Sufficiency of oppositely directed subthrehsold currents to explain excitability.

<p>(A) Responses from 3D model described in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000198#pcbi-1000198-g004" target="_blank">Figure 4B</a>. Without <i>I</i>_sub, the model operated at the interface between class 1 and 2 excitability (see (C)). Adding an outward current (<i>E</i>_sub = −100 mV) produced class 2 or 3 excitability, with the latter becoming more predominant (i.e. over a wider range of <i>I</i>_stim) as <i>ḡ</i>_sub was increased. Adding an inward current (<i>E</i>_sub = 50 mV) produced class 1 excitability. (B) Bifurcation diagrams show voltage at fixed point and at max/min of limit cycle as <i>I</i>_stim 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 <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000198#pcbi-1000198-g002" target="_blank">Figure 2B</a>). (C) Firing rate (color) is plotted against <i>I</i>_stim and <i>ḡ</i>_sub. These data are qualitatively identical to those for the 2D model (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000198#pcbi-1000198-g001" target="_blank">Figure 1D</a>) and indicate that direction and magnitude of <i>I</i>_sub are sufficient to explain different classes of excitability. The phasic-spiking that results from adaptation (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000198#pcbi-1000198-g001" target="_blank">Figure 1C</a>) 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 (<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000198#pcbi-1000198-g003" target="_blank">Figure 3A</a>), the class 3 version of the 3D model exhibited significantly greater spike amplitude variability than the class 1 version when driven by noisy stimulation (<i>p</i><0.001, respectively; Kolmogorov-Smirnov test). σ_noise = 10 µA/cm².