COMPARISON OF ONSET TIME AND MAGNITUDE OF ACTIVITY FOR PROXIMAL ARM MUSCLES AND MOTOR CORTICAL-CELLS BEFORE REACHING MOVEMENTS

The activity of motor cortical cells and proximal arm muscles during t he initiation of planar reaching movements were analyzed to identify w hether features of coordinated motor patterns of muscles spanning the elbow and shoulder were evident in the discharge patterns of motor cor tical cells. Shoulder and elbow muscles were divided into four groups, flexors and extensors at each joint. Features of the initial agonist activity, onset time and magnitude, st the shoulder and elbow were com pared for movements in different spatial directions. As observed for h uman movements, differences in the onset time and the relative magnitu de of electromyographic activity (EMG) of muscles acting about the sho ulder and elbow were dependent on the direction of movement. Motor cor tical cells were categorized as elbow or shoulder related on the basis of their response to passive movement of the joints. Differences in t he onset time and the relative magnitude of activity of cells related to the shoulder and elbow were both dependent on the direction of move ment and were similar to those observed for muscles spanning these joi nts. There was a modest, but significant correlation between the onset time and magnitude of EMG for individual muscles. A similar magnitude -time coupling was observed for individual motor cortical cells. Varia tions in the discharge pattern of motor cortical cells before movement that mirror those observed for muscles spanning the shoulder and elbo w support the potential role of primary motor cortex in the selection, timing, and magnitude of agonist motor patterns at the shoulder and e lbow to initiate reaching movements.