MOTOR AREAS OF THE CEREBRAL-CORTEX

The motor cortex includes several areas in the frontal agranular corte x. These areas receive inputs from sensory pathways, motor control str uctures, other cortical areas, and from ''modulatory'' pathways. Motor cortical outputs are widely distributed to many other parts of the ne rvous system and can thereby influence each of the major descending mo tor control pathways and spinal motor circuitry. The most intensively studied motor areas, the premotor area (PMA), supplementary motor area (SMA), and primary motor cortex (MI), appear to have different roles in movement. PMA is involved in coupling arbitrary cues to motor acts, whereas SMA appears to participate more in internal guidance or plann ing of movement. While MI has been implicated in control of muscle for ce or length, more recent data suggest that it encodes higher order pa rameters, such as movement direction. Two new views of motor cortex ar e presented. First, it is argued that MI contains functional subdivisi ons of the face, arm, and leg, and that each subdivision contains a hi ghly overlapping, extensively interconnected and non-topographic inter nal organization. Second, motor representations can reorganize rapidly as a consequence of experience or peripheral lesions. These changes m ay arise through modifications in synaptic coupling among motor cortex neurons. These features of motor cortex suggest a role for motor cort ex in learning and in performing voluntary movements.