Department of Exercise Sciences


Disinhibition in the human motor cortex is enhanced by synchronous upper limb movements

James W. Stinear and Winston D. Byblow Stinear JW, Byblow WD.
Published in The Journal of Physiology (2002) 543:307-316

Introduction and Aim

  • The phasic modulation of wrist flexor cortico-motor disinhibition has previously been demonstrated during the flexion phase of rhythmical passive flexion-extension of the human wrist.
  • Here we ask if rhythmical bimanual flexion-extension movements of the wrists of neurologically intact subjects, modulate inhibitory activity in the motor cortex.

Methods

  • In the first experiment intracortical inhibition was assessed when one wrist was passively flexed and extended on its own, with the addition of the opposite limb voluntarily moving synchronously in a mirror symmetric pattern, and also in a near-symmetric asynchronous pattern.
  • Dual-pulse transcranial magnetic stimulation was applied to the cortical representations of flexor carpi radialis (FCR) and extensor carpi radialis (ECR) of a test limb when each muscle was lengthening and when it was shortening during passive movement.
  • A second experiment investigated the modulation of spinal reflex pathway activity (H-reflexes in FCR) during the same three movement conditions.
  • A third experiment examined the effect of contralateral wrist movement alone on the excitability of corticomotoneuronal pathways to a static test limb.

Results

  • When the wrist flexors of both upper limbs were shortening simultaneously (i.e. synchronously), intracortical inhibition associated with flexor representations was suppressed to a greater extent than when the two muscles were shortening asynchronously (see Figure 1).
  • The analysis of H-reflexes failed to reveal an effect of movement condition.
  • Rhythmical flexion-extension of one wrist phasically modulated the excitability of CM pathways to FCR and ECR of the contralateral limb maintained at rest, however, the pattern of modulation was the reverse of that revealed in the first experiment.
iciIntact002
Figure 1. Cortical disinhibition in the representation of FCR (A.) and ECR (B.) from Experiment 1. Mean MEP amplitudes of conditioned responses expressed as a percent of non-conditioned responses. Filled bars represent mid-flexion responses, open bars represent mid-extension responses, the hatched bar represents static responses. Error bars represent 1 standard error of the mean. *, p = < 0.05; **, p = < 0.01, from one-tailed Student’s t-tests, df 1,7. The bracketed star indicates there is a difference between in-phase and novel means during flexion. The remaining levels of significance are from movement condition means compared with static means.

Conclusions

  • The results of the three experiments indicate that modulation of inhibitory activity was taking place at the cortical level.
  • These findings may have further application in the study of rehabilitation procedures where the effects of simultaneous activation of affected and unaffected upper limbs in hemiparetic patients are to be investigated.

For more information on this project contact:

Winston Bublow
Email: w.byblow@auckland.ac.nz