Over the last years,anatomical,electrophysiological and genetic studies have carefully dissected the pathways connecting the brain and the spinal cord.Lawrence and Kuypers (1968) described the organization of the descending motor pathways in the non-human primate spinal cord.Although there are some differences between species regarding the precise anatomical location of each spinal pathway and the selective connectivity onto spinal inteurons and motoneurons,the patt of organization described is conserved among the mammalian spinal cord (Courtine et al.,2007).