Heterogeneity of the contractile proteins and myosin-linked regulation in the superficial muscle of the lobster

Date of Completion

January 1998


Biology, Animal Physiology




Vertebrate skeletal muscles are composed of a population of fibers that are both functionally and structurally homogeneous. The filaments involved in contraction, myosin and actin, are arranged into a regular pattern within the contractile unit, the sarcomere. Within the filaments are the contractile proteins arranged such that a homogeneous population is composed. There are many isoforms of the contractile proteins, but these isoforms clearly correspond to the types of skeletal muscle fibers that exist. On the other hand, heterogeneity appears to be a key feature of many invertebrate skeletal muscle systems, particularly within the crustaceans. This study focuses on the heterogeneity that exists within a crustacean muscle system, the lobster superficial flexor muscle (SFM) which is a dually regulated muscle located within the abdominal musculature.^ Heterogeneity of both form and function appears to be a critical characteristic of this muscle system. Previous studies examining this muscle system have demonstrated that there is heterogeneity of the regulatory proteins, the contractile protein isoforms associated with actin, and mechanical properties within individual muscle fibers. This study further examines this heterogeneity. It demonstrates that structural and spatial heterogeneity extends from the level of the muscle fiber down to the level of the individual myosin molecule. In addition, heterogeneity also appears to exist in terms of the mode of myosin-linked regulation that exists within the muscle fibers. Correlations exist between the distribution of the fiber types that comprise the SFM and the distribution of the myosin filaments, the myosin light chains, and the modes of myosin-linked regulation within the SFM.^ Heterogeneity of form, structure, and function within the lobster SFM may have emerged as a means for controlling tensions. Adjustments of tensions within the SFM depends on localized excitations and on facilitation. It is possible that a more precise control may be achieved by varying the structural components and the proteins that are involved in the contractile process. In having such an arrangement, the lack of precision offered by the innervation of this muscle system may be offset by varying the activity of the various proteins within the muscle. ^