Biophysical Basis for Three Distinct Dynamical Mechanisms of Action Potential Initiation
Figure 3
Comparison of spikes initiated through different dynamical mechanisms.
(A) Spikes initiated through a QSC or SNIC bifurcation exhibit different spike amplitude variability. Data are from 2D models stimulated with noisy Istim (σnoise = 10 µA/cm2). V-nullclines are shown for rest (red) and for one stimulus intensity (blue) although Istim varies continuously during stimulation. Spikes initiated through a QSC exhibit variable amplitudes (yellow shading) because variations in Istim affect the V-w trajectory: trajectories starting close to the quasi-separatrix (produced by Istim fluctuations just exceeding rheobase) produce smaller spikes than trajectories starting further from the quasi-separatrix (produced by larger Istim fluctuations). Spikes initiated through an SNIC bifurcation exhibit little variability (pink shading) because all trajectories follow the invariant circle once the heteroclinic trajectories (green curves) fuse at the moment of the SNIC bifurcation to form a single homoclinic orbit. Histogram shows distribution of voltage maxima; maxima above cutoff (*) are considered spikes. Distributions differed significantly between cell classes after normalizing by maximum or by average spike amplitude (p<0.005 and p<0.001, respectively; Kolmogorov-Smirnov test). (B) As predicted, class 3 (single-spiking) neurons showed significantly greater variability in spike amplitude than class 1 (tonic-spiking) neurons (p<0.001 regardless of normalization by peak or average; Kolmogorov-Smirnov test). σnoise = 10 pA.