Structural origin of the series elastic component in horseshoe crab skeletal muscle fibers

Abstract

The structural origin of the highly compliant series elastic component (SEC) in arthropod skeletal muscle was investigated with intact muscle fibers and glycerinated myofibril bundles from the telson depressor muscle of a horseshoe crab Tachypleus tridentatus. In electrically stimulated muscle fibers, the extension of SEC at the maximum isometric force P 0 was about 6% of the slack fiber length L 0 (sarcomere length, 7 μm), i.e. about 210 nm per half-sarcomere, being too large to be explained by the cross-bridge and the thin filament elasticities. High-speed cinematography of the fibers during a quick release showed uniform distribution of the SEC in each sarcomere. In Ca 2+-activated myofibril bundles, the A-band length was observed to increase transiently by about 10% during Ca 2+-activated contraction. The above elastic A-band lengthening in the myofibril bundles amounted to 5–6% of the initial sarcomere length in the isometric condition, being almost equal to the extension of SEC at P 0 in isometrically contracting muscle fibers. Since the A-band lengthening is known to result from misalignment of the thick filaments, these results indicate that the highly compliant SEC mostly originates from the thick filament misalignment in each A-band during isometric force generation, and titin network providing elastic restoring force.