| 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." |
