A system of coupled neuronal oscillators, located in nearly all ganglia of the nerve cord, generates the rhythmic activity expressed as undulations in swimming medicinal leeches. Excitatory and inhibitory outputs from these segmental oscillators are conveyed to the dorsal and ventral longitudinal muscles of the body wall via a set of inhibitory and excitatory motor neurons. Contrary to published reports that electrical interactions alone interconnect the inhibitory motor neurons, the experiments described here reveal that chemical synaptic inhibition connects the dorsal inhibitory (DI) motor neurons to their ventral inhibitory (VI) counterparts.
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These inhibitory interactions apparently result from monosynaptic, non-spike-mediated connections. The DI-VI connections are functionally important, for membrane potential oscillations in VI motor neurons are greatly attenuated when phasic DI input is removed.
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Unlike previously described inhibition between the DI motor neurons and the dorsal excitatory (DE) motor neurons, the DI-VI inhibitory connections extend across the midline and hence provide synaptic links that insure that the left and right sides of the leech body make swimming movements in concert.
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Depolarizing current pulses injected into DI motor neurons during the expression of swimming activity delay the cycle phase; hence these inhibitory motor neurons are candidates for membership in the oscillator circuit that generates swimming movements in the leech.