Na+ imaging reveals little difference in action potential–evoked Na+ influx between axon and soma

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Abstract
The low threshold for action potential generation in the axon initial segment (AIS) is thought to reflect a high sodium channel density. Using high-speed sodium imaging, Fleidervish et al. estimate that sodium channel density in the AIS is only threefold higher than in the soma, a smaller difference than previously estimated. In cortical pyramidal neurons, the axon initial segment (AIS) is pivotal in synaptic integration. It has been asserted that this is because there is a high density of Na+ channels in the AIS. However, we found that action potential–associated Na+ flux, as measured by high-speed fluorescence Na+ imaging, was about threefold larger in the rat AIS than in the soma. Spike-evoked Na+ flux in the AIS and the first node of Ranvier was similar and was eightfold lower in basal dendrites. At near-threshold voltages, persistent Na+ conductance was almost entirely axonal. On a time scale of seconds, passive diffusion, and not pumping, was responsible for maintaining transmembrane Na+ gradients in thin axons during high-frequency action potential firing. In computer simulations, these data were consistent with the known features of action potential generation in these neurons.