| Abstract No.: | C-D3131 |
| Country: | Canada |
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| Title: | THRESHOLD CONTROL OF INTENTIONAL MOVEMENTS AND UNLOADING REFLEXES REVEALED BY TRANSCRANIAL MAGNETIC STIMULATION OF THE MOTOR CORTEX. |
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| Authors/Affiliations: | 1 Helli Raptis*; 2 Liziane Burtet; 2 Robert Forget; 1 Anatol G. Feldman;
1 U. de Montréal, Département de Physiologie; 2 U. de Montréal, École de réadaptation, Montreal, QC, Canada
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| Content: | Objectives: Numerous studies showed correlation of activity of cells in the motor cortex with mechanical variables or EMG signals characterizing the motor output. Correlations, however, do not necessarily imply causality. We tested the hypothesis that descending cortical signals can influence the output variables while remaining virtually independent of them.
Materials and Methods: Transcranial magnetic stimulation (TMS, single pulses, 1.2-1.4 motor threshold) of the M1 wrist area was used to evaluate the excitability of cortico-spinal pathways projecting to wrist muscles in 16 healthy subjects. Motor evoked potentials (MEPs) resulting from TMS were recorded from wrist flexors (FCR, FCU) and extensors (ECR, ECU). TMS was produced at two actively specified wrist positions (45º wrist flexion and 25º extension), before and after a wrist flexion or extension movement. By applying small loads compensating passive muscle tensions, it was possible to equalize the EMG activity at these positions at near zero levels.
Results: Extensors MEPs at the extension position were substantially bigger whereas flexors responses were smaller than at the flexion position (p<0.05). In other words, active changes in wrist position were associated with reciprocal changes in cortical facilitation of flexors and extensors motoneurons, and these changes were independent of EMG levels. When subjects relaxed wrist muscles and each of the two wrist positions was established passively, the MEP amplitudes significantly decreased and became less correlated with position. We also analyzed the changes in MEPs resulting from unloading of pre-loaded wrist extensors elicited either involuntary, by a torque motor, or intentionally, by subjects themselves. The wrist position changed in the former but not in the latter case. In both cases, extensors MEPs decreased but the decrease was bigger in self-initiated unloading (MEP amplitudes decreased by 60±15% in self-initiated and by 26±20% in involuntary unloading , p=0.02), even though the final EMG levels were the same.
Conclusion: The dissociation between EMG levels and corticospinal excitability suggests that motor cortex is not involved in the specification of EMG patterns and resulting mechanical variables. Rather, the primary effect of cortico-spinal facilitation is a change in the threshold position of body segments, i.e., the position at which muscles are silent but are ready to be activated in response to deviations from it. Results reiterate the finding (Feldman and Orlovsky 1972) that an adequate measure of influences of cortico-spinal and other descending systems on motoneurons is a shift in the threshold position of body segments. Thereby, to produce a motor action, descending systems reset the threshold position of appropriate body segments whereas the EMG patterns emerge following the difference between the actual and the threshold position of these segments. Our findings also imply that the effect of reflex and central influences on motoneurons strongly depends on this difference. This conclusion should be taken into account while using TMS as a prognostic and diagnostic tool in assessing the efficacy, integrity and reorganization of descending pathways in clinical populations. Funding : CIHR, NSERC, FQRNT |
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