FIBER-TYPE COMPOSITION OF HINDLIMB MUSCLES IN THE TURTLE, PSEUDEMYS (TRACHEMYS) SCRIPTA ELEGANS

A description is provided of the fiber-type composition of several hin dlimb muscles of the adult turtle, Pseudemys (Trachemys) scripta elega ns. In addition, cross-section areas of each fiber type and an estimat ion of the relative (weighted) cross-section area (wCSA) occupied by t he different fiber types are also provided. Seven muscles were selecte d for study, based on their suitability for future neurophysiological analysis as components of the segmental motor system, and on their hom ologies with muscles in other vertebrates. The test muscles were iliof ibularis (ILF), ambiens (AMB), external gastrocnemius (EG), extensor d igitorum communis (EDC), flexor digitorum longus (FDL), tibialis anter ior (TA), and peroneus anterior (PA). Serial sections of these muscles were stained for myosin adenosine triphosphatase (ATPase), NADH-diaph orase, and alpha-glycerophosphate dehydrogenase (alpha-GPDH), thereby enabling fiber-type classification on the basis of indirect markers fo r contraction speed and oxidative (aerobic) vs. glycolytic (anaerobic) metabolism. All muscles contained three fiber types: slow oxidative ( SO; possibly including some non-twitch tonic fibers); fast oxidative g lycolytic (FOG); and fast glycolytic (Fg). There were at least 30% FOG and 50% FOG + Fg fibers in the seven muscles, the extreme distributio ns being the predominantly glycolytic ILF vs. the predominantly oxidat ive FDL muscle (ILF-15.5% SO, 35.2% FOG, 49.3% Fg vs. FDL-49.1% SO, 41 .1% FOG, 9.8% Fg). As in other species, the test muscles exhibited var ying degrees of regional concentration (compartmentalization) of the d ifferent fiber types. This feature was most striking in ILF. Pronounce d compartmentalization was also observed in AMB, EG, PA, TA, and EDC, whereas the distribution of fiber types in the highly oxidative FDL wa s homogeneous. In five of the seven muscles, fiber size was ranked wit h Fg > FOG > SO. In terms of wCSA, which provides a coarse-grain measu re of the different fiber types' potential contribution to whole muscl e peak force, all muscles exhibited a higher Fg and lower SO contribut ion to cross-section area than suggested by their corresponding fiber- type composition. The largest relative increases in wCSA vs. fiber-typ e composition were in the ILF and AMB muscles. We conclude that the tu rtle hindlimb provides some interesting possibilities for testing for a division of labor among different muscles during different movements (e.g., sustained vs. ballistic), and for study of the behavior of the different fiber (and motor unit) types under normal and perturbed con ditions. The relationships between the present results and previous fi ndings on homologous muscles of the mammalian (cat, rat) and reptilian (lizard) hindlimb are discussed. (c) 1995 Wiley-Liss, Inc.