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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.

We stained sarcomere thin filaments with fluorescently labeled phalloidin, measured sarcomere and muscle length, and calculated sarcomere number in pyloric and gastric mill muscles. A wide range of sarcomere lengths (3.25-12.29 microm), muscle lengths (5.9-21.1 mm), and sarcomere numbers (648-3,036) were observed. Sarcomere number differences occurred both because of changes in sarcomere length and muscle length, and sarcomere and muscle length varied independently. This independence, the wide range of sarcomere numbers present, and the muscles being all 'slow', graded muscles allowed us to use these data to test Huxley and Neidergerke's (1954) hypothesis that muscle dynamics depend on sarcomere number. The time constants of exponential fits to contraction relaxations were used to measure muscle dynamics, and comparison of theoretical predictions and experimental results quantitatively confirm the predicted dependence. The differing dynamics of the various pyloric muscles are likely functionally important, and the dependence of muscle dynamics on sarcomere number implies that sarcomere number is likely closely regulated in these muscles. The stomatogastric system may thus be an excellent model system for studying the mechanisms regulating muscle sarcomere number.