A comparative pharmacologic-histologic study of slow and twitch fibers in the superior rectus muscle of the rabbit.

The superior rectus muscle of the rabbit can be separated into two plates: One is comprised, solely of F elder struktur ("slow") fibers and the other almost exclusively of Fibrillenstruktur ("twitch") fibers. Responses of isolated portions of the two muscle types to acetylcholine were compared under identical experimental conditions. The results indicate that the Felderstruktur fibers are responsible for tonic contractions of the extraocular muscles of the rabbit. It is reasonable to assume that they serve the same function in the extraocular muscles of other mammals. Felderstruktur fibers, although striated, demonstrate certain histologic and pharmacologic characteristics, ivhich are intermediate between those of striated and smooth muscle.