The limbs on different segments of the crayfish abdomen that drive forward swimming are directly controlled by modular pattern-generating circuits. These circuits are linked together by axons of identified coordinating interneurons. We described the distributions of these neurons in each abdominal ganglion and monitored their firing during expression of the swimming motor pattern. We analyzed the timing, the numbers of spikes, and the duration of each burst of spikes in these coordinating neurons. To see what information these neurons encoded, we correlated these parameters with the timing, durations, and strengths of bursts of spikes in motor axons from the same modules. During the power-stroke phase of each output cycle, the anterior-projecting neurons fired bursts of spikes that encoded information about the start-time, duration, and strength of each burst of spikes in power-stroke motor neurons from the same module. When the period and intensity of the motor output fluctuated, the bursts of spikes in these neurons tracked these fluctuations accurately. Each additional spike in these neurons signified an increase in the strength of the power-stroke burst. The posterior-projecting neurons that fired during the return-stroke phase encoded similar information about the return-stroke motor neurons. Although homologous neurons from different ganglia were qualitatively similar, neurons from posterior ganglia fired significantly more spikes per burst than those from more anterior ganglia, a segmental gradient that correlates with the normal posterior-to-anterior phase progression of limb movements. We propose that this gradient and a similar gradient in the durations of bursts in power-stroke motor neurons might reflect a hitherto-undetected difference in the excitation or intrinsic excitability of swimmeret modules in different segments.

Pyloric muscles of the stomatogastric neuromuscular system of the lobster Panulirus interruptus produce highly deterministic (range, less than +/- 6% of mean amplitude) contractions in response to motor nerve stimulation with unchanging spike bursts containing physiological (5-10) spike numbers. Intracellular recordings of extrajunctional potentials (EJPs) evoked in these muscles by motor nerve stimulation revealed a large, apparently stochastic amplitude variation (range, +/- 36% of mean amplitude). These observations raised the question of how do electrical responses with a large amplitude variation give rise to deterministic muscle output? We show here that this question is likely resolved by (1) combinatorial averaging within individual muscle fibers of the multiple EJPs that occur in motor neuron bursts, and (2) averaging across muscle fibers whose electrical responses are uncorrelated. Synapses with high inherent variability are also present in vertebrate CNSs. Combinatorial averaging in multispike inputs would also reduce variation in postsynaptic response at these synapses. The data reported here provide further support that bursting presynaptic activity could make such synapses functionally deterministic as well.

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.