Biophysical Basis for Three Distinct Dynamical Mechanisms of Action Potential Initiation
Figure 10
Depolarization-induced inactivation of a subthreshold outward current can also produce class 1 excitability.
(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 w-nullcline to give it a region of negative slope at subthreshold potentials (see [55]); this was done by changing Equation 5 so that where βw = −10 mV, γw = 10 mV, βw* = −60 mV, γw* = = 20 mV, and ξ = 0.1. Under these conditions, the V- and w-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–V curve that overlapped the apex of the instantaneous I–V curve.