ABSTRACT
To the point, only ventricular muscle cell action potentials have been discussed. But action potentials differ among the several types of heart muscle myocytes. Purkinje myocyte pacemaker activity can keep the ventricles beating, albeit at a very slow rate, if action potentials from the atria cannot get through to the ventricles. Sinoatrial (SA) node cells depolarize in the interval between action potentials and the most negative voltage level, the maximal diastolic potential, is -50 to -60 mV as compared with -80 to -90 mV in ventricular muscle cells. Diastole refers to the interval between heartbeats. The less negative level in SA node cells than in ventricular muscle cells is partly related to the limited expression of K1 channels in SA node cells. Diastolic depolarization during phase 4 brings the transmembrane potential to threshold and is responsible for repeated, rhythmic SA node cell action potentials.
