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Sliding Filament Theory of Skeletal Muscle Contraction

  
The sliding filament theory states that actin and myosin do not change length during skeletal muscle contraction.  Instead, actin filaments slide inward over myosin and pull the Z lines toward the sarcomere center.  The end result of this is a shortening of the muscle fiber. 

How is this accomplished?

Myosin is made up a series of rodlike structures.  The rodlike structures are, in turn, composed of a molecule called light meriomyosin.  The upstrokes, or crossbridges, of the myosin filament are composed of heavy meriomyosin.  A unidirectional hinge attaches the upstroke to the light meriomyosin filaments and is crucial in the sliding of the two filaments (as explained by the sliding filament theory). 

The heavy meriomyosin upstroke is topped by a globular head that is associated with the enzyme myosin ATPase and a molecule of ATP.  Together, the myosin ATPase and the ATP provide the energy that is necessary to perform skeletal muscle contraction.  When the myosin ATPase cleaves the ATP into ADP, a free floating phosphate, heat and free energy, the myosin globular head is able to bind to the active site of the actin molecule.  For a complete explanation of the contractile process of skeletal muscle, see the article "Sequence of Muscular Contraction and Relaxation."

 

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