Short-interval intracortical inhibition and facilitation targeting upper and lower limb muscles

Natalie Mrachacz-Kersting,Ulf Ziemann,Andrew James Thomas Stevenson

doi:10.1038/s41598-021-01348-6

Abstract

Transcranial magnetic stimulation (TMS) can be used to study excitability of corticospinal neurons in human motor cortex. It is currently not fully elucidated if corticospinal neurons in the hand vs. leg representation show the same or different regulation of their excitability by GABAAergic and glutamatergic interneuronal circuitry. Using a paired-pulse TMS protocol we tested short-interval intracortical inhibition (SICI) and short-interval intracortical facilitation (SICF) in 18 healthy participants. Motor evoked potentials were evoked in one hand (abductor digiti minimi) and one leg muscle (tibialis anterior), with systematic variation of the intensities of the first (S1) and second (S2) pulse between 60 and 140% resting motor threshold (RMT) in 10% steps, at two interstimulus intervals of 1.5 and 2.1 ms. For the hand and leg motor representations and for both interstimulus intervals, SICI occurred if the intensities of S1 < RMT and S2 > RMT, while SICF predominated if S1 = S2 ≤ RMT, or S1 > RMT and S2 < RMT. Findings confirm and extend previous evidence that the regulation of excitability of corticospinal neurons of the hand versus leg representation in human primary cortex through GABAAergic and glutamatergic interneuronal circuits is highly similar, and that corticospinal neurons of both representations are activated by TMS transsynaptically in largely identical ways.

Highlights

Transcranial magnetic stimulation (TMS) can be used to study excitability of corticospinal neurons in human motor cortex
For the typical setting for S1 and S2 used in upper limb muscles, S1 = 80% resting motor threshold (RMT) and S2 = 120% RMT, shortinterval intracortical inhibition (SICI) was seen in 9/10 participants for the ADM at both interstimulus intervals (ISI), while SICI was seen in 8/10 and 6/10 participants for the TA at the 1.5 ms and 2.1 ms ISIs, respectively
For the ADM at the 1.5 ms ISI, the optimal stimulation condition inducing SICI in all ten participants was S1 = 80% RMT and S2 = 140% RMT, which suppressed the motor evoked potentials (MEP) to 52 ± 26%

Summary

Introduction

Transcranial magnetic stimulation (TMS) can be used to study excitability of corticospinal neurons in human motor cortex. The expression of SICI and SICF in hand vs leg muscles has not been compared in the same individuals with very few exceptions[19,20] This comparison could address the question to what extent the excitability of corticospinal neurons in the hand versus leg representations of motor cortex are regulated or differently. We test this question by a sophisticated previously described ppTMS approach with systematic variation of S1 and S2 intensities (9 different intensities each, resulting in a matrix of 9 × 9 = 81 intensity conditions) at two different ISI (1.5 and 2.1 ms)[8], recording from one hand muscle (abductor digiti minimi) and one leg muscle (tibialis anterior) in 18 healthy young adult subjects. The ISI of 1.5 ms was chosen since it has been shown to induce pronounced SICF when S1 > RMT and S2 < R MT6, while 2.1 ms is known to produce distinct SICI when S1 < RMT and S2 > RMT21

Methods

Results

Conclusion