title: Slow-tonic muscle fibers and their potential innervation in the turtle, Pseudemys (Trachemys) scripta elegans.
abstract: A description is provided of the ratio of slow-tonic vs. slow- and fast-twitch fibers for five muscles in the adult turtle, Pseudemys (Trachemys) scripta elegans. The cross-sectional area of each fiber type and an estimation of the relative (weighted) cross-sectional area occupied by the different fiber types are also provided. Two hindlimb muscles (flexor digitorum longus, FDL; external gastrocnemius, EG) were selected on the basis of their suitability for future motor-unit studies. Three neck muscles (the fourth head of testo-cervicis, TeC4; the fourth head of retrahens capitus collique, RCCQ4; transversalis cervicis, TrC) were chosen for their progressively decreasing oxidative capacity. Serial sections were stained for myosin adenosine triphosphatase (ATPase), NADH-diaphorase, and alpha-glycerophosphate dehydrogenase (alpha-GPDH). Conventional fiber-type classification was then performed using indirect markers for contraction speed and oxidative (aerobic) vs. glycolytic (anaerobic) metabolism: i.e., slow oxidative (SO, including slow-twitch and possibly slow-tonic fibers), fast-twitch, oxidative-glycolytic (FOG), and fast-twitch glycolytic (Fg) fibers. Slow-tonic fibers in the SO class were then revealed by directing the monoclonal antibody, ALD-58 (raised against the slow-tonic fiber myosin heavy chain of chicken anterior latissimus dorsi), to additional muscle cross sections. All five of the tested muscles contained the four fiber types, with the ATPase-stained fibers including both slow-tonic and slow-twitch fibers. The extreme distributions of SO fibers were in the predominately glycolytic TrC vs. the predominately oxidative TeC4 muscle (TrC-SO, 9%; FOG, 20%; Fg, 71% vs. TeC4-SO, 58%: FOG, 16%; Fg, 25%). Across the five muscles, the relative prevalence of slow-tonic fibers (4-47%) paralleled that of the SO fibers (9-58%). TeC4 had the highest prevalence of slow-tonic fibers (47%). The test muscles exhibited varying degrees of regional concentration of each fiber type, with the distribution of slow-tonic fibers paralleling that of the SO fibers. In the five test muscles, fiber cross-sectional area was usually ranked Fg > FOG > SO, and slow-twitch always > slow-tonic. In terms of weighted cross-sectional area, which provides a coarse-grain measure of each fiber type's potential contribution to whole muscle force, all five muscles exhibited a higher Fg and lower SO contribution to cross-sectional area than suggested by their corresponding fiber-type prevalence. This was also the case for the slow-twitch vs. slow-tonic fibers. We conclude that slow-tonic fibers are widespread in turtle muscle. The weighted cross-sectional area evidence suggested, however, that their contribution to force generation is minor except in highly oxidative muscles, with a special functional role, like TeC4. There is discussion of: 1) the relationship between the present results and previous work on homologous neck and hindlimb muscles in other nonmammalian species, and 2) the potential motoneuronal innervation of slow-tonic fibers in turtle hindlimb muscles.
authors.affiliation: School of Biomedical Sciences, Faculty of Health, University of Newcastle, Callaghan, NSW 2308, Australia.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Callister
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Pierce
authors.forename: Jennifer C
authors.initials: JC
authors.lastname: McDonagh
authors.forename: Douglas G
authors.initials: DG
authors.lastname: Stuart
mesh_terms: D000818:Animals; D003712:Dendrites / Q000502:physiology / Q000648:ultrastructure; D005993:Glycerolphosphate Dehydrogenase / Q000378:metabolism; D006019:Glycolysis / Q000502:physiology; D006614:Hindlimb / Q000033:anatomy & histology / Q000502:physiology; D007150:Immunohistochemistry; D023421:Models, Animal; D009046:Motor Neurons / Q000166:cytology / Q000502:physiology; D009068:Movement / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000145:classification / Q000166:cytology* / Q000378:metabolism; D018482:Muscle, Skeletal / Q000166:cytology* / Q000294:innervation / Q000378:metabolism; D009218:Myosins / Q000378:metabolism; D009252:NADPH Dehydrogenase / Q000378:metabolism; D009334:Neck Muscles / Q000166:cytology / Q000294:innervation / Q000378:metabolism; D010085:Oxidative Phosphorylation; D014426:Turtles / Q000033:anatomy & histology* / Q000502:physiology
chemical_list: D005993:Glycerolphosphate Dehydrogenase; D009252:NADPH Dehydrogenase; D009218:Myosins