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This Article Full Text Full Text (PDF) All Versions of this Article: 580/2/685    most recent jphysiol.2006.126003v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Trevillion, L. Articles by Burke, D. Search for Related Content PubMed PubMed Citation Articles by Trevillion, L. Articles by Burke, D. Related Collections Integrative

¹ Institute of Clinical Neurosciences, University of Sydney and Royal Prince Alfred Hospital, Sydney, NSW 2006, Australia

A transient decrease in excitability occurs regularly during the S1 phase of threshold electrotonus to depolarizing conditioning stimuli for sensory and, less frequently, motor axons. This has been attributed to the outwardly rectifying action of fast K⁺ channels, at least in patients with demyelinating diseases. This study investigates the genesis of this notch in healthy axons. Threshold electrotonus was recorded for sensory and motor axons in the median nerve at the wrist in response to test stimuli of different width. The notch occurred more frequently the briefer the test stimulus, and more frequently in sensory studies. In studies on motor axons, the notch decreased in latency and increased in amplitude as the conditioning stimulus increased or the limb was cooled. Low-threshold axons displayed profound changes in strength–duration time constant even though the threshold electrotonus curves contained no detectable notch. When a 1.0 ms current was added to subthreshold conditioning stimuli to trigger EMG, the notch varied with the timing and intensity of the brief current pulse. This study finds no evidence for an outwardly rectifying deflection due to K⁺ channels, other than the slow accommodation attributable to slow K⁺ currents. In normal motor axons, a depolarization-induced notch during the S1 phase of threshold electrotonus is the result of the conditioning stimulus exceeding threshold for some axons. The notch is more apparent in sensory axons probably because of the lower slope of the stimulus–response curve and their longer strength–duration time constant rather than a difference in K⁺ conductances. This may also explain the notch in demyelinating diseases.

(Received 3 December 2006; accepted after revision 31 January 2007; first published online 1 February 2007)
Corresponding author D. Burke: Office of Research and Development, Medical Foundation Building – K25, University of Sydney, Sydney, NSW 2006, Australia.  Email: d.burke{at}med.usyd.edu.au