Coordination of reinnervated muscle and reorganization of spinal cord motoneurons after nerve transection in mice.

Combining physiological and anatomical methods, the dependence of muscle coordination on the alteration in connections between anterior horn motoneurons and muscle following peripheral nerve repair has been examined in adult mice. Fourteen weeks after transection and readaptation of the right common peroneal nerve electromyographic activity patterns of the tibialis anterior (TA) and medial gastrocnemius (MG) muscles were recorded simultaneously during free running. The number and location of motoneurons supplying the TA were determined bilaterally with horseradish peroxidase. In control animals with normal muscle coordination the number of TA motoneurons varied considerably, but was bilaterally symmetrical. The average distribution of cells in both anterior horns was nearly identical. After reinnervation muscle coordination was significantly impaired. Prolonged TA activity phases overlapped antagonistic MG activity with large inter-individual variability. The number of TA motoneurons was reduced to 70.2 +/- 24.0% compared to the contralateral control side. The distribution of the cells was significantly shifted to the caudal direction. This indicated inappropriate innervation of the TA by motoneurons which originally served the peroneal muscles, as suggested by the results in another control group. There was no correlation between the number of motoneurons reinnervating the TA and the restoration of motoneuron somatotopy. These two factors promoted the reestablishment of muscle coordination significantly and independently of one another.