Influence of different transcranial magnetic stimulation current directions on the corticomotor control of lumbar erector spinae muscles during a static task

Abstract

Different directions of transcranial magnetic stimulation (TMS) can activate different neuronal circuits. While posteroanterior current (PA-TMS) depolarizes mainly interneurons in primary motor cortex (M1), an anteroposterior current (AP-TMS) has been suggested to activate different M1 circuits and perhaps axons from the premotor regions. Although M1 is also involved in the control of axial muscles, no study has explored if different current directions activate different M1 circuits that may have distinct functional role. The aim of the study was to compare the effect of different current directions (PA- and AP-TMS) on the corticomotor control and spatial cortical organisation of the lumbar erector spinae muscle (LES). Thirthy-four healthy participants were recruited for two independent experiments and LES motor-evoked potentials (MEP) were recorded. In experiment 1 (n=17), active motor threshold (AMT), MEP latencies, recruitment curve (90 to 160% AMT), excitatory and inhibitory intracortical mechanisms using paired-pulse TMS (80% followed by 120% AMT stimuli at 2-3-10 and 15ms inter-stimulus intervals) were tested using a double cone (n=12) and a figure-of-eight (n=5) coils. In experiment 2 (n=17), LES cortical representations were tested using PA- and AP-TMS. AMT was higher for AP- compared to PA-TMS (p=0.002). Longer latencies with AP-TMS were compared to PA-TMS (p=0.017). AP-TMS produced more inhibition compared to PA-TMS at 2ms and 3ms (p=0.010), but no difference was observed for longer intervals. No difference was found for recruitment curve and mapping. Those findings suggest that each PA- and AP-TMS may activate different cortical circuits controlling low back muscles as proposed for hand muscles.

Influence of coil orientation on corticospinal excitability of trunk muscles during postural and volitional tasks in healthy adults

“…We found that the AMT was higher when the coil was oriented to induce the AP current than that to induce the PA current in the M1. This agrees with prior work investigating the TMS-induced currents in hand muscles with a figure-of-eight coil (Sakai et al, 1997 ; Cirillo and Byblow, 2016 ; Sale et al, 2016 ) as well as with a previous study using a double-cone coil in the lumbar ES muscles (Desmons et al, 2021 ) reporting lower motor thresholds with the PA current than the AP current. Additionally, our results obtained in the STE task revealed that the latency of ES MEP was 1.3 and 2.3 ms longer in the PA current and in the AP current compared with the latency elicited by the LM current during a sustained voluntary contraction of the ES muscle, in keeping with the differences in MEP latencies of early and late I-waves with respect to the D-wave from the epidural recordings (Di Lazzaro et al, 1998 , 2012 ) and from surface EMG recordings (Sakai et al, 1997 ; Hamada et al, 2013 ; Volz et al, 2015 ).…”

“…The influence of TMS-induced current directions on corticospinal excitability of the trunk muscles is less understood. Using single and paired-pulse TMS paradigms a previous study reported that the AP currents generated longer MEP latencies and greater motor cortical inhibition of the ES muscles during the voluntary trunk extension, compared with the PA currents (Desmons et al, 2021 ). However, it remains unclear the extent to which motor cortical circuits that are preferentially activated by different current direction contributed postural and voluntary control of the ES muscles.…”

A Comprehensive Review of Pain Interference on Postural Control: From Experimental to Chronic Pain

“…These results provide evidence that nociceptive sensory input can impact corticospinal excitability at the lower limb. Incoming research using the transcranial magnetic stimulation of the lumbar erector spinae muscles [84] will help to delineate corticospinal excitability modulation with pain.…”

The recruitment of indirect waves within primary motor cortex during motor imagery: A directional transcranial magnetic stimulation study

“…Crucially, M1 is known to integrate inputs from some of these structures, and the latter are differentially recruited according to the current orientation. For example, late I-waves evoked by AP orientation could activate axons of neurons of the premotor cortex projecting to the corticospinal cells (Aberra et al, 2020;Desmons et al, 2021;Groppa et al, 2012;Siebner, 2020;Volz et al, 2015). Recently, Oldrati et al (2021) reported that following off-line 1-Hz inhibitory repetitive TMS over the dorsal premotor cortex (PMd), corticospinal excitability assessed during kinesthetic MI was not significantly higher than rest condition (Oldrati et al, 2021).…”