Spinal cord atrophy and reorganization of motoneuron connections following long-standing limb loss in primates.

Primates with long-standing therapeutic amputations of a limb at a young age were used to investigate the possibility that deefferented motor nerves sprout to new muscle targets. Injections of anatomical tracers into the muscles proximal to the amputated stump labeled a larger extent of motoneurons than matched injections on the intact side or in normal animals, including motoneurons that would normally supply only the missing limb muscles. Although the total numbers of distal limb motoneurons remained normal, some distal limb motoneurons on the amputated side were smaller in size and simpler in form. These results suggest that deprived motoneurons survive and retain function by reinnervating new muscle targets. The sprouted motor efferents may account for some of the reorganization of primary motor cortex that follows long-standing amputation.