Catherine E. Lang1, Karen T. Reilly2 and Marc H. Schieber2,3
Program in Physical Therapy, Washington University School of Medicine, St. Louis, MO, USA and 2Departments of Neurobiology and Anatomy and 3Neurology, University of Rochester, Rochester, NY, USA
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.
2.1 The M1
Our thinking about Ml 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. Second, the concept of the upper motor neuron, which can be traced to Gowers (Phillips and Landau, 1990), has been interpreted to mean that cortical neurons are simply higher order neurons whose physiologic behavior is essentially like that of lower motoneurons. Following these two concepts, M1 has previously been viewed as a somatotopically organized sheet of separate groups of upper motor neurons, each of which controls a pool of spinal motoneurons, and thereby moves a particular body segment (illustrated schematically in Fig. 2.1(a)). The current view of M1 is quite different (Schieber, 2001) such that different spinal motoneuron pools receive input from broad, overlapping cortical territories, and many M1 neurons have projections that diverge to more than one motoneuron pool (Fig. 2.1(b)). In the current view, neurons distributed over a wide cortical region are active during the movement of a given body segment. This section reviews evidence that supports the current, more complex view of M1, and discusses how the current view indicates that M1 is a flexible control system with an inherent capacity for plastic reorganization after brain injury.
Three important points provide evidence for this current, more complex view of M1. First, whereas the prior view of M1 suggested that stimulation of different regions of M1 should elicit movement of different body segments, the current view indicates
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