Multiple motor systems in the extraocular muscles of man.

T, he culmination of more than a half century of research in frog muscle was the demonstration of the existence in that species of a separate and distinct slow or tonic striated muscle system that possessed its own peripheral nerve supply and motor units. Thus, the frog has two distinct contractile systems which have different biological functions: a fast or tetanic striate muscle system and a slow or tonic striate muscle system. The latter system is sensitive to acetylcholine and nicotine and yields small, irregular, nonpropagated electrical potentials. Its function is to maintain body postures for long periods without apparent fatigue, e.g., amplexus in the frog. Slow striate muscle systems have been demonstrated in amphibians and reptiles, in the red muscles of birds, in denervated mammalian striated muscle, and in the extraocular muscles of vertebrates (mainly the cat). As the phylogenetic scale ascends to the mammalian level it appears that the unique slow striated systems become limited to the extraocular muscles (per-