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Changes in intracortical excitability induced by stimulation of wrist afferents in man

Jean-Marc Aimonetti and Jens Bo Nielsen *

1. Inhibitory and facilitatory neuronal circuits may be explored in the human motor cortex by double pulse transcranial magnetic stimulation (TMS). At short interstimulus intervals (2-5 ms), conditioned motor-evoked potentials (MEPs) are reduced (intracortical inhibition, ICI), whereas they are facilitated at longer interstimulus intervals (8-25 ms; intracortical facilitation, ICF). The aim of this study was to investigate the effects of homonymous and antagonist nerve stimulation on the intracortical inhibition and facilitation in the cortical areas that control the wrist extensor and flexor radialis muscles.
2. Sixteen subjects were asked to contract either their wrist extensor or flexor muscles. The MEP evoked by a test TMS (at 1.2 MEP threshold) and recorded in the target muscle was then conditioned by subthreshold TMS (at 0.8 MEP threshold) 2 and 14 ms before the test TMS. The median and radial nerves were stimulated at 0.8 motor threshold (MT).
3. In both flexor and extensor muscles, antagonist nerve stimulation 40 ms before the test TMS decreased ICI and increased ICF. In contrast, homonymous nerve stimulation had no effect on ICI and ICF.
4. The intensity of the antagonist nerve stimulation required to alter ICI and ICF was as low as 0.6 MT, which suggests that thick diameter afferents may be involved. The nerve stimulation had to be applied 35-45 ms prior to the test TMS to alter significantly the intracortical excitability.
5. Cutaneous afferents were probably not responsible for the alterations of intracortical excitability, since cutaneous stimulation had no effect on either ICI or ICF at the investigated intervals.
6. The present data suggest that antagonist muscular afferent inputs may evoke reciprocal facilitation or disinhibition at the cortical level. This pattern of antagonist sensory afferent effects may be of significance for control of the wrist extensor and flexor muscles when used as synergists during manipulatory finger movements and gripping tasks.

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