Progressive transneuronal changes in the brainstem and thalamus after long-term dorsal rhizotomies in adult macaque monkeys

This study deals with a potential brainstem and thalamic substrate for the extensive reorganization of somatosensory cortical maps that occurs after c hronic, large-scale loss of peripheral input. Transneuronal atrophy occurre d in neurons of the dorsal column (DCN) and ventral posterior lateral thala mic (VPL) nuclei in monkeys subjected to cervical and upper thoracic dorsal rhizotomies for 13-21 years and that had shown extensive representational plasticity in somatosensory cortex and thalamus in other experiments. Volum es of DCN and VPL, number and sizes of neurons, and neuronal packing densit y were measured by unbiased stereological techniques. When compared with th e opposite, unaffected, side, the ipsilateral cuneate nucleus (CN), externa l cuneate nucleus (ECN), and contralateral VPL showed reductions in volume: 44-51% in CN, 37-48% in ECN, and 32-38% in VPL. In the affected nuclei, ne urons were progressively shrunken with increasing survival time, and their packing density increased, but there was relatively little loss of neurons (10-16%). There was evidence for loss of axons of atrophic CN cells in the medial lemniscus and in the thalamus, with accompanying severe disorganizat ion of the parts of the ventral posterior nuclei representing the normally innervated face and the deafferented upper limb. Secondary transneuronal at rophy in VPL, associated with retraction of axons of CN neurons undergoing primary transneuronal atrophy, is likely to be associated with similar with drawal of axons from the cerebral cortex and should be a powerful influence on reorganization of somatotopic maps in the somatosensory cortex.