Fate of the soma and dendrites of cord-projection central neurons after proximal and distal spinal axotomy: an intracellular dye injection study.

We used rat rubrospinal neurons as a model to study the soma-dendritic morphology of cord-projection neurons following spinal axonal injury. We examined lumbar-projection neurons following both upper cervical and lower thoracic axotomy to find out whether changes were dependent on the proximity of the lesion to the cell body. Axotomized neurons were marked with retrograde tracer and studied 4 and 8 weeks later with intracellular dye injection technique. Axotomy resulted in prominent shrinkage of their soma and relatively minor reduction of their dendritic spreads. The degree of soma shrinkage depended on both the duration of survival and the proximity of lesion. In addition, dendritic modification peaked 4 weeks following proximal lesion, which was also achieved 8 weeks following distal axotomy. Tractotomy at upper cervical and lower thoracic levels also allowed us to compare the effect of distal axotomy on cervical and lumbar-projection neurons. Results show that although cervical-projection neurons responded more quickly than lumbar-projecting ones, they however showed a similar degree of alteration in both their soma and dendrites 8 weeks following distal axotomy. In summary, cord-projection neurons survived 8 weeks following either upper cervical or lower thoracic axotomy with relatively intact dendritic features. Taken together, our data thus far suggest that cord-projection central neurons continue to integrate inputs and control supraspinal targets following spinal axotomy. The minor dendritic shrinkage within two months of spinal axotomy rejuvenates hopes for functional recovery if regeneration of their spinal axons can be achieved at least within this time frame.