| Abstract No.: | 312 |
| Country: | Canada |
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| Title: | THE CONTRIBUTION OF THE MOTOR CORTEX TO THE CONTROL OF LOCOMOTION AND REACHING IN THE CAT: MUSCLE SYNERGIES AS A MODEL FOR CONTROL |
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| Authors/Affiliations: | 1 Trevor Drew*;
1 University of Montreal, QC, Canada
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| Content: | Most studies directed at understanding motor cortical function are addressed at the question of what signal is encoded in the activity patterns of motor cortical neurones. This is an important step for understanding motor cortical function. However, equally important is to understand how that signal may interact with the spinal interneuronal networks to provide the required motor output. This issue, which has been much less studied, will form the focus of my presentation.
Cat locomotion provides an interesting model for the study of this question for several reasons. First, we have a century of detailed information on the organisation of the spinal cord that provides a foundation for understanding the pathways by which corticospinal neurones can initiate and modulate activity. Second, the fact that the isolated spinal cord can generate a basic locomotor rhythmicity has provided some insight into how these spinal circuits (commonly referred to as a central pattern generator: CPG) are functionally organised during motor activity. Third, the ability to record and stimulate in the motor cortex of awake, behaving cats allows an appreciation of how cortical signals interact with these spinal networks.
Experiments from several laboratories, including my own, suggest that the motor cortex makes a substantial contribution to the changes in muscle activation patterns responsible for the modifications in limb trajectory needed to step over obstacles. Our own recording studies strongly suggest that the descending signal from the motor cortex is organised in a muscle-based coordinate system and that it specifies the changes in the patterns of muscle activity that are needed to produce visually-guided modifications of gait. The results from microstimulation studies suggest that this signal is integrated into the base locomotor rhythm so as to modify locomotion while maintaining the basic rhythmic pattern. This suggests that the integration may occur via the corticospinal projections to interneuronal populations that are part of, or are regulated by, the activity of the spinal central pattern generator (CPG). In our previous work we have proposed a conceptual model based on the unit pattern generator model of Grillner. Recently, we have made a reinvestigation of the organisation of the locomotor pattern by performing a cluster analysis of the patterns of EMG activity in the fore- and hindlimbs. This analysis suggested that small groups of muscles, acting around multiple joints, are organised to discharge simultaneously (synergies) and sequentially during discrete behavioural events in the step cycle. During gait modifications, the composition of the synergies remained largely intact but the phase of activation, and the magnitude of the EMG responses, was modified. We suggest that sub-populations of motor cortical neurones, active during different parts of the swing phase, modulate the activity of these multi-articular synergies. Interestingly, we have found that the composition of these synergies is largely maintained during reaching. As we have found that motor cortical neurones discharge similarly during locomotion and reaching, we further suggest that common control mechanisms may be active during both behaviours. (Supported by the CIHR and the FRSQ).
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