Influence of Voluntary Contraction Level, Test Stimulus Intensity and Normalization Procedures on the Evaluation of Short-Interval Intracortical Inhibition.

Cécilia Neige,Alain Martin,Florent Lebon,Sidney Grosprêtre

doi:10.3390/brainsci10070433

Cécilia Neige, Alain Martin + Show 2 more

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https://doi.org/10.3390/brainsci10070433

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Journal: Brain sciences	Publication Date: Jul 8, 2020
Citations: 8	License type: CC BY 4.0

Affiliation: Université Bourgogne Franche-Comté, Inserm

Abstract

Short-interval intracortical inhibition (SICI) represents an inhibitory phenomenon acting at the cortical level. However, SICI estimation is based on the amplitude of a motor-evoked potential (MEP), which depends on the discharge of spinal motoneurones and the generation of compound muscle action potential (M-wave). In this study, we underpin the importance of taking into account the proportion of spinal motoneurones that are activated or not when investigating the SICI of the right flexor carpi radialis (normalization with maximal M-wave (Mmax) and MEPtest, respectively), in 15 healthy subjects. We probed SICI changes according to various MEPtest amplitudes that were modulated actively (four levels of muscle contraction: rest, 10%, 20% and 30% of maximal voluntary contraction (MVC)) and passively (two intensities of test transcranial magnetic stimulation (TMS): 120 and 130% of motor thresholds). When normalized to MEPtest, SICI remained unchanged by stimulation intensity and only decreased at 30% of MVC when compared with rest. However, when normalized to Mmax, we provided the first evidence of a strong individual relationship between SICI and MEPtest, which was ultimately independent from experimental conditions (muscle states and TMS intensities). Under similar experimental conditions, it is thus possible to predict SICI individually from a specific level of corticospinal excitability in healthy subjects.

Highlights

Cortical inhibitory and excitatory circuits within the primary motor cortex (M1) play an important role in the fine tuning of descending motor commands and in the neural adaptation following motor training [1]
An analyses of variance (ANOVA) performed on the mean Mmax amplitude obtained for each level of contraction showed no significant main effect (F(3,42) = 1.153; p = 0.323, Table 1), indicating that the increase in the level of muscle contraction from rest to 30% maximal voluntary isometric contraction (MVIC) did not affect Mmax amplitude
This study investigated the modulation of Short-interval intracortical inhibition (SICI) when normalizing or not with the proportion of spinal motoneurones activated (SICIMmax vs. SICIMEPtest ), with varying MEPtest amplitude

Summary

Introduction

Cortical inhibitory and excitatory circuits within the primary motor cortex (M1) play an important role in the fine tuning of descending motor commands and in the neural adaptation following motor training [1]. The reduction in intracortical inhibition within M1 is a crucial part of neural adaptation following acute and multi-session challenging strength training programs [2,3,4]. Paired-pulse transcranial magnetic stimulation (TMS) enables the non-invasive examination of these intracortical excitatory and inhibitory circuits at the time of the stimulation [5,6]. Short-interval intracortical inhibition (SICI) is a well-established paired-pulse measure to evaluate inhibitory circuits within the M1 area. SICI results from a subthreshold conditioning stimulus (CS). Followed 1 to 6 ms later by a suprathreshold test stimulus (TS) applied through the same coil over. The CS exerts a decrease in the conditioned MEP (MEPcond )

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