Characteristics of paravertebral muscles - fibre type distribution pattern in the pika, Ochotona rufescens (Mammalia: Lagomorpha)

Abstract

The evolution of the locomotor apparatus in vertebrates is marked by major reorganizations in trunk’s musculature. The hypothesized functions of mammalian back muscles in the literature are discussed under consideration of the distribution and proportion of oxidative, type-I-fibres, oxidativeglycolytic, type-IIa-fibres and glycolytic, type-IIb-fibres in paravertebral muscles of a small mammal. The fibre type distribution was examined fro ma complete series of histological sections maintaining topographical relationships between the muscles as well as within the muscle, in order to establish the overall distribution pattern. The deep and short muscles showed the highest percentage of oxidative fibres. The larger, superficial paravertebral muscles contained the highest percentage of glycolytic fibres. Two muscles were intermediate in their proportion of fibre types. All epaxial muscles together can be interpreted as an antigravity muscle–complex counteracting enduringly against the rebound tendency caused by gravitation, comparable with antigravity muscles in limbs. A gradient from deep to superficial, or a clear regionalization of oxidative muscle fibres in central deep regions around a large intramuscular tendon was found in the m. spinalis and the m. quadratus lumborum, respectively. Concepts of the function of human back muscles as those of A. Bergmark (1989: Acta Orthop. Scand. 230, 1) or S.G.T. Gibbons & M.J. Comerford (2001: Orthop. Division Rev. March/April, 21) were exposed to be more general within mammals. Functional specializations of different muscles and muscle parts are discussed under the consideration of evolutionary reorganization of the paravertebral musculature in tetrapods.Along the cranio-caudal axis, the percentage of oxidative fibres was decreased in caudal direction within the same muscles, whereas the proportion of glycolytic fibres was increased. Therefore, classifications of muscles as glycolytic or oxidative based on biopsies or analyses of single cross-sections may result in wrong interpretations. Changes in the proportions of the fibre type distribution pattern were mostly due to oxidative and glycolytic fibre types, whereas the percentage of oxidative-glycolytic fibres had only minor influence. A significant positive correlation between the cross-sectional area of the single fibre and its percentage in the area investigated were observed for oxidative fibres, whereby the size was positive correlated to the proportion of the oxidative fibres.