Interpreting muscle function in invertebrates.

Abstract

A brief historical survey of muscle studies is given to show how fundamental knowledge based on the use of invertebrate muscles became divorced from a “mainstream” of investigation of muscle, with unfortunate consequences. The diversity of muscles is emphasized, and the complexity of individual muscles described. For invertebrates, the graded contraction is the all-important functional unit and its intimate nature is not yet understood. An hypothesis of graded contraction is presented, taking into account the excitation-contraction coupling sequence. The “fundamental” functional unit of muscle is a “diad” and the underlying volume of contractile material reached by activator released from it. The hypothesis involves the postulate of a parallel elastic element within each sarcomere, which becomes a series elastic element during graded excitation owing to unequal contraction of individual sarcomeres.