Interhemispheric functional reorganization after cross nerve transfer: via cortical or subcortical connectivity?

It has been demonstrated that there could be long range interhemispheric reorganization between bilateral hemispheres after peripheral cross nerve transfer. Our previous studies found a striking dynamic process of interhemispheric functional reorganization in adult rats with cross seventh cervical nerve transfer. But it remains a question whether the extensive interhemispheric functional reorganization after cross nerve transfer depends on connectivities at the cortical or subcortical level. In the present study, 18 rats with cross C7 transfer were concurrently treated with corpus callosotomy while the other 18 were not. Intracortical microstimulation was performed in the primary motor cortex (M1) at intervals of 5, 7, and 10 months postoperatively. The neural electrophysiology study showed that the representation of the injured forepaw appeared in the ipsilateral cortex at 5 months after the cross nerve transfer combined with corpus callosotomy, and it shared great overlapping zones with the representation of the health forepaw. And then, at 7-10 months, the cortical representation of the paralyzed forepaw was still located in the ipsilateral motor cortex, although significantly contracted. In contrast, rats with mere cross nerve transfer still presented interhemispheric reorganization. The results indicated that corpus callosotomy in the early stage after cross C7 transfer may had interrupted the interhemispheric functional reorganization. Combined the present study with our previous research findings, we explored the possible pathway and mechanisms of the interhemispheric functional reorganization. Thus we came to the conclusion that interhemispheric connectivity at the cortical level was essential in establishing the new contralateral control of the paralyzed limb at the initial stage after cross nerve transfer.