Organization of the forelimb area in squirrel monkey motor cortex: representation of digit, wrist, and elbow muscles

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Abstract
The EMG in 8 to 14 hand, forearm, and arm muscles evoked by intracortical electrical stimulation was recorded at 433 sites in layer V in the region of the forelimb area of the primary motor cortex (MI) of three squirrel monkeys during ketamine anesthesia. At each site, the EMG was recorded at movement threshold (T) and at 1.5T and 2T at each site (but ≤60 μA), and the threshold movement was noted. In the animals examined, the total MI forelimb area identified by movements or EMG occupied about 25 to 35 mm2. At most sites from which a forelimb movement was evoked, EMG activity was evoked in one or more of the recorded muscles. One group of sites located rostrolaterally to the main forelimb area was separated by an intervening zone largely related to the face. The average area from which digit, wrist, elbow, or shoulder movement was evoked at threshold was nearly the same, and their movement thresholds were not significantly different. Average movement thresholds across the anterior-posterior extent of MI were also similar. All muscles recorded could be activated by cortical stimulation. Most commonly more than one muscle was activated from a single site. The highest individual EMG levels were produced at sites from which more than one muscle was activated. These results suggest that small regions of MI influence multiple muscles. Individual muscles were typically activated at multiple, spatially separated locations. For many muscles, increasing the stimulation intensity revealed additional separate areas of activation. Spatial locations of different muscles showed considerable interanimal variation. The size of most muscle representations was relatively large. The smallest representations always included the intrinsic hand muscles and the largest included the proximal muscles. Orderly topographic relationships among forelimb joints or muscles within the MI forelimb area were not apparent. Although distal muscle activation tended to be found posteriorly in the forelimb area and proximal muscles tended to be activated from anterior sites, both could be activated from broadly distributed and overlapping areas. The broad, overlapping nature of the muscle representation supports the concept that a small region of cortex is involved in controlling functional groups of muscles. The intermingling of muscle representations may provide a substrate for local cortical interactions among territories representing various muscle synergies or for changing associations of muscle groups. The representation plan derived from these mappings contains elements of all previously described summaries of MI organization. We argue that none of these adequately summarizes the internal organization of the MI forelimb area. Instead, patterns of organization that include functional combinations of muscles must be considered.

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