Fiber type population in limb muscles ofMicrocebus murinus

Populations and distributions of fiber types were studied in 19 limb muscles ofMicrocebus murinus. Proportions and cross-sectional areas of muscles fiber types were compared with data from the literature for other prosimians (Galago, Lemur, andNycticebus), another primate (Macaca cynomolgus), and the rat. Most muscles are heterogenous, with type I fibers (slow oxidative) localized in the deeper part, near the bone. Type IIA fibers (fast oxidative glycolytic) are more evenly distributed than type I and type IIB (fast glycolytic). The combination of large number and large size of type I fibers results in enhanced slow-twitch and oxidative properties as required for antigravity function of postural muscles. Compared with other primates,Microcebus shows relatively small cross-sectional areas of fibers and less numerous type I fibers, in every muscle, which is probably related to small body mass. The fiber type population of the different components of the quadriceps femoris is also related to the particular mode of locomotion of the mouse-lemur: running and leaping, climbing and hopping. M. vastus medialis and m. vastus lateralis are made up only of fast twitch fibers, IIA and IIB. A possible repercussion of hypothyroidism during the rest season and a decrease in locomotor activity was the subject of investigation of the fiber type proportion and section areas. No difference were found between individuals euthanized during the active period and those at rest period. Either a very low level of thyroxine associated with reduced activity is sufficient to maintain the processes controlling myosin expression, or the effects on muscles fibers of natural hypothyroidism and hypokinesia neutralize each other during the rest season.