Development and Plasticity of the Corticospinal System in Man

The young human brain is highly plastic, thus brain lesions occurring during development interfere with the innate development of the architecture, connectivity, and mapping of functions and trigger modifications in structure, wiring, and representations (for review see Payne & Lomber, 2001). In childhood, the motor cortex and/or the corticospinal tract is a common site of brain damage and the prenatal or immediately perinatal period is the most common time for brain damage to occur. It is now increasingly appreciated that the corticospinal system is capable of substantial reorganization after lesions and such reorganization is likely to underlie the partial recovery of function (Terashima, 1995; Eyre et al., 2001; Eyre et al., 2002). Clearly the developing nervous system has a much greater potential for plasticity, which can involve plasticity not only of the motor areas of the ipsi-lesional cerebral cortex but also of the contralesional cortex, the corticospinal tract, and the spinal cord network (Benecke et al., 1991; Carr et al., 1993; Cao et al., 1994; Lewine et al., 1994; Maegaki et al., 1995; Terashima, 1995; Nirrko et al., 1995; O’Sullivan et al., 1998; Balbi et al., 2000; Eyre et al., 2000a; Eyre et al., 2001). Functional and anatomical evidence demonstrates that spontaneous plasticity can be potentiated by activity and by