Motor units in cross-reinnervated fast and slow twitch muscle of the cat.

1. Isometric contractile properties of motor units were measured in cross-reinnervated fast (flexor digitorum longus) and slow (soleus) twitch muscles of the cat. All but one cross was at least 95% pure. 2. There was a reduction in the number of motor units in all muscles, but totals remained about equal in cross-reinnervated soleus and flexor digitorum longus. 3. Motor unit tensions (mean and maximum values) were higher in cross-reinnervated soleus than in cross-reinnervated flexor digitorum longus, reversing the differences between normal muscles. This was due to increases in muscle mass and in the tension developed per unit cross-sectional area. There were motor unit tensions larger and smaller than those seen in normal muscle, but the range was comparable with that seen in self-reinnervated muscle. 4. The changes in twitch time to peak of whole muscle following cross-reinnervations resulted from a change over the whole range of motor units. The conversion of soleus was less complete than that of flexor digitorum longus, and the time to peak of its fastest motor unit was twice as long as any seen in normal flexor digitorum longus. 5. In neither of the cross-reinnervated muscles were the fast contracting motor units larger than the slow contracting ones, and in cross-reinnervated soleus they were smaller. 6. Axonal conduction velocity was correlated with motor unit tension in both muscles and with twitch time to peak in cross-reinnervated flexor digitorum longus, but in all cases less clearly than in normal muscles. 7. The ratio of twitch to tetanic tension increased with increasing twitch time to peak, as in normal muscles.