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Excitation/contraction
coupling
Immediately
following depolarization of the plasma membrane and the
ensuing action potential, the heart muscle develops force
and then relaxes. The surface action potential is transmitted
to the interior of the muscle by means of the T tubular
system. Calcium ions enter the muscle cell during the plateau
phase of the action potential (phase 2), triggering the
release of calcium from the terminal cisternae of the sarcoplasmic
reticulum. Calcium diffuses to the myofilaments and combines
with the troponin-tropomyosin system (associated with the
thin actin filaments), producing a conformational change
that allows actin and myosin to interact. This interaction
in the presence of ATP results in cross-bridge cycling and
ATP hydrolysis. The force developed in the whole muscle
is the sum of all the forces developed by each of the millions
of cycling cross bridges of the muscle. The free calcium
ions in the cytosol are removed by an energy-dependent calcium
uptake system involving calcium ion pumps located in the
longitudinal sarcoplasmic reticulum. These calcium pumps
lower the concentration of free calcium in the cytosol,
resulting in the dissociation (release) of calcium from
the troponin-tropomyosin system. The troponin-tropomyosin
system is then transformed back to its original state, preventing
myosin and actin from interacting and thus causing relaxation
of the muscle. At the same time calcium is extruded from
the cell into the surrounding medium.
