Contractile properties and ultrastructure of extensor digitorum longus and soleus muscles in spinal cord transected rats.

The spinal cord of rats 25 days of age was transected at the thoracic level, and the contractile properties as well as the ultrastructure of their extensor digitorum longus (EDL) and soleus muscles was examined. In the normally slow-twitch soleus muscle, the operation produces a marked reduction of contraction time as well as the appearance of other contractile characteristics of a fast-twitch muscle, namely, post-tetanic potentiation and cooling potentiation of the isometric twitch. This operation has little effect on the fast-twitch EDL. The ultrastructure of the EDL is similarly unaffected by the motor paralysis introduced by transection, but little change to the soleus structure was found, except for an increase in sarcoplasmic reticulum surface to fibre volume ratio. These results provide little evidence that muscle structure, especially the amount of sarcoplasmic reticulum, plays a major role in the determination of contractile speed, and confirms that structure may be regulated independently by the nerve.