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

1. Inhibitory and facilitatory neuronal circuits may he explored in the hum an motor cortex by double pulse transcranial magnetic stimulation (TMS). At short interstimulus intervals (2-5 ms), conditioned motor-evoked potential s (MEPs) are reduced (intracortical inhibition, ICI), whereas they are faci litated at longer interstimulus intervals (8-25 ins; intracortical facilita tion, ICF). The aim of this study was to investigate the effects of homonym ous and antagonist nerve stimulation on the intracortical inhibition and fa cilitation in the cortical areas that. control the wrist. extensor and flex or radialis muscles. 2. Sixteen subjects were asked to contract either their wrist extensor or f lexor muscles. The MEP evoked by a test TMS (at 1.2 x MEP threshold) and re corded in the target muscle teas then conditioned by subthreshold TMS (at 0 .8 x MEP threshold) 2 and 14 ins before the test TMS. The median and radial nerves were stimulated at 0.8 x motor threshold (AFT). 3. In both flexor and extensor muscles, antagonist nerve stimulation 40 ins before the test TMS decreased ICI and increased ICF. In contrast, homonymo us nerve stimulation had no effect on ICI and ICF. 4. The intensity of the antagonist nerve stimulation required to alter ICI and ICF teas as loll as 0.6 x MT, which suggests that. thick diameter affer ents may be involved. The nerve stimulation had to be applied 35-45 ins pri or 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, oil either ICI or ICF at the investigated intervals. 6. The present. data suggest that antagonist muscular afferent inputs may e voke reciprocal facilitation or disinhibition at. the cortical level. This pattern of antagonist sensory afferent effects may be of significance for c ontrol of the wrist extensor and flexor muscles when used as synergists dur ing manipulatory finger movements and gripping tasks.