A network of specialized muscle cells
is found in the heart's walls.
These muscle cells send signals to
the rest of the heart muscle
causing a contraction.
This group of muscle cells is
called the cardiac conduction system.
The main parts of the system
are the SA node, AV node,
bundle of His, bundle branches,
and Purkinje fibers.
Let's follow a signal
through the contraction process.
The SA node starts the sequence by causing
the atrial muscles to contract.
That's why doctors
sometimes call it the anatomical pacemaker.
Next, the signal travels to the AV node,
through the bundle of His,
down the bundle branches,
and through the Purkinje fibers,
causing the ventricles to contract.
This signal creates an electrical current that
can be seen on a graph
called an electrocardiogram, or EKG.
Doctors use an EKG to see how well
the cardiac conduction system works.
Any changes on the EKG
can mean serious problems.