| 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." |
