LARGE-SCALE REORGANIZATION AT MULTIPLE LEVELS OF THE SOMATOSENSORY PATHWAY FOLLOWS THERAPEUTIC AMPUTATION OF THE HAND IN MONKEYS

Reorganization of somatosensory cortex after peripheral nerve damage t ypically has been attributed to cortical plasticity. Here we provide e vidence that much of the large-scale cortical reorganization that occu rs after a major Boss of peripheral inputs reflects the sprouting or e xpansion of afferents from the remaining forelimb into deprived territ ories of the spinal cord and brainstem. We examined sensory afferent t erminations in the spinal cord and brainstem, and determined the somat otopic organization of cortical area 3b in three adult monkeys with pr evious hand or forearm amputation, as veterinary treatment of forelimb injuries. In each monkey, the distribution of labeled sensory afferen t terminations from the remaining parts of the forelimb was much more extensive than the normal distribution of inputs from the forelimb, an d extended into portions of the dorsal horn of the spinal cord and the cuneate nucleus of the brainstem related to the amputated hand. In th e same animals, tactile stimulation of the forelimb activated much of the deprived hand representation in area 3b of cortex; the lateral por tion of the deprived region in area 3b appeared to be reactivated by i nputs from the face. These data provide important new evidence that on e of the mechanisms subserving large scale reorganization in cortex is a relay of topographic changes that occur subcortically. Presumably, the expanded primary sensory inputs activate postsynaptic neurons that are normally driven by inputs from the hand so that the neurons now h ave receptive fields on the forearm. Since the topographic representat ion of the body is greatly magnified in the relay to cortex, the subco rtical changes can result in dramatic cortical map changes.