Sharpness of Spike Initiation in Neurons Explained by Compartmentalization
A, Voltage across the axon as the soma is depolarized by steps of 3(5 values from −64 mV to −52 mV), with Na channels uniformly distributed between 25 and 40 µm away from the soma (compare with Fig. 1E, same total Na conductance). B, Same as A, but with Na channel density linearly decreasing between 25 and 40 µm. C, Spike threshold defined as the somatic voltage at which half of the sodium channels are activated as function of the location of Na channels when they are clustered at a single point (black). Red dots show the spike threshold when Na channels are uniformly distributed on a segment, as a function of the “effective” location, defined as a point inside the segment (x = 0.6 xstart+0.4 xend). Start points are taken among 1, 10, 20, 25, 30 and 35 µm; end points are taken among 40, 50 and 60 µm. D, Same as C but for initiation sharpness. E, Same as A but a tapering piece of axon is inserted at the beginning (the initial segment is moved), with length 10 µm and diameter linearly decreasing from 4 µm to 1 µm. F, Same as B with the initial tapering.