diagram of cell membraneHeart muscle cells are polarized when at rest.  This means that the net charge density of the fluid inside and outside of the cells is different, because ion concentrations are different on either side of the cell membranes.  The potential inside of the cells is approximately -90 mV with respect to the potential outside of the cell membranes.  At rest we find an excess an of positive sodium ions (Na+) outside of the membranes.
A typical cell membrane is relatively impermeable to sodium ions, but the stimulation of a muscle cell causes an increase in its permeability to Na+.  More sodium ions enter than leave the cell.  This causes a change in the cell potential (depolarization).  The potential inside of the cell becomes positive with respect to the potential outside of the cell membrane and then returns back to -90 mV, as ions pumps re-establish the resting potential.  The resulting voltage pulse is called the action potential.  In muscle cells, the action potential causes a muscle contraction.

Depolarization and repolarization of the entire heart can be measured on the skin surface.  Such a measurement is called an electrocardiogram (EKG or ECG).  Depolarization of the heart leads to the contraction of the heart muscles and therefore an EKG is an indirect indicator of heart muscle contraction.

The cells of the heart will depolarize without an outside stimulus.  This property of cardiac muscle tissue is called automaticity, or autorhythmicity.  The cells responsible for initiating and distributing the signal to contract are part of the conducting system of the heart, a network of specialized cardiac muscle cells that initiate and distribute electrical impulses.

electrical signal path in the heartThe group of cells that depolarizes fastest is called the pacemaker or sinoatrial (SA) node.  These cells are located in the right atrium of the heart. The cells of the atria are all physically connected and therefore the depolarization of the cells of the SA node causes all the cells of both atria to depolarize and contract almost simultaneously.

The atria and ventricles are electrically isolated from each other by connective tissue that acts like the insulation on an electric wire.  Therefore, the depolarization of the atria does not directly affect the ventricles.  However, there is another group of cells in the right atria called the AV node that will conduct the depolarization of the atria down a special bundle of conducting fibers (called the AV bundle or Bundle of His) to the ventricles.

In the muscle wall of the ventricles are the Purkinje fibers, a special system of muscle fibers that bring depolarization to all parts of the ventricles almost simultaneously.  This signal then causes the ventricular contraction.

There is a time delay inherent in this conduction process starting with the AV node and atria contraction and ending with ventricular contraction.  This time delay is very important because the atria must contract before the ventricles.  If all the chambers contracted at the same time, then blood could not flow through the heart because the AV valves would prevent blood flow between the atria and ventricles.

electrical fibers in the heartWhen one portion of the heart is polarized and an adjacent portion is depolarized, then an electrical current moves through the body.  This largest current flows when one half of the heart tissue is polarized and one half is not polarized.  The current decreases when the ratio of polarized tissue to non-polarized tissue is different from one-to-one.  The change in these current can be measured and displayed.  The EKG represents the sum of all the actions potentials from the heart detected on the surface of the body.  It does not measure the mechanical contractions of the heart directly.
A sample EKG voltage vs. time plot is shown below.

 example EKG

The small P wave accompanies the depolarization of the atria.  The atria begin contracting around 100 ms after the start of the P wave.
The QRS complex appears as the ventricles depolarize.  This is a relatively strong electrical signal because the mass of the ventricular muscle is much larger than that of the atria.  The ventricles begin contracting shortly after the R wave.

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