Motor areas of the cerebral cortex.

The motor cortex includes several areas in the frontal agranular cortex. These areas receive inputs from sensory pathways, motor control structures, other cortical areas, and from "modulatory" pathways. Motor cortical outputs are widely distributed to many other parts of the nervous system and can thereby influence each of the major descending motor 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 planning of movement. While MI has been implicated in control of muscle force or length, more recent data suggest that it encodes higher order parameters, such as movement direction. Two new views of motor cortex are presented. First, it is argued that MI contains functional subdivisions of the face, arm, and leg, and that each subdivision contains a highly overlapping, extensively interconnected and non-topographic internal organization. Second, motor representations can reorganize rapidly as a consequence of experience or peripheral lesions. These changes may arise through modifications in synaptic coupling among motor cortex neurons. These features of motor cortex suggest a role for motor cortex in learning and in performing voluntary movements.