Motor Unit Size of M. Stylohyoideus and M. Stapedius in the Mouse
Okajimas Folia Anatomica Japonica
To elucidate the relation between the caliber of myelinated nerve fibers (MNFs) and motor unit size (i.e., number of muscle fibers per motor unit), motor unit size was measured for the mouse m. stylohyoideus, innervated predominantly by extra large MNFs, and compared with that of the mouse m. stapedius, innervated predominantly by large MNFs. All muscle fibers and MNFs were counted morphologically, and mean motor unit size was calculated for each large MNFs. Edgeworth (1899) found on the dog
... vus facialis that nerve branches to the venter caudalis of the m. digastricus and m. stylohyoideus consisted of larger myelinated nerve fibers (MNFs) than nerve branches to the other facial muscles. In a study on the mouse n. facialis, Shimozawa and Nakamura (1985) also found that MNFs of the r. digastricus and r. stylohyoideus consisted of considerably larger MNFs than the truncus facialis distal to the foramen stylomastoideum innervating the skeletal muscles in the face (Shimozawa and Furuta, 1984) . It is generally considered that, in vertebrates, the large motor MNFs innervate the white (fast twitch) muscle fibers and the small motor MNFs innervate the red (slow twitch) muscle fibers (Tasaki and Mizutani, 1944; Kuffler and Williams, 1953) . Moreover, electrophysiological studies by Henneman and Olson (1965) in the cat m. soleus and m. gastrocnemius indicated that the diameter of the axon may be related to the number, type and size of the muscle fibers innervated by the axon. To examine the relationship between the caliber of MNFs and the types of muscle fibers, Shimozawa and Ishizuya-Oka (1987) performed a quantitative analysis of the muscle fiber types in the mouse m. digastricus and m. stylohyoideus, innervated by larger MNFs, and the m. zygomaticus and m. buccinator, innervated by smaller MNFs. However, the results showed that the m. digastricus and m. stylohyoideus do not have a higher ratio of white muscle fibers than the m. zygomaticus and m. buccinator. Shimozawa and Nakamura (1993) roughly classified the caliber of MNFs of the mouse into 4 classes: small (myelin sheath; MS 1 -2Ium, axon; A 0.5-1 iu,m), medium (MS 2-3.5 [tm, A 1-2 pm), large (MS 3.5-6 tun, A 2-4 (um) and extra-large (MS 6-12 tim, A 4-9 pm), according to electron microscopic measurements. The r. stylohyoideus consists of predominantly extra large MNFs (Shimozawa and Nakamura, 1985) and the r. stapedius consists of predominantly large MNFs (Shimozawa, 1978) in the mouse. To elucidate the relation between the caliber of MNFs and motor unit size, we performed a comparison between the motor unit size of the mouse m. stylohyoideus and that of the m. stapedius. Materials and Methods Twelve m. stapedius from 7 adult mice (ddy, 8 weeks old, 30.9 ± 0.9 g body weight) and 12 m. stylohyoideus from 6 adult mice (ddy, 8 weeks old, 29.1 0.4 g body weight) were used in this study. The muscle fibers and MNFs of each muscle were counted from the same side of each mouse.