Histology of highly-stretched beef muscle. II. Further evidence on the location and nature of gap filaments

Abstract

The response of beef muscle fibers to gross overstretch varied. In most fibers the A-filaments dislocated and stretched, and then a gap opened between the A- and I-bands. In a small proportion of fibers a wide gap opened between A- and I-bands while the A-band remained intact. These anomalous fibers appear to dislocate under extreme stretch, since transitions from such sarcomeres to the dislocated type were sometimes seen along a fiber. Thus there appear to be two routes to the same final state. A few fibers appeared to have gone into rigor before stretching, since cross-bridging of actin and myosin filaments survived, while thick filaments fractured. Extraction of myosin and actin from stretched fibers left the gap filaments running from an overlap in the center of the sarcomere through the Z-line, supporting the view that each gap filament was present as a core to the thick filament, emerging at one end only. Fibers released after stretch always sprang back, even after removal of myosin and actin. The order in “springbacks” varied greatly with the circumstances but the gap filaments appear to provide the restoring force in every case. A model is postulated in which each gap filament forms a core to two thick filaments, each in adjacent sarcomeres, linking them through the Zline and forming an elastic component in muscle, centered on the Z-line.