Depolarization-induced inactivation of a subthreshold outward current can also produce class 1 excitability.

<p>(A) Inactivation of an A-type K⁺ current by subthreshold depolarization should shift the balance of inward and outward currents the same way that depolarization-induced activation of an inward current does, and is therefore predicted to encourage class 1 excitability. To test this, we warped the <i>w</i>-nullcline to give it a region of negative slope at subthreshold potentials (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000198#pcbi.1000198-Rose1" target="_blank">[55]</a>); this was done by changing Equation 5 so that where <i>β</i>_w = −10 mV, <i>γ</i>_w = 10 mV, <i>β</i>_w* = −60 mV, <i>γ</i>_w* = = 20 mV, and ξ = 0.1. Under these conditions, the <i>V</i>- and <i>w</i>-nullclines intersected tangentially at rheobasic stimulation. (B) This phase plane geometry resulted in an SNIC bifurcation and class 1 excitability, as predicted. (C) Inactivation of the A-type K⁺ current at subthrehsold potentials gave a region of negative slope on the steady state <i>I</i>–<i>V</i> curve that overlapped the apex of the instantaneous <i>I</i>–<i>V</i> curve.