Figure 4

Induction of sudden fast rhythm by V _os and ThV _os on lowering high [K⁺] _o . In panels A and B, [K⁺]_o was decreased while the same SAN was driven. The bars mark the peak of V_os. The downward arrows mark the ThV_os that initiated an AP followed by the fast rhythm. The horizontal arrow points to the stimulus artifact of the last driven AP. The traces labeled with a dot and a rhombus are shown at higher gain in inset 1. In inset 2, the circle labels a ThV_os that missed the threshold during the subsiding of the fast rhythm. In panel C, the upward oblique arrows indicate driven APs that initiated runs of fast discharge. The downward oblique arrows indicate ThV_os that initiated an AP followed by runs of fast discharge.