Virtually all electrically active cells in the heart have the capacity to spontaneously and rhythmically open ion channels in the cell membrane. This capacity to periodically depolarise their cell membrane in the absence of an external stimulus is termed automaticity. As the membrane spontaneously depolarises a threshold is reached and an action potential is generated which propagates out to surrounding cells, ultimately triggering depolarisation of their membranes. In this way, once one region depolarises, a wave of depolarisation spreading through the entire organ is generated. The cells of the sinoatrial node (SA) node have the highest rate of periodic depolarisation (the highest automaticity) and, therefore, under normal circumstances, the SA node drives the heart rate.