The “kink” at spike onset.

<p>A, A pulse of current is injected at time 20(black), with Na channels opening almost all at once (dashed black: proportion of open channels), resulting in a discontinuity in the voltage derivative at the soma (the “kink”). B, Phase plot (voltage derivative vs. voltage) at the initiation site (black) and at the soma (red), showing a (modest) kink at the soma of about 5 mV/ms. C, Same as A, but Nav1.2 channels are placed at 15 µm from the soma, in addition to the Nav1.6 channels at 40 µm. D, Proportion of open Na channels as a function of somatic voltage, for both channel subtypes (Nav1.6 at initiation site, Nav1.2 closer to the soma). The two activation curves are shown in dashed with the same colors. E, Same as B, but with the additional Nav1.2 channels. The dashed red curve shows the somatic phase plot when Nav1.2 channels are also added at the soma. F, Enlargement of panel F, Onset rapidness is measured as the slope of the trajectories (green segments) when dVm/dt reaches 10 mV/ms (dashed line).