Interaction and synchronization between two abdominal motor systems in crayfish.

1. Extracellular and intracellular recordings from an isolated thoraco-abdominal preparation of the crayfish, Pacifastacus leniusculus, demonstrate that the swimmeret and the abdominal positioning systems can at times be spontaneously coordinated with each other. 2. Two forms of coordination were encountered between these two motor systems. First, some flexor and extensor motor neurons can burst in phase with the swimmeret power-stroke motor neurons. Second, when the flexor motor neurons displayed irregular bursting, the swimmeret rhythm was often inhibited. 3. Both of these two forms of coordination between the swimmeret and the abdominal positioning systems can be induced by depolarization of certain abdominal interneurons. 4. Bath application of oxotremorine increases the frequency of the swimmeret rhythm in a dose-dependent manner. The threshold concentration for this effect is 10(-8) M, and it persists for as long as oxotremorine is present in the bathing solution. 5. At a concentration of 10(-5) M, oxotremorine also induces slow rhythmic activity in the abdominal positioning system consisting of opposite firing between the flexor and extensor motor neurons. 6. Bath application of 10(-5) M oxotremorine also induces two types of interaction between these two abdominal motor systems. In cycle-by-cycle coordination the flexor motor neurons and one extensor motor neuron display rhythmic activity in phase with that of power-stroke motor neurons of the swimmeret system. A slow coordination also occurs with an inhibition of the swimmeret rhythm during the extensor bursts and an excitation during the flexor bursts. 7. Injection of similar doses of oxotremorine into the haemolymph of intact crayfish produces rhythmic abdominal movements that are comparable to the fictive pattern induced in the isolated preparation.