AZIZI, E.; BRAINERD, E.L.: Biomechanics of hypaxial myosepta in swimming salamanders

In contrast to the complex three dimensional shape of myomeres in teleost fishes, the lateral hypaxial muscles of salamanders are nearly planar and their myosepta run in a roughly straight line from mid-lateral to ventral. We used this relatively simple system as the basis for a mathematical model of segmented musculature. Results from our model highlight the importance of the deformation of the myosepta in determining the shortening characteristics of a segment. For example, if the myosepta prevent a segment from lengthening in the dorsoventral (DV) direction, then 10% muscle fiber shortening will result in 15.3% shortening of a segment with an initial muscle fiber angle of 35°. However, if the myosepta allow DV swelling of the segments in order to conserve the volume of the muscle, then 10% muscle fiber shortening produces 26.5% shortening of the same segment. We used sonomicrometry to measure the deformation of a hypaxial segment in a swimming salamander (Siren lacertina). Preliminary results show that the myosepta allow some DV lengthening, indicating an amplification of segment shortening that is greater than that produced by muscle fiber angle alone. The model shows that this shortening amplification is achieved with little trade-off of muscle force. The DV lengthening is, however, less than the lengthening predicted by a conservation of volume model, suggesting that the myosepta do constrain the DV swelling of the segments to some extent. We also examined the collagen fiber angles in hypaxial myosepta of S. lacertina and found that they are oriented in the mediolateral direction. The morphology of the myosepta is consistent with our discovery that during swimming the myosepta deform in the DV direction, perpendicular to the collagen fibers. Supported by NSF IBN9875245 to ELB.