In the heart, the atria receive blood and pump it into the ventricles, which then pump it back into the body. The atrioventricular (AV) node is a group of cardiac cells located in between the atria and the ventricles that stalls communication between these chambers to allow time for the atria to fully contract before ventricular contraction begins. This delay in communication increases the efficiency of contraction by ensuring that all blood entering the atria is pumped out. A secondary role of the AV node is that it will regulate contraction if the sinoatrial (SA) node that normally initiates contraction fails.
Heart contraction begins when the SA node fires an electrical impulse from its origin at the top of the right atrium. This electrical impulse propagates along the atria, initiating a slow-moving wave of contraction toward the ventricles. As the atria contract, there is an increase in pressure that propels blood into the ventricles. Simultaneously, this electrical impulse rapidly conducts through a chain of non-contractile cells to the AV node. Conduction is slowed through this node, delaying communication to the ventricles to allow for completion of the slower-moving contractile wave. This ensures that all blood is emptied from the atria before the ventricles begin to contract.
After the impulse passes the AV node, it travels down through non-contractile cells in the center of the ventricles to the apex of the heart. The impulse is then propagated upward, initiating a wave of contraction that squeezes blood from the ventricles into the body. The AV node plays a critical role in synchronizing the atria and the ventricles, and if conduction through this node is blocked, it results in a critical condition known as complete heart block.
In a healthy heart, the SA node functions as the pacemaker, firing a contraction-initiating impulse approximately 70 times per minute. This frequency of contraction is referred to as the heart rate. In some instances, the SA node will stop firing impulses, and the AV node is required to take over as the pacemaker. In this case, the AV node will fire an impulse that will cause the SA node to fire an impulse and initiate contraction. The impulses generated, however, are less frequent than the impulses of the SA node, at about 50 fires per minute.
When the AV node takes over as the pacemaker, it does not in itself initiate the contractile wave. Rather, the electrical impulse that it automatically generates will cause the SA node to fire the contraction-initiating impulse in reaction. Though an individual who has this condition will experience a lower heart rate, this situation ensures that heart failure will not result from failure of the SA node.