The action potential initiated in the SA
node spreads throughout the myocardium, passing from cell to cell by way of gap
junctions. The spread throughout the right atrium and from the right atrium to
the left atrium does not depend on fibers of the conducting system. The
conduction through atrial muscle cells is rapid enough that the two atria are
depolarized and contract at essentially the same time.
In the ventricles
The spread of the action potential to
the ventricles is more complicated and involves the rest of the conducting
system. The link between atrial depolarization and ventricular depolarization
is a portion of the conducting system called the atrioventricular node, located
at the base of right atrium. The action potential spreading through the muscle
cells of the right atrium causes depolarization of the AV node.
After leaving AV node, the impulses
enters the wall – the interventricular septum – between two ventricles. This pathway has
conducting-system fibers termed the bundle of His.
Within the interventricular septum the
bundle of His divides into right and left bundle branches, which eventually
leave the septum to enter walls of both ventricles. These fibers in turn make
contact with Purkinje fibers, large conducting cells that rapidly distribute
the impulse throughout much of the ventricles.
Finally , the Purkinje fibers make
contact with ventricular myocardial cells , by which the impulse spreads
through the rest of the ventricles. The rapid conduction along Purkinje fibers
and diffuse distribution of these fibers causes depolarization of all right and
left ventricular cells more or less simultaneously and ensure a single
contraction.