Dynamical explanation of the spike threshold dependence on d<i>V</i>/d<i>t</i> associated with each cell type.

<p>(A) and (B) respectively give the sample subthreshold and suprathreshold responses of each cell type to current ramp with <i>K</i> = 0.5μA/(cm²·ms) (blue), 1.5μA/(cm²·ms) (pink) and 5.0μA/(cm²·ms) (cyan blue). (C) and (D) respectively describe the separatrix (i.e., threshold) of two types of excitability on phase plane. The right panel in (C) is the enlarged view of the yellow region in its left panel. The separatrix of Type I neuron lies on the left side of Type II, which corresponds to a hyperpolarized spike threshold. As ramp slope <i>K</i> changes, the variable <i>w</i> in Type II neuron varies in a much larger scale compared to Type I. Then, varying d<i>V</i>/d<i>t</i> could produce an obvious change in the spike threshold of Type II neuron. The green dots denotes the spike threshold <i>V</i>th.