Excitatory and inhibitory responses to cervical root magnetic stimulation in healthy subjects

Abstract

Objectives: To characterize direct and reflex hand muscle responses to cervical root magnetic stimulation (CRMS) in healthy volunteers during sustained voluntary contraction.

Methods: In 18 healthy volunteers, we recorded from the first dorsal interosseous (FDI) muscle the responses to CRMS of progressively increasing intensity and level of muscle contraction. The compound muscle action potential (CMAP) and the silent period (SP) were compared to those obtained with plexus, midarm and wrist stimulation. Additionally, in a smaller number of subjects, we obtained the peristimulus time histogram (psth) of single motor unit firing in the FDI, examined the effects of vibration and recorded the modulation of sustained EMG activity in muscles of the lower limbs.

Results: Increasing CRMS intensity led to larger CMAP with no relevant changes in SP1 or SP2, except for lower amplitude of the burst interrupting the silent period (BISP). Increasing the level of muscle contraction led to reduced CMAP, shorter SP duration and increased BISP amplitude. The psth analysis showed the underlying changes in the motor unit firing frequency that corresponded to the changes seen in the CMAP and the SP with surface recordings. Progressively distal stimulation led to CMAPs of shorter latency and increased amplitude, SPs of longer latency and shorter duration, and a BISP of longer latency. Vibration led to reduction of the SP. CRMS induced SPs in muscles of the lower limb.

Conclusions: CRMS induces excitatory and inhibitory responses in hand muscles, fitting with the expected behavior of mixed nerve stimulation at very proximal sites.

Significance: Characterization of the effects of CRMS on hand muscles is of physiological and potentially clinical applicability, as it is a painless and reliable procedure.

Keywords: Cervical root magnetic stimulation; Mixed nerve stimulation; Peristimulus time histogram; Silent period.

Fig. 1

A: Sketch of the coil position during the experimental sessions. B and C: Schematic drawing of the responses obtained to cervical magnetic stimulation in the right first dorsal interosseous muscle at rest (B) and during contraction (C), labelled with the acronyms indicating the measures taken at each recording: CMAP amp = Compound muscle action potential peak amplitude; CMAP lat = Compound muscle action potential peak latency; EMG = electromyography; SP lat: Silent period onset latency; SPdur: Silent period total duration; BISP size: Burst interrupting the silent period amplitude, measured with respect to the level of EMG background. Only measuring details for the SP1 are marked to avoid confusion, but the same measures were taken for the SP2.

Fig. 2

CMAPs recorded from the right FDI to CRMS at rest using round coil face B (left side of the figure) and face A (right side of the figure). Numbers in the middle column indicate the stimulus intensity used for each recording, beginning at threshold intensity for face B. Note the progressive increase of peak amplitude when stimulating with face coil B, and the differences in threshold with respect to the recordings obtained when stimulating with coil face A. Each of the graphs shown in this figure and in the ones to follow result from averaging 10 rectified responses.

Fig. 3

Responses recorded from the right FDI to CRMS during sustained voluntary FDI contraction at 50% MVC. CRMS intensity was progressively increased from threshold to the intensity at which the maximum amplitude CMAP was obtained at rest (in this case, 70%). Note the progressive initial increase and late decrease of the BISP, without significant changes in the SPs.

Fig. 4

Responses recorded from the right FDI to CRMS at an intensity 200% RMT during sustained voluntary FDI contraction at progressively higher percentages of the MVC (25%, 50%, 75% and 100%). Note the progressive decrease of the CMAP amplitude, latency shortening of BISP, and decrease of duration in the SPs, with increasing levels of muscle contraction.

Fig. 5

Responses recorded from the right FDI during sustained voluntary contraction of mild intensity (50% of MVC) to magnetic stimuli applied to the cervical cord (first trace), brachial plexus (second trace) and midarm (third trace), as well as to electrical stimulation of the ulnar nerve at the wrist (fourth trace). Stimulus intensity was 200% RMT for CRMS. Note the progressive shortening of CMAP latency and lengthening of BISP, with maintenance of SP duration from proximal to distal stimulation. F = F wave.

Fig. 6

Recording from the right tibialis anterior to cervical (upper trace) and plexus magnetic stimulation (lower trace) at an intensity of 300% RMT, during voluntary activation of the tibialis anterior at 75% of maximum voluntary contraction. Note the silent period appearing in this subject at an onset latency of 48 ms, without significant differences between the two stimulation sites.

Fig. 7

Recordings from the right FDI to CRMS during sustained voluntary contraction of mild intensity (50% of MVC) when vibration was applied to the forearm (bottom trace) in comparison to a control condition, without vibration (top trace). Stimulation intensity was 200% RMT. Note the decreased duration of SP2 with vibration.

Fig. 8

Needle recordings of motor unit action potentials (A) and the peristimulus time histogram (B) representing the number of motor unit action potentials per 5 ms bins in 25 out of the 100 epochs of 500 ms recorded around the time of stimulus application (S) in one representative subject. The upper recording in A was done at low CRMS intensity, insufficient to recruit any CMAP, while the lower recording in A was done at higher CRMS intensity, which recruited a tiny response in every trace (arrowhead). Notice the motor unit action potential firing in the middle of an otherwise empty stretch, coinciding, respectively, with the BISP and the SP1 and SP2. Labels indicating where the corresponding events observed in the main study occurred are shown in B.

Fig. 8

Needle recordings of motor unit action potentials (A) and the peristimulus time histogram (B) representing the number of motor unit action potentials per 5 ms bins in 25 out of the 100 epochs of 500 ms recorded around the time of stimulus application (S) in one representative subject. The upper recording in A was done at low CRMS intensity, insufficient to recruit any CMAP, while the lower recording in A was done at higher CRMS intensity, which recruited a tiny response in every trace (arrowhead). Notice the motor unit action potential firing in the middle of an otherwise empty stretch, coinciding, respectively, with the BISP and the SP1 and SP2. Labels indicating where the corresponding events observed in the main study occurred are shown in B.