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.

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