Thalamic and brainstem contributions to large-scale plasticity of primate somatosensory cortex.

After long-term denervation of an upper limb in macaque monkeys, the representation of the face in somatosensory cortex expands over many millimeters into the silenced representation of the hand. Various brainstem and cortical mechanisms have been proposed to explain this phenomenon. Reorganization in the thalamus has been largely ignored. In monkeys with deafferented upper limbs for 12 to 20 years, it was found that the brainstem cuneate and the thalamic ventral posterior nuclei had undergone severe transneuronal atrophy, and physiological mapping in the thalamus revealed that the face and trunk representations were adjoined while the normally small representation of the lower face had expanded comparable to the expansion in cortex. Reorganization of brainstem and thalamic nuclei associated with slow transneuronal atrophy is likely to be a progressive process. When coupled with divergence of ascending connections, it is likely to make a substantial contribution to representational changes in cortex.