2 - Human voluntary motor control and dysfunction

Summary

The ability to promote functional recovery after nervous system injury depends in part on understanding how the normal brain works and how the damaged brain reorganizes itself. This chapter discusses the current understanding of how areas of the cerebral cortex and their descending pathways contribute to voluntary motor control in humans in the context of how these areas may provide compensatory control for each other in the damaged brain. Primary motor cortical (M1) areas and non-primary motor cortical areas (NPMAs) are presented as a flexible control system for voluntary movement, with an inherent capacity for reorganization. In part, this capacity for flexible reorganization arises from the intrinsic organization of cortical areas, in part from the network of connections among areas, and in part from the availability of more than one descending pathway. The concluding section of this chapter discusses how, in addition to lesion size and lesion location, territories and tracts that are spared after a lesion can affect the capacity for functional recovery of movement.

The M1

The current view of M1 organization

Our thinking about M1 has been shaped largely by the oversimplification of two related concepts. First, the concept of motor somatotopy, which was carried farthest by the work of Penfield and his memorable cartoon, the homunculus (Penfield and Boldrey, 1937; Penfield and Rasmussen, 1950), has been interpreted to mean that different segments of the body are controlled from spatially separate regions of M1, down to the level of a different region for each finger of the hand.