Action potentials generated by different parts of conduction system

The sinoatrial (SA) and atrioventricular (AV) nodes generate a slow action potential, mediated by calcium ions. In comparison, the tissues of the atria, ventricles, and the His-Purkinje system generate a fast action potential mediated by sodium ions. Sequential activation of these structures results in the characteristic waveforms visible on the surface electrocardiogram (ECG). The AV node and bundle of His are small structures; as a result, no electrical activity is recorded on the surface ECG during their activation.