The social rank of an animal is distinguished by its behavior relative to others in its community. Although social-status-dependent differences in behavior must arise because of differences in neural function, status-dependent differences in the underlying neural circuitry have only begun to be described. We report that dominant and subordinate crayfish differ in their behavioral orienting response to an unexpected unilateral touch, and that these differences correlate with functional differences in local neural circuits that mediate the responses. The behavioral differences correlate with simultaneously recorded differences in leg depressor muscle EMGs and with differences in the responses of depressor motor neurons recorded in reduced, in vitro preparations from the same animals. The responses of local serotonergic interneurons to unilateral stimuli displayed the same status-dependent differences as the depressor motor neurons. These results indicate that the circuits and their intrinsic serotonergic modulatory components are configured differently according to social status, and that these differences do not depend on a continuous descending signal from higher centers.
Many macruran decapod crustaceans show sexual dimorphism of abdominal appendages adapted for use as secondary reproductive organs. Not only does the Australian crayfish, Cherax destructor, show no external, abdominal dimorphism, but both males and females have lost the pleopods of the first abdominal segment entirely. The first nerves of the abdominal ganglia of crayfish and lobsters carry the axons of the pleopod motor neurons. We used intracellular cobalt infusion into the first nerves of the first and second abdominal ganglia to reveal the motor neuron complement of these ganglia in males and females. The first nerves of the second abdominal ganglia of both males and females have approximately 37 motor neurons associated with them. The homologous nerves in the first abdominal segment, where there are no pleopods, have only 8 or 9 motor neurons associated with them. The evolutionary implications of this difference are discussed.
Associated with the abdominal muscle receptor organs of crayfish are accessory neurons that inhibit the activity of the stretch receptors. Cobalt infusion into their cut axons reveals four accessory somata associated with each hemiganglion in the abdomen of the crayfish Cherax destructor. These conform to the pattern described previously for these neurons: The cell bodies are in the ganglion posterior to the one from which they exit. We recorded intracellularly from the largest accessory neurons, Acc-1 and Acc-2, and stained them with intracellular dye to establish unambiguously the characteristics defining their identity and structure. We describe their branching patterns in the ganglion of origin and the ganglion of exit. This morphological information permitted us to distinguish all four accessory neurons in preparations with dye infused through their cut axons, and we propose a revised, unambiguous nomenclature for the two smaller ones. Our intracelluar recordings allowed us to reexamine the physiological relationships of Acc-1 and Acc-2, the only accessory neurons for which there are data in the literature. In general, the connections and inputs described in previous studies were substantiated, although there has clearly been confusion between the two, and they differ in a number of significant ways. We found that they are seldom active together, have different firing patterns, and may operate with different clusters of extensor and flexor motorneurons. The results illustrate the level at which the accessory neurons operate within the abdominal control system but do not distinguish between competing hypotheses concerning their role in behavior. The data are consistent with the view that accessory neurons assist in timing between adjacent segments.
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