Figure 1: Spontaneous electrical activity of rabbit and human sinoatrial node (SAN) pacemaker cells. (a) and (b) Typical examples (a) and average characteristics (b) of action potentials. (c) and (d) Typical examples (c) and average characteristics (d) of the intracellular free concentration (). Data are ; of cells; diastolic potential; potential amplitude; depolarization rate over the 100 ms time interval starting at mV; upstroke velocity; , and potential duration at 20, 50, and 90% repolarization; diastolic ; transient amplitude; d rate of rise of transient; , and transient duration at 20, 50, and 90% decay. .