Muscle fiber type analysis in the mouse m. digastricus, m. stylohyoideus, m. zygomaticus and m. buccinator.

It is generally accepted in vertebrates that large motor nerve fibers innervate the white (fast) muscle fibers and small motor nerve fibers innervate the red (slow) muscle fibers. In the previous study (SHIMOZAWA, NAKAMURA 1985), we showed that nerve branches to the venter caudalis m. digastrici and m. stylohyoideus of the mouse consist of considerably larger myelinated nerve fibers compared with the truncus facialis distal to the foramen stylomastoideum innervating the other skeletal muscles in the face. To examine the relationship between the size of nerve fibers and the type of muscle fibers, we performed, in this investigation, quantitative analysis of muscle fiber types (white, intermediate and red) in the venter rostralis (RD) et venter caudalis (CD) m. digastrici, m. stylohyoideus (St), m. zygomaticus (Zy) and m. buccinator (Bu) of the mouse with the Sudan Black B stain preparations. Ratios of the number of muscle fibers of 3 types were RD (28.1%), CD (24.6%), St (27.5%), Bu (28.5%) less than Zy (40.8%) for the white muscle fibers, the RD (58.5%), CD (62.9%), St (53.9%), Bu (60.3%) greater than Zy (47.3%) for the red muscle fibers, while differences between the ratios for these muscles were not significant for the intermediate muscle fibers. Mean major and minor diameters and average transverse areas of the muscle fibers of 3 types were white greater than intermediate greater than red muscle fiber in all muscles examined in this study. When compared between these muscles, they were RD, CD, St greater than Zy, Bu for any type of muscle fibers.(ABSTRACT TRUNCATED AT 250 WORDS)