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

Sensorimotor Integration in Chronic Low Back Pain

OBJECTIVE

Chronic low back pain (CLBP) impacts on spine movement. Altered sensorimotor integration can be involved. Afferents from the lumbo-pelvic area might be processed differently in CLBP and impact on descending motor control. This study aimed to determine whether afferents influence the corticomotor control of paravertebral muscles in CLBP. Fourteen individuals with CLBP (11 females) and 13 pain-free controls (8 females) were tested with transcranial magnetic stimulation (TMS) to measure the motor-evoked potential [MEP] amplitude of paravertebral muscles. Noxious and non-noxious electrical stimulation, and magnetic stimulation in the lumbo-sacral area were used as afferent stimuli and triggered 20 to 200 ms prior to TMS. EMG modulation elicited by afferent stimulation alone was measured to control net motoneuron excitability. MEP/EMG ratio was used as a measure of corticospinal excitability with control of net motoneuron excitability. MEP/EMG ratio was larger at 60, 80 and 100-ms intervals in CLBP compared to controls, and afferent stimulations alone reduced EMG amplitude greater in CLBP than controls at 100 ms. Our results suggest alteration in sensorimotor integration in CLBP highlighted by a greater facilitation of the descending corticospinal input to paravertebral muscles. Our results can help to optimise interventions by better targeting mechanisms.

Corticomotor Control of Lumbar Erector Spinae in Postural and Voluntary Tasks: The Influence of Transcranial Magnetic Stimulation Current Direction

Lumbar erector spinae (LES) contribute to spine postural and voluntary control. Transcranial magnetic stimulation (TMS) preferentially depolarizes different neural circuits depending on the direction of electrical currents evoked in the brain. Posteroanterior current (PA-TMS) and anteroposterior (AP-TMS) current would, respectively, depolarize neurons in the primary motor cortex (M1) and the premotor cortex. These regions may contribute differently to LES control. This study examined whether responses evoked by PA- and AP-TMS are different during the preparation and execution of LES voluntary and postural tasks. Participants performed a reaction time task. A Warning signal indicated to prepare to flex shoulders (postural; n = 15) or to tilt the pelvis (voluntary; n = 13) at the Go signal. Single- and paired-pulse TMS (short-interval intracortical inhibition-SICI) were applied using PA- and AP-TMS before the Warning signal (baseline), between the Warning and Go signals (preparation), or 30 ms before the LES onset (execution). Changes from baseline during preparation and execution were calculated in AP/PA-TMS. In the postural task, MEP amplitude was higher during the execution than that during preparation independently of the current direction (p = 0.0002). In the voluntary task, AP-MEP amplitude was higher during execution than that during preparation (p = 0.016). More PA inhibition (SICI) was observed in execution than that in preparation (p = 0.028). Different neural circuits are preferentially involved in the two motor tasks assessed, as suggested by different patterns of change in execution of the voluntary task (AP-TMS, increase; PA-TMS, no change). Considering that PA-TMS preferentially depolarize neurons in M1, it questions their importance in LES voluntary control.