Automaticity in cardiac cells.

Automaticity is the result of dynamic changes in transmembrane currents during electrical diastole. It is readily demonstrated in most cardiac cell types. In all four cardiac cell types studied by the voltage clamp technique (Purkinje, ventricular, atrial, and sino-atrial node fibers), the major change detected during diastolic depolarization is a decrease in outward current. This decrease in a repolarizing current (largely potassium mediated) permits an inward current (sodium and/or calcium mediated) to depolarize the cell.All four cardiac cell types appear to possesess a time-dependent potassium conductance which controls the decrease in outward current over the ?70 to ?30 mV potential range. Purkinje fibers manifest an additional conductance which is responsible for automaticity in this type of cell at potentials between ?100 and ?70 mV.