Insect muscles innervated by single motoneurons: Structural and biochemical features

Abstract

SYNOPSIS. Three locomotory muscles of adult male cockroaches have been biochemically and structurally compared. All fibers of two muscles are innervated by the same motoneuron; both muscles are monofunctional-used only in running. Fibers of the third muscle are also innervated by a single, but different motoneuron. This muscle is bifunctional- used in both walking and flying. Histochemical observations of enzymes associated with energy production indicate that the three muscles are each comprised of a homogeneous population of fibers. However, qualitative differences do correlate with muscle use. The bifunctional muscle shows high oxidative and glycolytic enzyme localization; the monofunctional muscles show a low profile for these enzymes. Quantitative determinations of specific activity for similar enzymes corroborate the histochemical observations. Light and electron microscope observations also indicate that fibers from each of the two muscle groups are structurally homogeneous and distinct. The two monofunctional muscles parallel one another for all structures examined and are different from the bifunctional muscle. The bifunctional muscle fibers have cross-sectional areas twice that of the monofunctional muscles, have a greater myofibrillar diffusion distance, and have about half as many actin per myosin filaments. Stereometric analyses show volume densities for mitochondria and tracheoles to be five times greater; membrane systems associated with excitation- contraction coupling, however, are half as extensive; and myofibrillar volume about 1.3 times less. These data form the basis for studies of denervation and cross-reinnervation, and the role of individual motoneurons in specifying muscle fiber properties. © 1987 by the American Society of Zoologists.