Inflections in threshold electrotonus to depolarizing currents in sensory axons

Authors

  • David Burke MD, DSc,

    Corresponding author
    1. Institute of Clinical Neurosciences, Royal Prince Alfred Hospital and Office of Research & Development, Medical Foundation Building – K25, University of Sydney, Sydney, NSW 2006, Australia
    • Institute of Clinical Neurosciences, Royal Prince Alfred Hospital and Office of Research & Development, Medical Foundation Building – K25, University of Sydney, Sydney, NSW 2006, Australia
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  • James Howells BSc,

    1. Institute of Clinical Neurosciences, Royal Prince Alfred Hospital and Office of Research & Development, Medical Foundation Building – K25, University of Sydney, Sydney, NSW 2006, Australia
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  • Louise Trevillion BSc,

    1. Institute of Clinical Neurosciences, Royal Prince Alfred Hospital and Office of Research & Development, Medical Foundation Building – K25, University of Sydney, Sydney, NSW 2006, Australia
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  • Matthew C. Kiernan MD, PhD,

    1. Prince of Wales Medical Research Institute and Prince of Wales Clinical School, University of New South Wales, Sydney, NSW, Australia
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  • Hugh Bostock PhD

    1. Sobell Department of Neurophysiology, Institute of Neurology, London, United Kingdom
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Abstract

Threshold electrotonus involves tracking the changes in axonal excitability produced by subthreshold polarizing currents and is the only technique that allows insight into the function of internodal conductances in human subjects in vivo. There is often an abrupt transient reversal of the threshold change as excitability increases in response to conditioning depolarizing currents (S1 phase). In recordings from motor axons, it has been recently demonstrated that this notch or inflection is due to activation of low-threshold axons. We report that a notch is frequently seen in sensory recordings (in 33 of 50 healthy subjects) using the standard threshold electrotonus protocol. When large, the notch can distort subsequent phases of threshold electrotonus and could complicate quantitative measurements and modeling studies. Muscle Nerve, 2007

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