RAPID COMMUNICATION Comparison of Onset Time and Magnitude of Activity for Proximal Arm Muscles and Motor Cortical Cells Before Reaching Movements

Scott, Stephen H. Comparison of onset time and magnitude of magnitude of motor cortical cell discharge covaried with activity for proximal arm muscles and motor cortical cells before the magnitude of muscle force (Evarts 1968), and that the reaching movements. J. Neurophysiol. 77: 1016–1022, 1997. The ensemble activity of cells could roughly predict the temporal activity of motor cortical cells and proximal arm muscles during the pattern of EMG (Fetz et al. 1989; Humphrey 1972). Further, initiation of planar reaching movements were analyzed to identify a study of motor cortical activity during reaching movements whether features of coordinated motor patterns of muscles spanning to grasp an object in space showed that cells related to the the elbow and shoulder were evident in the discharge patterns of proximal arm were active earlier than cells related to the motor cortical cells. Shoulder and elbow muscles were divided into distal arm (Murphy et al. 1985). These shifts in cell onset four groups, flexors and extensors at each joint. Features of the time mirrored the associated shift in EMG onset between initial agonist activity, onset time and magnitude, at the shoulder and elbow were compared for movements in different spatial direcproximal and distal muscles related to the reach and grasp tions. As observed for human movements, differences in the onset phases of movement, respectively, suggesting that motor time and the relative magnitude of electromyographic activity cortex was involved in the selection and timing of muscle (EMG) of muscles acting about the shoulder and elbow were activity to generate volitional movements. dependent on the direction of movement. Motor cortical cells were However, recent studies on motor cortical activity during categorized as elbow or shoulder related on the basis of their rereaching have focused not on its role in motor coordination, sponse to passive movement of the joints. Differences in the onset but on how spatial information related particularly to the time and the relative magnitude of activity of cells related to the direction of hand movement influenced neuronal activity (Fu shoulder and elbow were both dependent on the direction of movement and were similar to those observed for muscles spanning these et al. 1993; Georgopoulos et al. 1982; Schwartz 1992; joints. There was a modest, but significant correlation between the Schwartz et al. 1988). While some studies continued to show onset time and magnitude of EMG for individual muscles. A simithe importance of variables related to muscle activity level lar magnitude-time coupling was observed for individual motor or arm geometry (Caminiti et al. 1990; Kalaska et al. 1989; cortical cells. Variations in the discharge pattern of motor cortical Scott and Kalaska 1995, 1997), it has become less clear cells before movement that mirror those observed for muscles spanhow or whether motor cortex is specifically involved in coorning the shoulder and elbow support the potential role of primary dinating the activity of the elbow and shoulder musculature motor cortex in the selection, timing, and magnitude of agonist during reaching. At the same time, studies on spinalized motor patterns at the shoulder and elbow to initiate reaching movefrogs have demonstrated that microstimulation in the spinal ments. cord could coactivate groups of muscles resulting in a force at the end of the limb whose direction and magnitude varied I N T R O D U C T I O N with limb posture (Bizzi et al. 1991; Giszter et al. 1993). As in studies of motor cortical activity during reaching, these Reaching movements provide a useful model for understudies focused on the relation between spinal function and standing motor coordination because muscle activity acting variables related to the entire limb as a whole rather than at the shoulder must be controlled and sequenced with that related to a portion of the peripheral motor apparatus. From at the elbow to move the hand in space. Several studies have these perspectives, descending commands provide global inshown that the activity of proximal arm muscles is broadly formation related to movement direction, while circuitry in tuned to the direction of movement (Flanders et al. 1994; the spinal cord converts this information into the appropriate Turner et al. 1995; Wadman et al. 1980). Further, the onset output commands to muscles (Bizzi et al. 1991; Georgotime and magnitude of electromyographic activity (EMG) poulos 1996). of muscles spanning the shoulder are coordinated with those A recent study demonstrated that the activity of motor of muscles spanning the elbow and are influenced not only cortical cells was altered during reaching movements using by the initial start and final target positions, but also by limb similar hand trajectories but different arm orientations (Scott geometry (Karst and Hasan 1991). and Kalaska 1997). Those results indicated that the disAn important question remains as to how these coordicharge of single motor cortical cells did not covary exclunated motor patterns are generated by the CNS. Of particular sively with global parameters of movement related to the interest is whether or not supraspinal structures such as the motion of the hand in space. One of the arm orientations primary motor cortex are involved directly in developing used in that study maintained the hand and the elbow at these temporal and spatial patterns of motor output. Traditionally, this view was supported by observations that the nearly shoulder level during reaching. These horizontal