A nanometer difference in myofilament lattice spacing of two cockroach leg muscles explains their different functions

Abstract

Muscle is highly organized across scales. Consequently, small changes in arrangement of myofilaments can influence macroscopic function. Two leg muscles of a cockroach, have identical innervation, mass, twitch responses, length-tension curves, and force-velocity relationships. However, during running, one muscle is dissipative, while the other produces significant positive mechanical work. Using time resolved x-ray diffraction in intact, contracting muscle, we simultaneously measured the myofilament lattice spacing, packing structure, and macroscopic force production of these muscle to test if nanoscale differences could account for this conundrum. While the packing patterns are the same, one muscle has 1 nm smaller lattice spacing at rest. Under isometric activation, the difference in lattice spacing disappeared explaining the two muscles identical steady state behavior. During periodic contractions, one muscle undergoes a 1 nm greater change in lattice spacing, which correlates with force. This is the first identified feature that can account for the muscles different functions.