Normally, the electrical impulse originates in 2 - 3 separate clusters of electrically coupled cells, which contain approximately 5000 cells each and occupy an area of approximately 0.1 square mm. These cells spontaneously depolarize during diastole to reach the threshold potential. The threshold potential is that transmembrane voltage at which the voltage-sensitive sub-units of the proteins, which comprise the ionic channels, change their configuration. This results in an increased conductance of sodium and calcium ions, the ions responsible for rapid depolarization. An action potential is then generated which propagates throughout the atria.