Successful transplantation of motoneurons into the peripheral nerve depends on the number of transplanted cells.

Transplantation of motoneurons (MN) into the peripheral nerve to provide a source of neurons for muscle reinnervation, termed motoneuron integrated striated muscle (MISM), may provide the potential to restore functional muscle activity, when combined with computer-programmed functional electrical stimulation (FES). The number of MNs required to restore innervation to denervated muscles in adult Fischer 344 rats was investigated by comparing two groups, one transplanted with 2 × 10(5) cells (group A) and the other with 1 × 10(6) cells (group B). Twelve weeks after transplantation, electrophysiological analysis, muscle function analysis, and tissue analysis were performed. The mean motor nerve conduction velocity was faster (12.4 ± 1.0 m/s vs. 8.5 ± 0.7 m/s, P = 0.011) and the mean amplitude of compound muscle action potential was larger (1.6 ± 0.4 mV vs. 0.7 ± 0.2 mV, P = 0.034) in group B. The dorsiflexed ankle angle was larger in group B (27 ± 5° vs. 75 ± 8°, P = 0.02). The mean myelinated axon number in the peroneal nerve and the proportion of reinnervated motor end plates were also greater in group B (317 ± 33 vs. 104 ± 17, 87.5 ± 3.4% vs. 40.6 ± 7.7%; P < 0.01, respectively). When sufficient MNs are transplanted into the peripheral nerve, MISM forms functional motor units. MISM, in conjunction with FES, provides a new treatment strategy for paralyzed muscles.