Fig. 4

a Averaged transfer functions from sympathetic nerve stimulation (SNS) to heart rate (HR), coherence functions (Coh.), and the corresponding step responses before (thin lines) and after (bold lines) the administration of ivabradine. All plots, excepting the coherence in the frequency range above approximately 0.2 Hz, were not significantly different between before and after ivabradine. b Averaged transfer functions from SNS to R–R interval (RRI), coherence function, and the corresponding step response before (thin lines) and after (bold lines) the administration of ivabradine. Ivabradine increased dynamic gain of the RRI transfer function and augmented the negative step response of RRI. c The mean transfer functions calculated for the HR response and the RRI response are only displaced along the ordinate in the gain plots, both before (top panel) and after (bottom panel) ivabradine. bpm beats/min. The solid and dashed lines in panels a and b indicate mean and mean ± SE values (n = 7)