MECHANISMS OF CORTICAL REORGANIZATION IN LOWER-LIMB AMPUTEES

The human motor system undergoes reorganization after amputation, but the site of motor reorganization and the mechanisms involved are unkno wn. We studied the site and mechanisms of motor reorganization in 16 s ubjects with traumatic lower-limb amputation. Stimulation at different levels in the CNS was used to determine the site of reorganization. T he mechanisms involved were evaluated by measuring the thresholds for transcranial magnetic stimulation (TMS) and by testing intracortical i nhibition and facilitation. With TMS, the threshold for muscle activat ion on the amputated side was lower than that of the intact side, but with transcranial electrical stimulation there was no difference in mo tor threshold between the two sides. TMS at the maximal output of the stimulator activated a higher percentage of the motor neuron pool (%MN P) on the amputated side than on the intact side. The %MNP activated b y spinal electrical stimulation was similar on the two sides. Paired T MS study showed significantly less intracortical inhibition on the amp utated side. Our findings suggest that motor reorganization after lowe r-limb amputation occurs predominately at the cortical level. The mech anisms involved are likely to include reduction of GABAergic inhibitio n.