After-effects of near-threshold stimulation in single human motor axons.
Subthreshold electrical stimuli can generate a long-lasting increase in axonal excitability, superficially resembling the phase of superexcitability that follows a conditioning nerve impulse. This phenomenon of 'subthreshold superexcitability' has been investigated in single motor axons in six healthy human subjects, by tracking the excitability changes produced by conditioning stimuli of different amplitudes and waveforms. Near-threshold 1 ms stimuli caused a mean decrease in threshold at 5 ms of 22.1 +/- 6.0% (mean +/-s.d.) if excitation occurred, or 6.9 +/- 2.6% if excitation did not occur. The subthreshold superexcitability was maximal at an interval of about 5 ms, and fell to zero at 30 ms. It appeared to be made up of two components: a passive component linearly related to conditioning stimulus amplitude, and a non-linear active component. The active component appeared when conditioning stimuli exceeded 60% of threshold, and accounted for a maximal threshold decrease of 2.6 +/- 1.3%. The passive component was directly proportional to stimulus charge, when conditioning stimulus duration was varied between 0.2 and 2 ms, and could be eliminated by using triphasic stimuli with zero net charge. This change in stimulus waveform had little effect on the active component of subthreshold superexcitability or on the 'suprathreshold superexcitability' that followed excitation. It is concluded that subthreshold superexcitability in human motor axons is mainly due to the passive electrotonic effects of the stimulating current, but this is supplemented by an active component (about 12% of suprathreshold superexcitability), due to a local response of voltage-dependent sodium channels.
Bostock
Hugh
H
Sobell Department of Neurophysiology, Institute of Neurology, Queen Square, London WC1N 3BG, UK. h.bostock@ion.ucl.ac.uk
Lin
Cindy Shin-Yi
CS
Howells
James
J
Trevillion
Louise
L
Jankelowitz
Stacey
S
Burke
David
D
D000200:Action Potentials / Q000502:physiology*; D000222:Adaptation, Physiological / Q000502:physiology; D001369:Axons / Q000502:physiology*; D004056:Differential Threshold / Q000502:physiology*; D004558:Electric Stimulation / Q000379:methods*; D006801:Humans; D009046:Motor Neurons / Q000502:physiology*; D009473:Neuronal Plasticity / Q000502:physiology*
Muscle Nerve. 2000 Mar;23(3):399-409
10679717
Muscle Nerve. 1998 Feb;21(2):137-58
9466589
J Neurophysiol. 2002 Feb;87(2):995-1006
11826063
J Physiol. 2002 Aug 1;542(Pt 3):951-61
12154192
J Physiol. 2004 Sep 15;559(Pt 3):953-64
15272032
J Physiol. 1982 Feb;323:117-44
6980272
Neuroscience. 1985 May;15(1):1-12
4010930
Neuroscience. 1985 May;15(1):13-31
2409473
Neuroscience. 1987 May;21(2):585-93
3497361
Brain Res. 1988 Oct 18;462(2):354-8
3191395
Muscle Nerve. 1994 Jan;17(1):74-9
8264705
Neurology. 1996 May;46(5):1387-90
8628487
Brain. 1996 Aug;119 ( Pt 4):1099-105
8813274
J Physiol. 1997 Jan 1;498 ( Pt 1):277-94
9023784
Muscle Nerve. 2000 Sep;23(9):1365-73
10951439