Interactions between segmental leg central pattern generators during fictive rhythms in the locust.

1. Rhythmic activity of leg motor neurons could be evoked in isolated locust thoracic ganglia as well as in preparations of two or three connected thoracic ganglia superfused with the muscarinic agonist pilocarpine. Rhythms were always more regular and reliably elicited in single isolated ganglia. When the ganglia were connected, rhythmic activity of leg motor neurons was not usually simultaneously evoked in all six hemiganglia. Typically, some of the hemiganglia were rhythmically active, whereas others showed tonic or highly irregular activity. 2. Action potentials from leg motor neuron pools were recorded extracellularly from motor nerves and cross-correlated with the use of standard algorithms. The following correlations were observed between activities of motor neurons in different hemisegments. 1) Within a segment, trochanteral levators were coactive with contralateral trochanteral depressors. This correlation was strong in the metathoracic ganglion, and weaker in the pro- and mesothoracic ganglia. 2) Coupling between levators on opposite sides of the same segment was variable in the pro- and mesothoracic ganglia, because phase relationships between levators were different in each preparation and could also change during the course of an experiment. In the metathoracic ganglion, levators on opposite sides were never coactive. 3) Trochanteral levators were often active within a short latency of levator bursts in an ipsilateral adjacent hemiganglion. In addition, levators in one segment were often inhibited during levators bursts in the ipsilateral adjacent segment. 4) Trochanteral levators were strongly coupled to ipsilateral adjacent trochanteral depressors, for all three thoracic ganglia. 3. The phase relationships between motor neuron activities revealed by cross-correlation are discussed in the context of what is known about the mechanisms involved in the control of intersegmental coupling during legged locomotion.