The Journal of Neurophysiology Vol. 80 No. 4 October 1998, pp. 2089-2101
Copyright ©1998 The American Physiological Society

High Safety Factor for Action Potential Conduction Along Axons But Not Dendrites of Cultured Hippocampal and Cortical Neurons

Paul J. Mackenzie and Timothy H. Murphy

Kinsmen Laboratory of Neurological Research, Departments of Psychiatry and Physiology, University of British Columbia, Vancouver V6T 1Z3, Canada

Mackenzie, Paul J. and Timothy H. Murphy. High safety factor for action potential conduction along axons but not dendrites of cultured hippocampal and cortical neurons. J. Neurophysiol. 80: 2089-2101, 1998. By using a combination of Ca²⁺ imaging and current-clamp recording, we previously reported that action potential (AP) conduction is reliably observed from the soma to axonal terminals in cultured cortical neurons. To extend these studies, we evaluated Ca²⁺ influx evoked by Na⁺ APs as a marker of AP conduction under conditions that are expected to lower the conduction safety factor to explore mechanisms of axonal and dendritic excitability. As expected, reducing the extracellular Na⁺ concentration from 150 to ~60 mM decreased the amplitude of APs recorded in the soma but surprisingly did not influence axonal conduction, as monitored by measuring Ca²⁺ transients. Furthermore, reliable axonal conduction was observed in dilute (20 nM) tetrodotoxin (TTX), despite a similar reduction in AP amplitude. In contrast, the Ca²⁺ transient measured along dendrites was markedly reduced in low Na⁺, although still mediated by TTX-sensitive Na⁺ channels. Dendritic action-potential evoked Ca²⁺ transients were also markedly reduced in 20 nM TTX. These data provide further evidence that strongly excitable axons are functionally compartmentalized from weakly excitable dendrites. We conclude that modulation of Na⁺ currents or membrane potential by neurotransmitters or repetitive firing is more likely to influence neuronal firing before AP generation than the propagation of signals to axonal terminals. In contrast, the relatively low safety factor for back-propagating APs in dendrites would suggest a stronger effect of Na⁺ current modulation.

This article has been cited by other articles:

				K. Vervaeke, N. Gu, C. Agdestein, H. Hu, and J. F. Storm Kv7/KCNQ/M-channels in rat glutamatergic hippocampal axons and their role in regulation of excitability and transmitter release J. Physiol., October 1, 2006; 576(1): 235 - 256. [Abstract] [Full Text] [PDF]

				P. Monsivais, B. A. Clark, A. Roth, and M. Hausser Determinants of Action Potential Propagation in Cerebellar Purkinje Cell Axons J. Neurosci., January 12, 2005; 25(2): 464 - 472. [Abstract] [Full Text] [PDF]

				J. P. Meeks and S. Mennerick Selective Effects of Potassium Elevations on Glutamate Signaling and Action Potential Conduction in Hippocampus J. Neurosci., January 7, 2004; 24(1): 197 - 206. [Abstract] [Full Text] [PDF]

				W. B Levy and R. A. Baxter Energy-Efficient Neuronal Computation via Quantal Synaptic Failures J. Neurosci., June 1, 2002; 22(11): 4746 - 4755. [Abstract] [Full Text] [PDF]

				Y. He, C. F. Zorumski, and S. Mennerick Contribution of Presynaptic Na+ Channel Inactivation to Paired-Pulse Synaptic Depression in Cultured Hippocampal Neurons J Neurophysiol, February 1, 2002; 87(2): 925 - 936. [Abstract] [Full Text] [PDF]

				W. Graf, R. Spencer, H. Baker, and R. Baker Vestibuloocular Reflex of the Adult Flatfish. III. A Species-Specific Reciprocal Pattern of Excitation and Inhibition J Neurophysiol, September 1, 2001; 86(3): 1376 - 1388. [Abstract] [Full Text] [PDF]

				D. J. Hagler Jr. and Y. Goda Properties of Synchronous and Asynchronous Release During Pulse Train Depression in Cultured Hippocampal Neurons J Neurophysiol, June 1, 2001; 85(6): 2324 - 2334. [Abstract] [Full Text] [PDF]

				C. L. Cox, W. Denk, D. W. Tank, and K. Svoboda Action potentials reliably invade axonal arbors of rat neocortical neurons PNAS, August 6, 2000; (2000) 170278697. [Abstract] [Full Text]

				P. Varona, J. M. Ibarz, L. Lopez-Aguado, and O. Herreras Macroscopic and Subcellular Factors Shaping Population Spikes J Neurophysiol, April 1, 2000; 83(4): 2192 - 2208. [Abstract] [Full Text] [PDF]

				D. L. Brody and D. T. Yue Release-Independent Short-Term Synaptic Depression in Cultured Hippocampal Neurons J. Neurosci., April 1, 2000; 20(7): 2480 - 2494. [Abstract] [Full Text] [PDF]

				L. Lopez-Aguado, J. M. Ibarz, and O. Herreras Modulation of Dendritic Action Currents Decreases the Reliability of Population Spikes J Neurophysiol, February 1, 2000; 83(2): 1108 - 1114. [Abstract] [Full Text] [PDF]

				J. C. Magee and M. Carruth Dendritic Voltage-Gated Ion Channels Regulate the Action Potential Firing Mode of Hippocampal CA1 Pyramidal Neurons J Neurophysiol, October 1, 1999; 82(4): 1895 - 1901. [Abstract] [Full Text] [PDF]

				C. L. Cox, W. Denk, D. W. Tank, and K. Svoboda From the Cover: Action potentials reliably invade axonal arbors of rat neocortical neurons PNAS, August 15, 2000; 97(17): 9724 - 9728. [Abstract] [Full Text] [PDF]