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Nat Neurosci. 2010 Jul;13(7):852-60. doi: 10.1038/nn.2574. Epub 2010 Jun 13.

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

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Department of Physiology, Faculty of Health Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel.


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.

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