The spread of depolarization from atria to ventricles occurs at the AV node. The AV node is the only point of electrical contact between the atria and ventricles and so is the only way an action potential can spread to the ventricles.

Action potentials are conducted relatively slowly through the AV node tissue. This delay (about 0.1 sec) permits the atria to become fully depolarized and to contract well before the subsequent ventricular contraction. This results in a more complete filling of the ventricles.