CYCLIC POSTURAL BEHAVIOR IN THE CRAYFISH, PROCAMBARUS-CLARKII - PROPERTIES OF THE PATTERN-INITIATING NETWORK

Crayfish exhibit complex cyclical adjustments in abdominal posture dur ing certain forms of backward walking. An isolated nerve cord preparat ion was used to investigate the properties of the interneurons which d irect this alternation of abdominal flexion and extension. The command function for this cyclic postural behavior appears to be the domain o f a distributed network of multiple pattern-initiating interneurons: e ach interneuron may be viewed as a command element within a command sy stem. The cyclic pattern may be elicited by stimulation of small axon bundles pulled from the ventrolateral margins of any of the abdominal connectives. As few as one stimulus pulse to the axon bundle can elici t a single cycle of patterned output, although more pulses are general ly necessary. This suggests some convergence or amplification step in the pattern-initiating interneurons. The amplification may be accompli shed by several pattern-initiating interneurons that are coupled to on e another and converge on pattern generating circuits in each ganglion . Evidence supporting this interpretation is presented. Experiments in volving resection of the cord reveal that the pattern-initiating signa ls transfer laterally across all of the abdominal ganglia, but the net work contains a bias for descending signal conduction once a lateral t ransfer is made. This finding agrees with other results. For example, recordings from pattern-initiating axon bundles at rostral and caudal locations in the abdominal nerve cord show several descending but only one ascending unit activated during cyclic pattern generation. We als o show that an isolated ganglion is capable of producing the cyclic mo tor program, although the outputs are much weaker than those elicited in the intact abdominal cord. Therefore, the pattern-initiating system is both central and distributed. (C) 1993 Wiley-Liss, Inc.