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Ion channel properties underlying axonal action potential initiation in pyramidal neurons

Nature Neuroscience volume 5, pages 533–538 (2002)Cite this article

An Erratum to this article was published on 01 October 2002

Abstract

A high density of Na+ channels in the axon hillock, or initial segment, is believed to determine the threshold for action potential initiation in neurons. Here we report evidence for an alternative mechanism that lowers the threshold in the axon. We investigated properties and distributions of ion channels in outside-out patches from axons and somata of layer 5 pyramidal neurons in rat neocortical slices. Na+ channels in axonal patches (〈30 μm from the soma) were activated by 7 mV less depolarization than were somatic Na+ channels. A-type K+ channels, which were prominent in somatic and dendritic patches, were rarely seen in axonal patches. We incorporated these findings into numerical simulations which indicate that biophysical properties of axonal channels, rather than a high density of channels in the initial segment, are most likely to determine the lowest threshold for action potential initiation.

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Figure 1: Total ensemble currents in outside-out patches from visually identified somata and axons of neocortical layer 5 pyramidal neurons.
Figure 2: Na+ channel properties differ between the soma and axon.
Figure 3: Effect of Na+ channel properties on action potential threshold: computer simulations.
Figure 4: Spatial patterns of action potential initiation: computer simulations.
Figure 5: Threshold increases when a pyramidal neuron is cut at the end of the initial segment.

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Acknowledgements

This work was supported by a National Institute of Neurological Disorders and Stroke (NINDS) grant (NS36982) to C.M.C. We thank J. Stringer, D. Johnston, M. Rea, A. Eskin, G. Cahill and S. Dryer for reading earlier versions of the manuscript, and L. Cleary for the use of the Neurolucida system.

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  1. Department of Biology and Biochemistry, University of Houston, 4800 Calhoun Road, Houston, 77204-5513, Texas, USA

    Costa M. Colbert & Enhui Pan

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Correspondence to Costa M. Colbert.

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Colbert, C., Pan, E. Ion channel properties underlying axonal action potential initiation in pyramidal neurons. Nat Neurosci 5, 533–538 (2002). https://doi.org/10.1038/nn0602-857

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