Modulation of single motor unit discharges using magnetic stimulation of the motor cortex in incomplete spinal cord injury

  • Smith H
  • Davey N
  • Savic G
 et al. 
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Abstract

OBJECTIVES—Motor
evoked potentials (MEPs) and inhibition of voluntary contraction to
transcranial magnetic stimulation (TMS) of the motor cortex have longer
latencies than normal in patients with incomplete spinal cord injury
(iSCI) when assessed using surface EMG. This study now examines the
modulation of single motor unit discharges to TMS with the aim of
improving resolution of the excitatory and inhibitory responses seen
previously in surface EMG recordings.
METHODS—A group of
five patients with iSCI (motor level C4-C7) was compared with a group
of five healthy control subjects. Single motor unit discharges were
recorded with concentric needle electrodes from the first dorsal
interosseus muscle during weak voluntary contraction (2%-5%
maximum). TMS was applied with a 9 cm circular stimulating coil centred
over the vertex. Modulation of single motor unit discharges was
assessed using peristimulus time histograms (PSTHs).
RESULTS—Mean (SEM)
threshold (expressed as percentage of maximum stimulator output
(%MSO)) for the excitatory peak (excitation) or inhibitory trough
(inhibition) in the PSTHs was higher (p(excitation=47.1 (5.9) %MSO; inhibition=44.3 (3.2) %MSO) than in
controls (excitation=31.6 (1.2) %MSO; inhibition=27.4 (1.0) %MSO).
Mean latencies of excitation and inhibition were longer (pthe patients (excitation=35 (1.8) ms; inhibition=47.1 (1.8) ms) than in
the controls (excitation=21.1 (1.6) ms; inhibition=27 (0.4) ms).
Furthermore, the latency difference (inhibition−excitation) was
longer (p(6.2 (0.6) ms).
CONCLUSION—Increased
thresholds and latencies of excitation and inhibition may reflect
degraded corticospinal transmission in the spinal cord. However, the
relatively greater increase in the latency of inhibition compared with
excitation in the patients with iSCI may reflect a weak or absent early
component of cortical inhibition. Such a change in cortical inhibition
may relate to the restoration of useful motor function after iSCI.

Author-supplied keywords

  • and inhibitory
  • corticospinal
  • evoked potentials
  • in incomplete spinal cord
  • injury
  • isci
  • magnetic stimulation
  • meps
  • motor
  • motor unit
  • spinal cord injury

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Authors

  • H Smith

  • N Davey

  • G Savic

  • D Maskill

  • P Ellaway

  • M Jamous

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