Motor Areas on the Medial Wall of the Hemisphere

The primary motor cortex (M1) receives input from four premotor areas on the medial wall of the hemisphere: the supplementary motor area (SMA) and three cingulate motor areas located on the banks of the cingulate sulcus (CMAr, CMAd and CMAv). All four premotor areas have maps of the body containing distinct proximal and distal representations of the arm. Surprisingly, the size of the distal representation is comparable to or larger than the size of the proximal representation in each area. Thus, contrary to some previous hypotheses, the anatomical substrate exists for the premotor areas on the medial wall to be involved in the control of distal, as well as proximal arm movements. Each of the premotor areas on the medial wall also has substantial direct projections to the spinal cord. Corticospinal axons from these premotor areas terminate in the intermediate zone of the spinal cord. Some corticospinal axons from SMA, CMAd, and CMAv terminate around motoneurons. In this respect, these motor areas are like M1 and appear to have direct connections with spinal motoneurons, particularly those innervating muscles of the fingers and wrist. These results suggest that the premotor areas on the medial wall are an important source of descending commands for the generation and control of movement. In recent experiments we examined the pattern of functional activation in the premotor areas on the medial wall during the performance of sequences of pointing movements. The patterns of activation were then compared with the body maps revealed by our anatomical studies. Overall, our initial results indicate that the attributes of motor control are unequally represented across the premotor areas. For example, one of the areas on the medial wall, the CMAd, was strongly and selectively activated during the performance of highly practised, remembered sequences of movement. Further insights into the function of the premotor areas are likely to come from examining their participation in a broad range of behavioural paradigms. These initial results support our hypothesis that each premotor area makes some unique contribution to the planning, initiation and/or execution of movement.