We anatomically and physiologically identify four interneurones which inhibit the motor giant neurone (MoG) and an interneurone which inhibits both the MoG and the segmental giant (SG) neurone of crayfish. We term these the MoG-I1, -I2, -I3, -I4 and MoG/SG-I neurones. MoG-I1 is almost always very strongly dye-coupled to its bilateral homologue. It is one of the interneurones mediating recurrent feedforward inhibition from the giant fibres (GFs) to the MoG. The GFs activate MoG-I1 by a disynaptic path through the SGs (GF --> SG --> MoG-I1), which is entirely mediated by rectifying electrical synapses. The resulting trisynaptic path (i.e. GF --> SG --> MoG-I1 --> MoG), in which the first two synapses are electrical, ensures reliable and constant short-latency inhibition of the MoGs following their monosynaptic electrical activation by the GFs (GF --> MoG). The remaining MoG-Is receive input from the GFs and other sources through unidentified polysynaptic pathways. Each interneurone inhibits the MoG and/or SG through depolarising IPSPs, which can be as large as 25 mV in amplitude. These IPSPs can effectively block transmission from the GFs to the MoG. The unique morphology of the MoG allows the inhibitory connections from the MoG-Is to be visualised at the light microscope level following staining with Lucifer Yellow. The MoG-Is project a high-density cobweb-like network of fine synaptic branches over the surface of the MoG, which spread from the region of the electrical input from the GFs within the connectives, across the expanded integrating region of the MoG, and onto its axon in the proximal region of the third root. The extensiveness of this anatomical connection correlates well with the high effectiveness of the inhibition mediated by some of the MoG-Is.

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