Isometric contractions of motor units in self-reinnervated fast and slow twitch muscles of the cat.

1. Isometric contractions of motor units have been studied in self-reinnervated soleus and flexor digitorum longus muscles, the motor nerves of which had been transected 6 months earlier.2. The distributions of motor unit tetanic tensions in both muscles were skewed towards large values and showed greater variance than normal: there were motor units which produced more tension and others which developed less tension than in control muscles. Mean tensions of motor units are compared with those of the controls.3. The variance of times to peak of motor units in both muscles was significantly smaller than normal.4. Conduction velocities of axons were slower than normal above the point of nerve section. A number of axons did not functionally cross this point and those that did conducted slowly below the neuroma.5. Many of the correlations seen in normal muscle between axon conduction velocity, motor unit tetanic tension, twitch time to peak and ratio of twitch to tetanic tension were demonstrated in reinnervated muscle.6. We suggest that the pattern of motor unit organization in self-reinnervated muscle can in part be due to random regrowth of axons but is largely produced by specific processes related to axon size which are not identical in fast and slow muscle.