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The Journal of Neurophysiology Vol. 85 No. 2 February 2001, pp. 605-619
Copyright ©2001 by the American Physiological Society
Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Saltiel, Philippe,
Kuno Wyler-Duda,
Andrea D'Avella,
Matthew C. Tresch, and
Emilio Bizzi.
Muscle Synergies Encoded Within the Spinal Cord: Evidence From
Focal Intraspinal NMDA Iontophoresis in the Frog. J. Neurophysiol. 85: 605-619, 2001. This paper relates
to the problem of the existence of muscle synergies, that is whether
the CNS command to muscles is simplified by controlling their activity
in subgroups or synergies, rather than individually. We approach this
problem with two methods that have been recently introduced:
intraspinal N-methyl-D-aspartate (NMDA)
microstimulation and a synergy-extracting algorithm. To search for a
common set of synergies encoded for by the spinal cord whose
combinations would account for a large range of electromyographic (EMG)
patterns, we chose, rather than examining a large range of natural
behaviors, to chemically microstimulate a large number of spinal cord
interneuronal sites in different frogs. A possible advantage of this
complementary method is that it is task-independent. Visual inspection
suggested that the NMDA-elicited EMG patterns recorded from 12 leg
muscles might indeed be constructed from smaller subgroups of muscles
whose activity co-varied, suggestive of synergies. We used a
modification of our extracting computational algorithm whereby a
nonnegative least-squares method was employed to iteratively update
both the synergies and their coefficients of activation in
reconstructing the EMG patterns. Using this algorithm, a limited set of
seven synergies was found whose linear combinations accounted for more
than 91% of the variance in the pooled EMG data from 10 frogs, and
more than 96% in individual frogs. The extracted synergies were
similar among frogs. Further, preferred combinations of these synergies
were clearly identified. This was found by extracting smaller sets of
four, five, or six synergies and fitting each synergy from those sets
as a combination from the set of seven synergies extracted from the
same frog; the synergy combinations from each frog were then pooled
together to examine their frequency of occurrence. Concordance with
this method of identifying synergy combinations was found by examining
how the synergies from the set of seven correlated pair-wise as they
reconstructed the EMG data. These results support the existence of
muscle synergies encoded within the spinal cord, which through
preferred combinations, account for a large repertoire of spinal cord
motor output. These findings are contrasted with previous approaches to
the problem of synergies.
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