The parameters G
ss and G
sa, which determine the pacing ability, are used to construct the parameter plane. G
s,as = 5 nS for all ASs if they exist. Notations PND, PD and NP denote the SAN behaviors. A: The situation without an AS. When either G
ss is small or G
sa is small, impulse propagation fails (see the gray regions labeled “PND”). The inset is a magnification of the dashed squared frame. If G
ss is very large, the pacing of the SAN fails (see the black region labeled “NP”). B: A single AS (11–32) is added to the system. C: Two ASs (11–32) and (23–34) are added. D: Four ASs (17–32), (20–34), (23,36) and (26,38) are added. In all cases with AS(s), the PND regions are completely eliminated.