Abstract
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.
This is a preview of subscription content, access via your institution
Access options
Subscribe to this journal
Receive 12 print issues and online access
$209.00 per year
only $17.42 per issue
Buy this article
- Purchase on Springer Link
- Instant access to full article PDF
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
References
Kole, M.H. & Stuart, G.J. Is action potential threshold lowest in the axon? Nat. Neurosci. 11, 1253–1255 (2008).
Colbert, C.M. & Pan, E. Ion channel properties underlying axonal action potential initiation in pyramidal neurons. Nat. Neurosci. 5, 533–538 (2002).
Kole, M.H. et al. Action potential generation requires a high sodium channel density in the axon initial segment. Nat. Neurosci. 11, 178–186 (2008).
Hu, W. et al. Distinct contributions of Na(v)1.6 and Na(v)1.2 in action potential initiation and backpropagation. Nat. Neurosci. 12, 996–1002 (2009).
Stuart, G. & Sakmann, B. Amplification of EPSPs by axosomatic sodium channels in neocortical pyramidal neurons. Neuron 15, 1065–1076 (1995).
Astman, N., Gutnick, M.J. & Fleidervish, I.A. Persistent sodium current in layer 5 neocortical neurons is primarily generated in the proximal axon. J. Neurosci. 26, 3465–3473 (2006).
Mainen, Z.F., Joerges, J., Huguenard, J.R. & Sejnowski, T.J. A model of spike initiation in neocortical pyramidal neurons. Neuron 15, 1427–1439 (1995).
Colbert, C.M. & Johnston, D. Axonal action-potential initiation and Na+ channel densities in the soma and axon initial segment of subicular pyramidal neurons. J. Neurosci. 16, 6676–6686 (1996).
Minta, A. & Tsien, R.Y. Fluorescent indicators for cytosolic sodium. J. Biol. Chem. 264, 19449–19457 (1989).
Callaway, J.C. & Ross, W.N. Spatial distribution of synaptically activated sodium concentration changes in cerebellar Purkinje neurons. J. Neurophysiol. 77, 145–152 (1997).
Palmer, L.M. & Stuart, G.J. Site of action potential initiation in layer 5 pyramidal neurons. J. Neurosci. 26, 1854–1863 (2006).
Lelievre, L., Zachowski, A., Charlemagne, D., Laget, P. & Paraf, A. Inhibition of (Na+ + K+)–ATPase by ouabain: involvement of calcium and membrane proteins. Biochim. Biophys. Acta 557, 399–408 (1979).
Kushmerick, M.J. & Podolsky, R.J. Ionic mobility in muscle cells. Science 166, 1297–1298 (1969).
Clark, B.A., Monsivais, P., Branco, T., London, M. & Hausser, M. The site of action potential initiation in cerebellar Purkinje neurons. Nat. Neurosci. 8, 137–139 (2005).
Rose, C.R., Kovalchuk, Y., Eilers, J. & Konnerth, A. Two-photon Na+ imaging in spines and fine dendrites of central neurons. Pflugers Arch. 439, 201–207 (1999).
Peters, A. The node of Ranvier in the central nervous system. Q. J. Exp. Physiol. Cogn. Med. Sci. 51, 229–236 (1966).
Fleidervish, I.A. & Gutnick, M.J. Kinetics of slow inactivation of persistent sodium current in layer V neurons of mouse neocortical slices. J. Neurophysiol. 76, 2125–2130 (1996).
Alzheimer, C., Schwindt, P.C. & Crill, W.E. Modal gating of Na+ channels as a mechanism of persistent Na+ current in pyramidal neurons from rat and cat sensorimotor cortex. J. Neurosci. 13, 660–673 (1993).
Kole, M.H., Letzkus, J.J. & Stuart, G.J. Axon initial segment Kv1 channels control axonal action potential waveform and synaptic efficacy. Neuron 55, 633–647 (2007).
Engel, D. & Jonas, P. Presynaptic action potential amplification by voltage-gated Na+ channels in hippocampal mossy fiber boutons. Neuron 45, 405–417 (2005).
Royeck, M. et al. Role of axonal NaV1.6 sodium channels in action potential initiation of CA1 pyramidal neurons. J. Neurophysiol. 100, 2361–2380 (2008).
Carter, B.C. & Bean, B.P. Sodium entry during action potentials of mammalian neurons: incomplete inactivation and reduced metabolic efficiency in fast-spiking neurons. Neuron 64, 898–909 (2009).
Bender, K.J. & Trussell, L.O. Axon initial segment Ca2+ channels influence action potential generation and timing. Neuron 61, 259–271 (2009).
Lasser-Ross, N. & Ross, W.N. Imaging voltage and synaptically activated sodium transients in cerebellar Purkinje cells. Proc. Biol. Sci. 247, 35–39 (1992).
Alle, H., Roth, A. & Geiger, J.R. Energy-efficient action potentials in hippocampal mossy fibers. Science 325, 1405–1408 (2009).
Nevian, T., Larkum, M.E., Polsky, A. & Schiller, J. Properties of basal dendrites of layer 5 pyramidal neurons: a direct patch-clamp recording study. Nat. Neurosci. 10, 206–214 (2007).
Acker, C.D. & Antic, S.D. Quantitative assessment of the distributions of membrane conductances involved in action potential backpropagation along basal dendrites. J. Neurophysiol. 101, 1524–1541 (2009).
Bean, B.P. The action potential in mammalian central neurons. Nat. Rev. Neurosci. 8, 451–465 (2007).
Sugihara, I., Lang, E.J. & Llinas, R. Uniform olivocerebellar conduction time underlies Purkinje cell complex spike synchronicity in the rat cerebellum. J. Physiol. (Lond.) 470, 243–271 (1993).
Markram, H., Lubke, J., Frotscher, M. & Sakmann, B. Regulation of synaptic efficacy by coincidence of postsynaptic APs and EPSPs. Science 275, 213–215 (1997).
Shrager, P. The distribution of sodium and potassium channels in single demyelinated axons of the frog. J. Physiol. (Lond.) 392, 587–602 (1987).
Zhang, C.L., Wilson, J.A., Williams, J. & Chiu, S.Y. Action potentials induce uniform calcium influx in mammalian myelinated optic nerves. J. Neurophysiol. 96, 695–709 (2006).
Attwell, D. & Iadecola, C. The neural basis of functional brain imaging signals. Trends Neurosci. 25, 621–625 (2002).
Mata, M., Fink, D.J., Ernst, S.A. & Siegel, G.J. Immunocytochemical demonstration of Na+,K+-ATPase in internodal axolemma of myelinated fibers of rat sciatic and optic nerves. J. Neurochem. 57, 184–192 (1991).
Larkum, M.E., Watanabe, S., Nakamura, T., Lasser-Ross, N. & Ross, W.N. Synaptically activated Ca2+ waves in layer 2/3 and layer 5 rat neocortical pyramidal neurons. J. Physiol. (Lond.) 549, 471–488 (2003).
Stuart, G.J., Dodt, H.U. & Sakmann, B. Patch-clamp recordings from the soma and dendrites of neurons in brain slices using infrared video microscopy. Pflugers Arch. 423, 511–518 (1993).
Helmchen, F., Imoto, K. & Sakmann, B. Ca2+ buffering and action potential–evoked Ca2+ signaling in dendrites of pyramidal neurons. Biophys. J. 70, 1069–1081 (1996).
Neher, E. & Augustine, G.J. Calcium gradients and buffers in bovine chromaffin cells. J. Physiol. (Lond.) 450, 273–301 (1992).
Hodgkin, A.L. & Keynes, R.D. Experiments on the injection of substances into squid giant axons by means of a microsyringe. J. Physiol. (Lond.) 131, 592–616 (1956).
Hines, M.L. & Carnevale, N.T. The NEURON simulation environment. Neural Comput. 9, 1179–1209 (1997).
Stuart, G., Schiller, J. & Sakmann, B. Action potential initiation and propagation in rat neocortical pyramidal neurons. J. Physiol. (Lond.) 505, 617–632 (1997).
Acknowledgements
We thank S. Manita for excellent help with the preparation of slices. This work was supported by a US-Israel Binational Science Foundation grant (2003082), a grant from the Israel Science Foundation (1376-06), a Grass Faculty grant from the Marine Biological Laboratory, a US National Institutes of Health grant (NS16295), a Multiple Sclerosis Society grant (PP1367) and a fellowship from the Gruss Lipper Foundation.
Author information
Authors and Affiliations
Contributions
I.A.F., N.L.-R., M.J.G. and W.N.R. designed the study, performed the cortical experiments and wrote the paper. N.L.-R. and W.N.R. performed the cerebellar experiments. I.A.F. constructed the computational models.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing financial interests.
Supplementary information
Supplementary Text and Figures
Supplementary Figures 1–5 (PDF 1489 kb)
Supplementary Movie 1
Single action potential elicited a prominent change in [Na+]i in the AIS. The changes in [Na+]i in the soma and in the nearby basal dendrites were too small to be detected. (MPG 942 kb)
Supplementary Movie 2
In a model with the equal Na+ channel density in soma and AIS (250 pS μm−2), somatic current injection (1 nA, 3 ms) produces a relatively slowly rising action potential that initiates simultaneously in soma and in nearby processes. Waveform represents membrane potential across the axo-dendritic axes at different time points starting 1 ms before the beginning of the current step and ending 0.5 ms after its end. (WMV 568 kb)
Supplementary Movie 3
In a model that assumes the AIS Na+ channel density threefold higher than in the soma, the time constant of Na+ channels activation accelerated (τm × 0.2; see Fig. 8a) and persistent Na+ conductance which comprises 5% of the total AIS Na+ conductance, the spike initiates in the AIS, propagates rapidly into the axon and more slowly into the soma and the apical dendrite. Same time window as in Supplementary Movie 2. (WMV 603 kb)
Rights and permissions
About this article
Cite this article
Fleidervish, I., Lasser-Ross, N., Gutnick, M. et al. Na+ imaging reveals little difference in action potential–evoked Na+ influx between axon and soma. Nat Neurosci 13, 852–860 (2010). https://doi.org/10.1038/nn.2574
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/nn.2574
This article is cited by
-
Spatial and temporal aspects of neuronal calcium and sodium signals measured with low-affinity fluorescent indicators
Pflügers Archiv - European Journal of Physiology (2024)
-
Physiological Aspects of the Use of the Hodgkin–Huxley Model of Action Potential Generation for Neurons in Invertebrates and Vertebrates
Neuroscience and Behavioral Physiology (2017)
-
Hyperphosphorylated tau causes reduced hippocampal CA1 excitability by relocating the axon initial segment
Acta Neuropathologica (2017)
-
Functional implications of axon initial segment cytoskeletal disruption in stroke
Acta Pharmacologica Sinica (2016)
-
Synaptic GABA release prevents GABA transporter type-1 reversal during excessive network activity
Nature Communications (2015)