Fig. 1.

The cardiac electrical activity and cardiac ion currents. a The electrical activity of the heart is represented on the surface electrocardiogram (ECG), and results from coordinated action potential generation in individual cardiomyocytes. The electrical activity starts by the spontaneous generation of action potentials in pacemaker cells in the sinoatrial node. Propagation of these action potentials creates an excitation wave through the atria, leading to atrial depolarization. After traveling through the atrioventricular node, the excitation wave reaches the ventricles, and leads to ventricular depolarization. b The cardiac action potential is generated by transmembrane inwardly and outwardly directed ion currents. The inward (depolarizing) sodium and calcium currents are pointed downwards and colored blue. The outward (repolarizing) potassium currents are pointed upwards and colored green