Motor units and immunohistochemistry of cat soleus muscle after long periods of cross-reinnervation.

1. Cat soleus (slow twitch) was cross-reinnervated with nerve to flexor hallucis or digitorum longus muscle (fast twitch). More than 3 years later motor unit isometric contractions and muscle immunohistochemistry and histochemistry were investigated. All muscles differed from normal fast or slow muscle.2. The motor units could be divided into two groups: one with fast twitches and low tetanic tension, the other with slow twitches and high tension. This is the reverse of the relation between motor unit twitch time and tetanic tension in normal muscle (fast or slow).3. Motor unit twitch time to peak decreased with axonal conduction velocity, as in normal muscle, but so did tetanic tension, which is abnormal.4. Twitch-tetanus ratio increased with twitch time to peak in the group of slow units but not in the fast group (although the range of ratios was as great).5. A tetanus depressed the twitch tension of slow motor units and potentiated fast ones as in normal muscle but the potentiation was often accompanied by an abnormal prolongation of the twitch.6. The mean conduction velocity of axons was slightly higher than at 6 months' reinnervation but below the normal value for fast muscle.7. Antibody to slow myosin was bound strongly to Type I fibres but not to Type II fibres, confirming the histochemical division of fibres into Types I and II.8. More than 95% of the fibres were oxidative, with Type I predominating over Type II a in the ratio of about 2:1.9. The higher tension of the slow motor units was the result of three factors: the number of fibres per motor unit was at least three times that in the fast; Type I fibres had cross-sectional areas little less than Type II (a and b together) and were estimated to develop more tension per unit area. All three findings were different from those in normal fast muscle.10. One flexor hallucis longus muscle was self-reinnervated and examined histochemically. This muscle was abnormal in that a large majority of the fibres were Type I.