Limb movements during locomotion: Tests of a model of an intersegmental coordinating circuit.

During normal forward swimming, the swimmerets on neighboring segments of the crayfish abdomen make periodic power-stroke movements that have a characteristic intersegmental difference in phase. Three types of intersegmental interneurons that originate in each abdominal ganglion are necessary and sufficient to maintain this phase relationship. A cellular model of the intersegmental coordinating circuit that also produces the same intersegmental phase has been proposed. In this model, coordinating axons synapse with local interneurons in their target ganglion and form a concatenated circuit that links neighboring segmental ganglia. This model assumed that coordinating axons projected to their nearest-neighboring ganglion but not farther. We tested this assumption in two sets of experiments. If the assumption is correct, then blocking synaptic transmission in an intermediate ganglion should uncouple swimmeret activity on opposite sides of the block. We bathed individual ganglia in a low Ca(2+)-high Mg(2+) saline that effectively silenced both motor output from the ganglion and the coordinating interneurons that originated in it. With this block in place, other ganglia on opposite sides of the block could nonetheless maintain their normal phase difference. Simultaneous recordings of spikes in coordinating axons on opposite sides of the blocked ganglion showed that these axons projected beyond the neighboring ganglion. Selective bilateral ablation of the tracts in which these axons ran showed that they were necessary and usually sufficient to maintain coordination across a blocked ganglion. We discuss revisions of the cellular model of the coordinating circuit that would incorporate these new results.