Ipsilateral Motor Cortex Reorganization For Controlling Bilateral Upper Limbs Movements Following The Crossing Of C7 Peripheral Nerves:in Vivo Optogentics And Electrophysiology Study

Background:As one of the most serious sequelae of central nervous system injury, the results of treating upper extremity spastic paralysis are far from satisfactory. Our previous studies showed that changing the peripheral nerve pathways to increase the ipsilateral fibers in controlling the hemiplegia limb could enable the intact hemisphere to control bilateral upper extremities. However, the specific neural pathways involved in limb functional recovery and the underlying central mechanism is still not clear. In the present study, we innovatively applied in vivo optogenetic-electrophysiology techniques to dynamically assess the plastic changes on the intact motor cortex of Thyl-ChR2-EYFP transgenic mice following the cross nerve transfer(C7 spinal root from the intact cortex to C7 of the paralyzed upper limb using sural nerve graft) after traumatic brain injury, together with behavioral assessment and retrograde trans-multisynaptic viral tracing, to elucidate the central mechanism underlying the extremity function recovery. The recent development of optogenetics offers the brain or neuronal activation at higher precision and repeatability, as well as advantages in the time efficiancy, and these advantegous properties can allow to monitor dynamic changes in the bilateral extremity representations and the motor cortex excitability at different stages following the C7 crossing surgery. The results from the optogentic-electrophysiolgical researches are likely to reveal a cortical functional reorganization mechanism underlying the ipsilateral hemisphere controling bilateral extremeties, thus providing a neural basis for ensuring active intervention of the cortical reorganization towards a better functional recovery.Methods:We conducted the rotarod and skilled walking tests to evaluate recovery of bilateral upper limb function at different stages after the controlled cortical impact (CCI) injury of the left primary motor cortex in the mice, which received contralateral C7-ipsilateral C7 transfer or bilateral C7 cut without repairing models. These mice were also subjected to in vivo optogenetic-electrophysiology examination to map the the normal upper limb representation in the motor cortex and its dynamics changes at different stages after the surgery. Pseudorabies virus PRV-Bartha strain dsred were applied to the C7 nerve root of the paralysed limb, combined with serial immunofluoscence imaging of whole brain sections, to anatomically analyse the potential connectivity pathway from the peripheral never to the central cortex.Results:After the introduction of CCI, mice showed a sharp decrease in the score of rotarod and skilled walking tests of the contralateral limb, followed by spontaneous improvement at some extent in the during 1-10 months in the control groups(CCI group and CCI plus bilateral C7 cut group). In contrast, the CCI mice, in which received the C7 transfer operations, showed better scoring in the gesture, head position and advance in comparison with the control groups in the 5th month postsurgery, and the significant difference in these behavorial assays between these two groups was more evident in the period of 6-10 months after the surgery. In the 7th month, the paralysed forelimb exhibited better scores in the carry and skilled walking tests, and the difference is significant during 8-10 months. Although there was a decrease in the scoring for the intact forelimb within 1 month postsurgery, it quickly recovered to the level before the surgery. Furthermore, we utilized optogenetic method to map the normal forelimb representations in the control or ipsilateral motor cortex, and revealed that a small anterior area is involved in controlling the movements of wrist and toes, while one large posterior area controls the movements of the shoulder, elbow and partial wrist in the normal Thyl-ChR2 transgenic mice. Within 4 months after the C7 transfer surgery in the CCI mice, optogenetic activation of the motor cortex on the intact side could only evoke motor evoked potentials (MEPs) in muscles of the contralateral limbs. However, in the 5th month, MEPs on the bilateral triceps could be recorded following the similar optogentic activation of the intact motor cortex, and the cortical representation of the ipsilateral triceps showed a gradual shrinkage in its size and juxtaposition to the original contralateral triceps representation. Meanwhile,7 months after the surgery, similar changes of MEPs and the cortical representations for the bilateral forelimb extensors were also observed. In contrast, MEPs in the ipsilateral biceps could not be evoked throughout the period of 10 months after the surgery. The area and amplitude of the contralateral intact forelimb underwent little change throughout the 10 months after the surgery. Revealed by retrograde trans-synaptic viral tracing from the cut-end of C7 peripheral nerves, in the control groups none of labeled neurons was observed in the intact cortex throughout the 10 months postsurgery, while in the CCI plus C7 transfer group, a few labeled neurons in the intact cortex were first found at the 5th month after surgery and the number of labeled neurons increased over the time.Conclusion:In severe CCI mice, contralateral C7 transfer could promote the functional recovery of ipsilateral elbow and wrist extensor, and the elbow shows a more significant recovery. After the surgery, the intact hemisphere participates in the controlling the movements of the bilateral upper limbs, and at early period after the surgery, the cortical representation for the healthy forelimb is largely over-lapped with that for the paralysed forelimb, which undergoes a gradual shrinkage and juxtaposition towards the healthy forelimb representation over the time after the surgery.