Peripheral nerves to flexor (common peroneal) and extensor (tibial) nerves in a hind limb of seven 2‐6 month old cats were cut and cross‐united to study the plasticity in the spinal cord. The extent to which motoneurones from extensor and flexor motor pools were misdirected to their antagonistic muscles was determined by measuring the potentials generated at the spinal roots from the crossed nerves. The axons contributing to the extensor nerves normally leave the cord in the L7 and S1 ventral and dorsal roots while the axons contributing to the flexor nerves normally leave the cord in the L6 and L7 ventral and dorsal roots. Following cross‐union, medial gastrocnemius (m.g.) and lateral gastrocnemius‐soleus (l.g.s.) nerves were primarily supplied by L6 and L7 ventral and dorsal roots, and common peroneal (c.p.) nerves were primarily supplied by L7 and S1 ventral and dorsal roots. A method for quantifying the completeness of cross‐reinnervation was developed. The pattern of e.m.g. activity in cross‐reinnervated muscles during locomotion was primarily determined by the innervating nerve with the reinnervated flexor muscles being activated during the extensor phase. However, the cross‐reinnervated extensor muscles showed evidence of extensor activity in addition to the double‐burst pattern typical of flexor nerves. This extensor activity was more prominent when the nerve cross was less complete. We conclude that during locomotion the activity of spinal motoneurones was not substantially modified by inappropriate peripheral connexions, even when the nerve cross was carried out in young animals. This conclusion is discussed in relation to previous studies which suggested some degree of functional modification.
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