Outwardly rectifying deflections in threshold electrotonus due to K+ conductances
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 demyelin...
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Veröffentlicht in: | The Journal of physiology 2007-04, Vol.580 (2), p.685-696 |
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Sprache: | eng |
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Zusammenfassung: | 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 strengthâduration 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 stimulusâresponse curve and their longer strengthâduration time constant rather than a difference
in K + conductances. This may also explain the notch in demyelinating diseases. |
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ISSN: | 0022-3751 1469-7793 |
DOI: | 10.1113/jphysiol.2006.126003 |