross-bridges open to "pull" on the actin filaments. With fewer available cross-bridges less tension and force can be produced.

When the muscle lengthens too far beyond resting length the force potential will also be small, because the actin filaments are too far away from the cross-bridges to the able to join and shorten the muscle.

Contractile force diminishes as the muscle length becomes either shorter or longer than resting length. The highest force output occurs when the contraction begins at a joint angle of approximately 110-120 degrees (resting length).

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