Figure 1. The cardiac cycle starts when the sinoatrial node fires an action potential which spreads like an electrical impulse through the atria and – via the atrioventricular node – the ventricles. As the impulse spreads over the myocardium, it activates the cells, which responds by contracting. Moreover, the impulse is actually an electrical impulse, which gives rise to the classical ECG waveforms. Activation of the atria gives rise to the P-wave and activation of the ventricles gives rise to the QRS-complex. The T-wave reflects the recovery (repolarization) of the ventricles.