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Muscles respond to imposed length changes with rapid, large force changes followed by slow relaxations to new steady-state forces. These responses were originally believed to arise from background levels of actomyosin binding. Discovery of giant sarcomere-spanning proteins suggested muscle passive responses could arise from length changes of elastic domains present in these proteins. However, direct evidence that actomyosin plays little role in passive muscle force responses to imposed length changes has not been provided. We show here that a poison of actomyosin interaction, thiourea, does not alter initial force changes or subsequent relaxations of lobster stomatogastric muscles. These data provide direct evidence that background actomyosin cross-bridge formation likely plays, at most, a small role in muscle passive responses to length changes. Thiourea does not alter lobster muscle electrical responses to motor nerve stimulation, although in this species it does cause tonic motor nerve firing. This firing limits the utility of thiourea to study lobster muscle electrical responses to motor nerve stimulation. However, it is unclear whether thiourea induces such motor nerve firing in other animals. Thiourea may therefore provide a convenient technique to measure muscle electrical responses to motor nerve input without the confounding difficulties caused by muscle contraction.

Abstract: This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca++ binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved. [Copyright &y& Elsevier]