Morphology and Function of Lateral Hypaxial Musculature in Salamanders

Abstract

SYNOPSIS. The lateral hypaxial musculature (LHM) of salamanders may serve as a useful model for understanding the functions of LHM in tetrapods more generally. Salamanders have between two and four layers of LHM, arranged segmentally in myomeres. These layers produce three primary mechanical actions: they bend the body, pressurize the body, and produce or resist torsion about the long axis of the body. The optimum muscle fiber angle for forceful bending is 0؇ to the long axis, the optimum angle for pressurization is 90؇, and the optimum angle for torsion is 45؇. For generating bending and torsional moments, lateral (superficial) muscle layers have greater mechanical advantage than medial (deep) layers. For increasing body pressure, by contrast, medial layers have greater mechanical advantage. A comparison of muscle fiber angles in aquatic and terrestrial salamanders reveals that some aquatic salamanders have one muscle layer with a low fiber angle which may represent a specialization for swimming. Overall, however, the fiber angles in the LHM of terrestrial and aquatic salamanders are surprisingly similar. In contrast, the pattern of fiber angles in caecilians is different, suggesting that these amphibians use their LHM differently. The fiber angle models and morphological observations presented here form a framework which may be useful in future studies of lateral hypaxial musculature.

Muscle fiber angle, segment bulging and architectural gear ratio in segmented musculature

“…The TA will experience smaller longitudinal segment strains, but we still expect that the dorsoventral bulging of the segments will cause the muscle fibers of the TA to undergo some active lengthening during swimming. Our calculations predict that the EOS and TA contribute little to axial bending, indeed they may generate forces that oppose lateral bending, but they may function to balance torsional moments and modulate body pressure and connective tissue stiffness during swimming (Brainerd and Simons, 2000;Bennett et al, 2001).…”

“…We can compare these maximum initial muscle fiber angles with the actual muscle fiber angles observed in the lateral hypaxial musculature of salamanders. In eight representative species from eight families, the fiber angles in the external and internal oblique layers are generally in the range of 20-40° (Brainerd and Simons, 2000;Simons and Brainerd, 1999). This range of initial muscle fiber angles would allow maximum muscle fiber strains of up to ~15% for most of the bulging conditions defined by our models.…”

“…Muscle fiber angles in the transverse abdominis (TA) and the external oblique superficialis (EOS; present only in some salamanders) are generally higher, ranging from 60 to 80°in the TA and from 50 to 70°in the EOS (Simons and Brainerd, 1999;Brainerd and Simons, 2000). These high angles indicate that the fibers in these layers probably undergo very low strains or even active lengthening during segment shortening (EMG studies indicate that the EOS and TA are active during swimming; Carrier, 1993;Bennett et al, 2001).…”

Bases ósseas e musculares dos cortes comerciais do tronco de jacaré-do-Pantanal (Caimanyacare Daudin, 1802)

“…Embora haja variação no número de camadas presentes nessa musculatura em salamandras (Brainerd & Simons 2000), o arranjo da parede latero-ventral da região abdominal dos tetrápodes é altamente conservador (Bhullar 2009).…”

“…A disposição do m. oblíquo abdominal externo em duas camadas foi descrita na salamandra D. ensatus (Carrier 1993) e identificada em outras salamandras por Brainerd & Simons (2000) aonde quatro camadas de músculos hipaxiais laterais estavam presentes, com diferentes ângulos das fibras musculares em cada uma delas.…”