Action
potentials initiate the contraction of our hearts. The contraction of
cardiac muscle is triggered in a different fashion then contraction of the skeletal
muscle, which is caused by the electrical impulses from motor nerve terminals
located at individual muscle fibers. These motor nerve terminals are networked
and coordinated by the central nervous system.
Contrary to the skeletal
muscle cells, the heart cells have no motor nerve terminals attached to them.
If each of the millions of cardiac muscle cells were to have a separate nerve,
the heart would be a huge organ. The cells of the heart remedy this lack of
motor nerve terminals by using their own type of stimulus. They communicate
with each other directly by passing electrical impulse to their neighbors through
special channels called gap-junctions. This enables the heart to be stimulated
at only one location, with the electrical impulse passing on to the rest of
the heart muscle, causing it to contract.
What exactly happens to the
muscle cells when they are stimulated? Inside each cardiomyocyte
are hundreds of myofibrils which are thin, elongated structures. Each myofibril,
in turn, consists of thin filaments and thick filaments. Each of the thin filaments
is composed of a protein called actin. Each of the thick filaments is composed
of a protein called myosin. Each myosin filament is composed of about 200 myosin
molecules. Each myosin molecule contains what is called a myosin head. Inside
each cardiomyocyte there are compartments filled with calcium. The action potential
causes these compartments to release the calcium into the cell. This calcium
allows myosin heads to bind to actin filaments and pull them by a process called
a power stroke. That is how action potential causes the individual muscle cells
to contract.
This
movie shows a calcium wave passing through a cardiomyocyte. The cell was loaded
with a special dye which changes color from blue to red when the concentration
of the calcium inside the cell raises. The result is a visible "calcium
wave" which passes through the cell.
This movie
shows
calcium being released as a result of an action potential. The release of calcium
is fast and uniform because the electrical impulse rapidly spreads through the
cell membrane.
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