Contribution of neural circuits tested by transcranial magnetic stimulation in corticomotor control of low back muscle: a systematic review

Abstract

IntroductionTranscranial magnetic stimulation (TMS) is widely used to investigate central nervous system mechanisms underlying motor control. Despite thousands of TMS studies on neurophysiological underpinnings of corticomotor control, a large majority of studies have focused on distal muscles, and little is known about axial muscles (e.g., low back muscles). Yet, differences between corticomotor control of low back and distal muscles (e.g., gross vs. fine motor control) suggest differences in the neural circuits involved. This systematic review of the literature aims at detailing the organisation and neural circuitry underlying corticomotor control of low back muscles tested with TMS in healthy humans.MethodsThe literature search was performed in four databases (CINAHL, Embase, Medline (Ovid) and Web of science) up to May 2022. Included studies had to use TMS in combination with EMG recording of paraspinal muscles (between T12 and L5) in healthy participants. Weighted average was used to synthesise quantitative study results.ResultsForty-four articles met the selection criteria. TMS studies of low back muscles provided consistent evidence of contralateral and ipsilateral motor evoked potentials (with longer ipsilateral latencies) as well as of short intracortical inhibition/facilitation. However, few or no studies using other paired pulse protocols were found (e.g., long intracortical inhibition, interhemispheric inhibition). In addition, no study explored the interaction between different cortical areas using dual TMS coil protocol (e.g., between primary motor cortex and supplementary motor area).DiscussionCorticomotor control of low back muscles are distinct from hand muscles. Our main findings suggest: (i) bilateral projections from each single primary motor cortex, for which contralateral and ipsilateral tracts are probably of different nature (contra: monosynaptic; ipsi: oligo/polysynaptic) and (ii) the presence of intracortical inhibitory and excitatory circuits in M1 influencing the excitability of the contralateral corticospinal cells projecting to low back muscles. Understanding of these mechanisms are important for improving the understanding of neuromuscular function of low back muscles and to improve the management of clinical populations (e.g., low back pain, stroke).

Corticomotor control of lumbar erector spinae in postural and voluntary tasks: the influence of transcranial magnetic stimulation current direction

“…Neural control of trunk muscles is unique ( Desmons et al, 2023 ) and necessitates proper testing. Indeed, it is critical to adapt our interventions to the different health conditions that impact its control [e.g., low back pain ( Tsao et al, 2011a , b ; Massé-Alarie et al, 2016a ; Schabrun et al, 2017 )].…”

“…Indeed, it is critical to adapt our interventions to the different health conditions that impact its control [e.g., low back pain ( Tsao et al, 2011a , b ; Massé-Alarie et al, 2016a ; Schabrun et al, 2017 )]. Nonetheless, there are only a limited number of studies that specifically tested neural control of trunk muscles ( Desmons et al, 2023 ), showing it is organized differently from that of hand muscles ( Brinkman and Kuypers, 1973 ; Strutton et al, 2004 ). For example, stimulation of the primary motor cortex (M1) elicits a higher prevalence of ipsilateral motor-evoked potentials for trunk muscles compared with hand muscles in nonhuman primates and humans ( Brinkman and Kuypers, 1973 ; Strutton et al, 2004 ).…”

“…Nonetheless, transcranial magnetic stimulation (TMS) studies of human trunk muscles also suggest some similarities with the control of hand muscles. For example, when applied over M1, TMS led to selective activation of the erector spinae muscle M1 representation with short latencies (≍15–20 ms), suggesting a monosynaptic corticospinal projection ( Ferbert et al, 1992 ; O’Connell et al, 2007 ; Chiou et al, 2018a ; Desmons et al, 2023 ) and to the observation of intracortical inhibition and facilitation mechanisms ( Massé-Alarie et al, 2016b ; Chiou et al, 2018a ). Altogether, evidence suggests that neural control of back muscles is unique with both differences and similarities when compared with control of hand muscles and then requires specific testing in humans.…”

“…For example, when applied over M1, TMS led to selective activation of the erector spinae muscles M1 representation with short latencies (≈15-20 ms), suggesting a monosynaptic corticospinal projection (Chiou et al, 2018a;Desmons et al, 2023;Ferbert A. et al, 1992;O'Connell et al, 2007) and to the observation of intracortical inhibition and facilitation mechanisms (Chiou et al, 2018a; e N e u r o A c c e p t e d M a n u s c r i p t…”