Biophysical Basis for Three Distinct Dynamical Mechanisms of Action Potential Initiation
Figure 7
Sufficiency of oppositely directed subthrehsold currents to explain excitability.
(A) Responses from 3D model described in Figure 4B. Without Isub, the model operated at the interface between class 1 and 2 excitability (see (C)). Adding an outward current (Esub = −100 mV) produced class 2 or 3 excitability, with the latter becoming more predominant (i.e. over a wider range of Istim) as ḡsub was increased. Adding an inward current (Esub = 50 mV) produced class 1 excitability. (B) Bifurcation diagrams show voltage at fixed point and at max/min of limit cycle as Istim was increased. Class 1, 2, and 3 versions of the 3D models exhibited exactly the same spike initiating dynamics seen in class 1, 2 and 3 versions of the 2D models (compare with Figure 2B). (C) Firing rate (color) is plotted against Istim and ḡsub. These data are qualitatively identical to those for the 2D model (see Figure 1D) and indicate that direction and magnitude of Isub are sufficient to explain different classes of excitability. The phasic-spiking that results from adaptation (see Figure 1C) can be understood in terms of slowly activating outward current (or inactivating inward current) causing a shift from class 2 to class 3 excitability. (D) As with the 2D model (Figure 3A), the class 3 version of the 3D model exhibited significantly greater spike amplitude variability than the class 1 version when driven by noisy stimulation (p<0.001, respectively; Kolmogorov-Smirnov test). σnoise = 10 µA/cm2.