What Is the Most Realistic Single-Compartment Model of Spike Initiation?
(A) Spikes of real neurons have a distinct kink at onset (top: human pyramidal cell, adapted from ), which does not appear in single-compartment HH models (bottom: HH models of squid giant axon and rat hippocampal interneuron, adapted from ). (B) The peak current in somatic voltage-clamp shows a discontinuity as a function of voltage (red), not seen when axonal Na channels are inactivated (black) (adapted from ). (C) Spiking responses of cortical neurons in vitro to somatic current injection (black) are well predicted by integrate-and-fire models with sharp spike initiation (blue, first two traces; percentages are proportion of correctly predicted spikes) but not by the Izhikevich model, a variation of the quadratic model (blue, third trace) (adapted from ). (D) Phase locking to high frequencies: cortical neurons fire at preferred phases of a sinusoidal current added on top of noise (left), at frequencies up to several hundred Hz (right, vector strength is a measure of phase locking) (adapted from ).