Mechanisms of pattern generation underlying swimming in Tritonia. I. Neuronal network formed by monosynaptic connections.

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Abstract
The motor program underlying swimming in the marine mollusk, T. diomedea, is generated by a network consisting of at least 3 populations of cerebral interneurons. These interneuron populations are termed C2, dorsal swim interneurons (DSI) and ventral swim interneurons (VSI). The monosynaptic connectivity among the 10 identified swim interneurons is reported. Monosynaptic connections were assessed by 5 criteria including: persistence in high-Ca/high-Mg seawater, one-for-one following, gradation with presynaptic stimulus strength, modulation by presynaptic tetraethylammonium (TEA) ion injection and modulation by presynaptic voltage. Chemically mediated monosynaptic connections were observed only between ipsilateral swim interneurons. These connections fell into 3 categories: excitatory, inhibitory or multicomponent synaptic potentials combining excitation with inhibition. The pattern of monosynaptic connectivity closely parallels the functional interactions observed between the swim interneurons in quiescent preparations. Contralateral monosynaptic interactions are restricted to electronic coupling between bilaterally homologous cells. No chemically mediated contralateral monosynaptic connections were observed. The time courses of the monosynaptic potentials differ by a factor of up to 30, suggesting that some of the synaptic interactions are effective only during a single presynaptic burst or a single swim cycle, while others may act over many cycles. The role of multicomponent synaptic potentials in pattern generation is discussed in terms of the time course of each component relative to cycle period. The pattern-generating network formed by the monosynaptic connections relies on the interplay of synaptically mediated excitation with inhibition.