The catch property of ordinary muscle.

Three background issues motivate the present study. First, it has long been known that a special category of muscle, found in certain molluscs, can maintain tension for long periods after brief excitation with very little energy expenditure.'-3 Relaxation in these catch muscles may be separately initiated by inhibitory or relaxing neural input, by 5-hydroxytryptamine, by temperatures over 30'C, and perhaps by low Ca++ concentration.4' Molluscan muscles contain large amounts of a special protein, paramyosin (tropomyosin A), which might be crucial in the maintenance of prolonged contractions,6 7 or it might only contribute to the unusually great tensile strength of the thick filaments.3 During long-maintained contractions in intact animals there appears to be a constant, low level of neurally evoked electrical activity.8'-0 In spite of the large number of diverse experiments that have been performed on these special muscles, it is still said that "the basis of catch tension is not yet understood."4 Second, there are the phenomena we may call the Blaschko effects, observed in certain crustacean muscles." Here, a single impulse in an excitatory motor axon to the crayfish claw closer muscle produces only a small contraction, while a low-frequency train of impulses gives rise to a slowly developed but much larger tension plateau. But a single extra impulse interpolated within this lowfrequency train produces a marked extra contraction that is maintained beyond the normal twitch duration. If a brief high-frequency train of impulses is added to the low-frequency background, then the muscle may contract to a new tension plateau from which it does not fully relax for long periods after the return to lowfrequency stimulation. Finally, Partridge'2 has recently shown that a mammalian skeletal muscle preparation shows a slowly decaying hysteresis when the nerve is stimulated by sinusoidally frequency-modulated impulse trains. His technique did not, however, allow the localization of the site of the hysteretic process. We feel that the evidence presented below suggests that these three categories of muscle response may be mechanistically related. We have chosen arthropod nerve-muscle preparations for the examination of Partridge's hysteresis effect because they make it relatively easy to work with single motor units and single muscle fibers. These preparations have not only exhibited the hysteresis phenomenon but have additionally allowed us to verify the old Blaschko effects and to relate them to modern studies. And our work with one type of insect muscle has also convinced us that the phenomena we have observed are not specific to crayfish muscle. Materials and Methods.-The claw opener muscle of the crayfish (Procambarus cdarkii) has one excitatory axon." It can be selectively stimulated by electrodes placed under the thin bundle of the nerve coursing through the meropodite. Stimulation of that axon gives rise to small junctional potentials which may summate up to a 30-mv depolarization in all the muscle fibers. Summation is necessary before measurable tension changes