Interactions between the tonic and cyclic postural motor programs in the crayfish abdomen.

1. In the crayfish (Procambarus clarkii) abdomen, the superficial flexor and extensor muscles and the motoneurons that innervate them are employed during two completely different modes of behavior: (1) tonic postural adjustments and (2) cyclic movements associated with backwards terrestrial walking. We have tested the possibility that these two behavioral subsystems share at least some of the same tonic premotor interneurons. 2. Of the 108 tonic flexion- and extension-producing interneurons monitored during cyclic pattern generation, only 25 were recruited while 36 were inhibited. None of the recruited interneurons made a measurable contribution to the cyclic motor output. Similarly, none of the 20 inhibitory interneurons of the tonic subsystem recorded in this study was found to play a role in shaping the cyclic motor pattern. 3. Simultaneous activation of single tonic postural interneurons with the cyclic motor pattern revealed that the two behavioral subsystems interact in complex ways. Some tonic interneurons produced motor outputs that overrode the cyclic motor outputs while the motor outputs of other tonic interneurons were completely overwhelmed by the cyclic motor program. Still other tonic interneurons generated motor outputs that predominated over cyclic patterned outputs in some ganglia but were masked by the cyclic motor pattern in other ganglia. 4. Although weak interactions between the two subsystems occur at the premotor level, they have little effect on the normal generation of the cyclic pattern. Stronger interactions apparently occur at the level of the motoneurons and these interactions presumably may form the basis of switching from one behavior to the other. We conclude, therefore, that each behavioral subsystem relies upon its own unique set of premotor interneurons. Finally, those interneurons contributing to the cyclic motor pattern have not yet been identified.