Figure 1. Schematics of action potential waveforms and underlying ionic currents in human ventricular (left) and atrial (right) myocytes.

Note that relative inward (downward) and outward (upward) current densities and waveforms, estimated from voltage-clamp data and modeling studies, in non-diseased ventricular and atrial are shown. The voltage-gated inward Na⁺ (Nav) and Ca²⁺ (Cav) currents in human atrial and ventricular myocytes are similar. In contrast, there are multiple types of outward K⁺ currents, particularly Kv currents, contributing to atrial and ventricular action potential repolarization. In addition, the time- and voltage-dependent properties of the various Kv currents are distinct. Differences in the densities and in the detailed time- and voltage-dependent of the repolarizing Kv and Kir channels contribute to differences in the waveforms of atrial and ventricular action potentials. The genes encoding the pore-forming (α) subunits underlying the various cardiac ion channels are also indicated (on the right).