Motor organization of Tritonia swimming. II. Synaptic drive to flexion neurons from premotor interneurons.

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Abstract
The central pattern generator underlying swimming in Tritonia consists of at least 3 classes of premotor interneurons (C2, DSI and VSI). How these interneurons control the activity of the flexion neurons that drive the swim movements was studied. Individual flexion neurons were voltage clamped during a swim to assess the compound synaptic drive underlying the recurrent bursts. The synaptic drive to cells active during ventral flexions consists of large, rapid, discrete excitatory postsynaptic potentials (EPSPs). The synaptic drive to cells active during dorsal flexions consists of a slow-wave depolarization, with no discernible discrete EPSPs. Simultaneous intracellular recordings from identified pattern-generator interneurons and flexion neurons in high Ca, high Mg seawater showed that all 3 classes of interneurons make specific monosynaptic connections to flexion neurons in the contralateral pedal ganglion. Four types of monosynaptic connections were found: a moderately fast (70 ms to peak) EPSP due to an increase in Na conductance; a very slow (2 s to peak) EPSP, which appears to be due to a decrease in conductance; a fast (50 ms to peak) inhibitory postsynaptic potential (IPSP) due to an increase in Cl conductance; and a very slow (1 s to peak) IPSP due to an increase in K conductance. Many flexion neurons receive multicomponent postsynaptic potentials that are combinations of the 4 types. The identified monosynaptic connections from pattern-generator interneurons to the flexion neurons can account for much but not all of the burst activity of the flexion neurons. At least 2 additional as yet unidentified premotor interneurons probably contribute to the synaptic drive to flexion neurons during a swim.

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