Morphological types of motor nerve terminals in rat hindlimb muscles, possibly innervating different muscle fiber types

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Summary

With methylene blue stain, three morphological types of motor nerve terminals were distinguished in hindlimb muscles (extensor digitorum longus, semitendinosus, soleus, and tibialis anterior muscles) of rat. Type A terminals possess many long thin branches with numerous minute swellings, type C ones possess few and thick terminal branches with few large swellings, and type B ones possess intermediate characteristics. The frequencies of occurrence of these types are characteristic for each muscle and differ between different hindlimb muscles. Specifically, soleus muscle contains a much higher proportion of type B terminals than other hindlimb muscles, and the superficial part of tibialis anterior muscle contains a higher proportion of type A terminals and a lower proportion of type C ones than the deep part. The frequencies of occurrence of types A, B, and C terminals closely parallel the frequencies of “white”, “intermediate”, and “red” muscle fibers (as determined by a fat stain), respectively, in the different muscles.

As also suggested previously for the diaphragm, it is concluded that three morphological types of motor nerve terminals each may innervate one of three types of muscle fibers.