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Smooth
muscle. Initiation of contraction
Smooth
muscle cells contract in response to neuronal or hormonal
stimulation, either of which results in an increase in intracellular
calcium as calcium enters through membrane channels or is
released from intracellular storage sites. The elevated
level of calcium in the cell cytoplasm results in force
generation. The rise in the level of intracellular calcium,
however, initiates contraction through a mechanism that
differs substantially from that in striated muscle. In striated
muscle, myosin cross bridges are prevented from attaching
to actin by the presence ofthe troponin-tropomyosin system
molecules on the actin filament (see above Striated muscle).In
smooth muscle, although tropomyosin is present, troponin
is not, which means that an entirely different regulatory
scheme operates in smooth muscle. Regulation of the contractile
system in smooth muscle is linked to the myosin filament;
regulation in striated muscle is linked to the actin filament.
In
order for the smooth-muscle myosin cross bridge to interact
cyclically with actin, a small protein on the myosin molecule,
called the light chain, must be phosphorylated (receive
a phosphate group). This phosphorylation is the result of
a series of interdependent biochemical reactions that are
initiated by the rise in intracellular calcium. For the
cell to relax, the concentration of intracellular calcium
falls, thus inactivating these biochemical processes associated
with light chain phosphorylation. The phosphate molecule
that was added in the previous steps, however, still must
be removed from the light chain so that attachment of the
cross bridge to actin is prevented. Phosphatases are enzymes
in the muscle cell that cleave the phosphate group from
the myosin light chain
