The heart conduction system propagates electrical activity through the heart muscles that pump blood. It is organized by the spontaneous pacemaker activity of the sinoatrial node (SAN), which sends pulses to other cardiac fibers that relay them from cell to cell. All heart muscle, called cardiac myocyte, is electrically excitable and responds to the pulses from the SAN. The phases of the heart's relaxation and contraction all have specific electrical profiles that are represented graphically by the electrocardiogram (ECG).
Heart muscle is innervated by the neurons of the autonomic nervous system, especially the vagus nerve. Cardiac myocytes, like neurons, communicate with each other through electrical signals called action potentials. A heartbeat is a cycle in the heart conduction system that cause the cardiac chambers to relax and contract. For each heartbeat, electrical stimulation opens ionic channels in the cell's membrane, allowing positively charged ions in to depolarize the cell, triggering an action potential. Another set of ion channels depolarize the cell with an influx of potassium, following a brief period called the resting state.
In the wall of the heart's right atrium, the sinoatrial node regulates the heart rate by sending out electrical impulses to the cardiac myocytes. The SAN is called the pacemaker since it regulates the heart conduction system through cells that fire at paced intervals. Sometimes direct signals from the autonomic nervous system or hormone surges change the pace of signaling during exercise and stress. While all cardiac myocytes convey electrical activity, the pacemaking cells of the sinoatrial node actively produce synchronized, spontaneous pulses.
Impulses from the SAN travel to the atrioventricular node, the point where further conduction across the heart begins. There is a brief delay here between contractions so that the blood in the atria flows into the ventricles. The heart conduction system uses bundled conduction fibers to carry the SAN's signals through the heart. The right and left atria contract first as current spreads to the ventricles, which depolarize next. The entire sequence of depolarization for one heartbeat takes less than a third of a second.
The electrical signals of the heart conduction system are measured by an electrocardiogram that plots myocyte activity as a wave on a graph. Signals passing through the left and right atria appear on the ECG as the P wave, while the activity of the sinoatrial node is represented by the PR segment. The QRS wave of the ECG corresponds to depolarization of the ventricles, while its T wave signifies their depolarization. Many disorders of the heart conduction system, called arrhythmias, produce irregular wave patterns that can be observed on the ECG. For this reason, some cardiac disorders are named according to their abnormal ECG readings.