Mechanisms underlying stabilization of temporally summated muscle contractions in the lobster (Panulirus) pyloric system

Muscles are the final effectors of behavior. The neural basis of behavior t herefore cannot be completely understood without a description of the trans fer function between neural output and muscle contraction. To this end, we have been studying muscle contraction in the well-investigated lobster pylo ric system. We report here the mechanisms underlying stabilization of tempo rally summating contractions of the very slow dorsal dilator muscle in resp onse to motor nerve stimulation with trains of rhythmic shock bursts at a p hysiological intraburst spike frequency (60 Hz), physiological cycle period s (0.5-2 s), and duty cycles from 0.1 to 0.8. For temporal summation to sta bilize, the rise and relaxation amplitudes of the phasic contractions each burst induces must equalize as the rhythmic train continues. Stabilization could occur by changes in rise duration, rise slope, plateau duration, and/ or relaxation slope. We demonstrate a generally applicable method for quant ifying the relative contribution changes in these characteristics make to c ontraction stabilization. Our data show that all characteristics change as contractions stabilize, but their relative contribution differs depending o n stimulation cycle period and duty cycle. The contribution of changes in r ise duration did not depend on period or duty cycle for the 1-, 1.5-, and 2 -s period regimes, contributing similar to 30% in all cases; but for the 0. 5-s period regime, changes in rise duration increased from contributing 25% to contributing 50% as duty cycle increased from 0.1 to 0.8. At all cycle periods decreases in rise slope contributed little to stabilization at smal l duty cycles but increased to contributing similar to 80% at high duty cyc les. The contribution of changes in plateau duration decreased in all cases as duty cycle increased; but this decrease was greater in long cycle perio d regimes. The contribution of changes in relaxation slope also decreased i n all cases as duty cycle increased; but for this characteristic, the decre ase was greatest in fast cycle period regimes, and in these regimes at high duty cycles these changes opposed contraction stabilization. Exponential f its to contraction relaxations showed that relaxation time constant increas ed with total contraction amplitude; this increase presumably underlies the decreased relaxation slope magnitude seen in high duty cycle, fast cycle p eriod regimes. These data show that changes in no single contraction charac teristic can account for contraction stabilization in this muscle and sugge st that predicting muscle response in other systems in which slow muscles a re driven by rapidly varying neuronal inputs may be similarly complex.