Transplantation of embryonic spinal motoneurones into an injured adult spinal cord results in reinnervation of the denervated hindlimb muscles via the avulsed and reimplanted L4 ventral root. The aim of our study was to investigate the integration of the grafted motoneurones into the host spinal cord circuits. Embryonic spinal cord pieces were grafted into the L4 segment of adult rats, the L4 ventral root was avulsed and reimplanted into the graft. In control animals the L4 ventral root was avulsed but they received no grafts. Six months after grafting EMG recordings were performed from the Extensor Digitorum Longus, Soleus and Tibialis Anterior muscles. After EMG recordings the anterograde tracer Phaseolus vulgaris was injected iontophoretically into the L4 segment, either into the graft or into the host cord near the graft. The EMG analysis of recordings showed that grafted motoneuron activity was related to rhythmic locomotor limb movement. In the EMG activity recorded in grafted animals during sitting 2 populations of motor unit action potentials (MUAPs) were distinguished. MUAPs in the first group were recruited for a long period of time and firing below 50 Hz, while MUAPs of the second group (with relatively higher amplitude) were recruited for short period of time (<200 ms) and firing between 50 and 200 Hz. This latter group may be characteristic for the grafted motoneurones. The morphological analysis of our anterograde tracing studies has shown that while processes of neurones of the host spinal cord are only able to enter the periphery of the graft, grafted neurones send great number of processes into the host spinal cord and establish functional connections with the host spinal cord circuits. These results indicate improved functional recovery induced by the grafted embryonic motoneurones following depletion of the host motoneurones and suggest that there is a functionally meaningful rewiring between the neurones of the graft and the host spinal cord.
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