Somatic stimulation: Combined effect of varying crossover location (κ) and Na_V separation (x) in the axon initial segment.

The threshold for forward AP propagation is the same as for backpropagation. Varying the separation parameter “x” from x = 0 to x = 1, the distribution of Na_V channels goes from flat (homogeneous) to separated, the latter approximating the distribution observed in developing pyramidal neurons (see Fig 1A). Note that curves for all values of κ converge to a single point at x = 0, since κ can have no effect when the two Na_V subtypes are uniformly distributed along the AIS. The lines have been drawn to guide the eye.