Depolarization occurs in the four chambers of the heart: both atria first, and then both ventricles.
# The sinoatrial (SA) node on the wall of the right atrium initiates depolarization in the right and left atria, causing contraction, which corresponds to the P wave on an electrocardiogram.
# The SA node sends the depolarization wave to the atrioventricular (AV) node which—with about a 100 ms delay to let the atria finish contracting—then causes contraction in both ventricles, seen in the QRS wave. At the same time, the atria re-polarize and relax.
# The ventricles are re-polarized and relaxed at the T wave.
This process continues regularly, unless there is a problem in the heart.