| Abstract No.: | B-D2144 |
| Country: | Canada |
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| Title: | EVOLVING MOTOR CORRECTIONS CAN BE RAPIDLY SUPPRESSED BY A COMMON SPINAL MECHANISM |
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| Authors/Affiliations: | 1 Isaac Kurtzer*; 1 Andrew Pruszynski; 1 Stephen Scott;
1 Queen's University, Kingston, ON, Canada
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| Content: | Objectives: Rapid corrections to an imposed muscle stretch involve a series of distinct muscular events by increasingly sophisticated neural mechanisms: short-latency reflex (~25-50 ms), long-latency reflex (50-100ms), and a voluntary reaction (100-180 ms). Here we describe how these rapid corrections are affected by perturbation duration. Such perturbations sample the temporal range of naturally occurring disturbances and can help clarify whether rapid corrections are largely pre-planned and triggered or continuously incorporate sensory information.
Materials and Methods: Subjects maintained a steady posture of their upper arm against torque perturbations at the elbow joint (±2Nm). These mechanical perturbations were applied by a planar robot while surface EMG was recorded from the subject’s elbow flexor and extensor muscles. Importantly, the duration of the applied perturbation was varied unpredictably from trial to trial (pulse width = 25ms – 1500ms) so we could examine when corrective responses incorporate perturbation-offset relative to a maintained perturbation. ROC analysis determined the timing of suppression.
Results: We found that perturbation offset lead to a strong attenuation of muscular activity relative to a sustained perturbation. This effect began at a relatively fixed and rapid time (20-30 ms from perturbation offset) for both long-latency reflexes (50-100ms post-perturbation) and early voluntary responses (>100 ms post-perturbation). In fact, rapid suppression was observed for every offset-time we examined – even within long-latency or voluntary epochs – which indicates that sensory input plays a continuous role in sculpting the net motor correction.
Conclusions: Corrective actions involve a series of qualitatively different epochs of activity. In particular, short-latency reflexes, long-latency reflexes, and voluntary actions are generated by increasingly sophisticated (though slower acting) neural mechanisms. In contrast, we show that a rapid spinal mechanism continuously filters these commands with a strong attenuating effect following perturbation offset. That is, motor commands can be generated via slow and intelligent pathways but suppressed at fast notice. Such flexibility is well-suited to a world which includes unpredictable perturbations of variable duration.
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