Figure 1. Termination modes of spontaneous activity in a mathematical model of the mouse sinoatrial node cell.

(A) Irregular activation and periodic skipped beat runs are apparent prior to termination as [Na⁺]_o is decreased stepwise from 70 mM to 58 mM. (B) In contrast, a gradual decline in regular activity occurs as g_Kr is decreased from 0.37 to 0.31 times its control value. (C-E) Representative spontaneous action potentials under (C) control conditions, and during stepwise decrease in (D) [Na⁺]_o and (E) g_Kr (corresponding time periods marked by red bars in panels A and B).