Differential Modulation of Intracortical Inhibition in Human Motor Cortex during Selective Activation of an Intrinsic Hand Muscle
- 1 August 2003
- journal article
- clinical trial
- Published by Wiley in The Journal of Physiology
- Vol. 550 (3), 933-946
- https://doi.org/10.1113/jphysiol.2003.042606
Abstract
Paired‐pulse transcranial magnetic stimulation (TMS) was used to assess the effectiveness of intracortical inhibition (ICI) acting on corticospinal neurons controlling three intrinsic hand muscles in humans. We hypothesised that the suppression of ICI with selective activation of a muscle would be restricted to corticospinal neurons controlling the muscle targeted for activation. Surface EMG was recorded from abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles of the left hand. Subjects were tested at rest and during weak selective activation of APB or ADM, while they attempted to keep the other muscles relaxed using visual feedback. Paired‐pulse TMS was applied with a circular coil oriented to produce antero‐posterior (AP) current flow in the right motor cortex (to preferentially evoke I3 waves in corticospinal neurons) and with postero‐anterior (PA) currents (to preferentially evoke I1 waves). Paired‐pulse TMS was less effective in suppressing the muscle evoked potential (MEP) when the muscle was targeted for selective activation, with both AP and PA stimulation. The mechanism for this includes effects on late I waves, as it was evident with a weak AP test TMS pulse that elicited negligible I1 waves in corticospinal neurons. ICI circuits activated by TMS, which exert their effects on late I waves but do not affect I1 waves, are strongly implicated in this modulation. With AP stimulation, paired‐pulse inhibition was not significantly altered for corticospinal neurons controlling other muscles of the same hand which were required to be inactive during the selective activation task. This differential modulation was not seen with PA stimulation, which preferentially activates I1 waves and evokes a MEP that is less influenced by ICI. The observations with AP stimulation suggest that selective activation of a hand muscle is accompanied by a selective suppression of ICI effects on the corticospinal neurons controlling that muscle. The pattern of differential modulation of ICI effectiveness with voluntary activation suggests that the ICI circuits assist the corticospinal system in producing fractionated activity of intrinsic hand muscles.Keywords
This publication has 38 references indexed in Scilit:
- Descending volleys evoked by transcranial magnetic stimulation of the brain in conscious humans: effects of coil shapeClinical Neurophysiology, 2001
- Direct demonstration of the effect of lorazepam on the excitability of the human motor cortexClinical Neurophysiology, 2000
- Differential effects on motorcortical inhibition induced by blockade of GABA uptake in humansThe Journal of Physiology, 1999
- Demonstration of facilitatory I wave interaction in the human motor cortex by paired transcranial magnetic stimulationThe Journal of Physiology, 1998
- Paired‐pulse magnetic stimulation of the human motor cortex: differences among I wavesThe Journal of Physiology, 1998
- Effects of voluntary contraction on descending volleys evoked by transcranial stimulation in conscious humansThe Journal of Physiology, 1998
- The glutamate antagonist Riluzole suppresses intracortical facilitationJournal of Neural Transmission, 1997
- Short latency facilitation between pairs of threshold magnetic stimuli applied to human motor cortexElectroencephalography and Clinical Neurophysiology/Electromyography and Motor Control, 1996
- Changes in the balance between motor cortical excitation and inhibition in focal, task specific dystonia.Journal of Neurology, Neurosurgery & Psychiatry, 1995
- Changes in excitability of motor cortical circuitry in patients with parkinson's diseaseAnnals of Neurology, 1995