Transformation of contraction speed in muscle following cross-reinnervation; dependence on muscle size.

The characteristics of isometric contractions and the force-velocity relation were studied in flexor digitorum longus, flexor hallucis longus and soleus muscles of the cat, in situ, at 37 degrees C and with nerve stimulation. The two flexors were identified as typical fast twitch muscles and the soleus as a typical slow twitch muscle. Following self-reinnervation, both fast and slow muscles retained, to a large extent, their basic contraction characteristics. The soleus muscle, when cross-reinnervated with the nerve of either flexor hallucis longus muscle or extensor digitorum longus muscle exhibited a more complete slow-to-fast transformation than when cross-reinnervated with the nerve of flexor digitorum longus muscle. The flexor digitorum longus muscle underwent a greater degree of fast-to-slow transformation than the flexor hallucis longus muscle, when each was cross-reinnervated with the soleus nerve. The data previously reported for sarcomere shortening velocities of the cross-reinnervated muscles in the rat, the rabbit and the cat are reviewed in the light of present findings. It is found that the discrepancies obtained between species and between different muscles in the same species, with respect to the degree of muscle-speed transformation following cross-reinnervation, are correlated with the differences in the size-ratio of the muscles used in the cross-reinnervation procedure.