Summary: 1. Electrical stimulation of the ventromedial (VM) abdominal nerve cord region in crayfish causes inhibition of afferent EPSPs in many near-field mechanosensory interneurons (MSIs). This inhibition occurs without any detectable change in interneuronal membrane potential or conductance (Figs. 1 and 2). In an earlier report this region was shown to contain multisegmental proprioceptive interneurons that receive input from walking legs and swimmerets. 2. IPSPs evoked in one identified mechanosensory interneuron (Interneuron A) by VM stimulation, are too brief to account for the full period of EPSP suppression (Fig. 3). 3. In five recordings from unidentified MSIs, EPSPs that normally decrement during repetitive low frequency stimulation were able to facilitate during sustained periods of inhibition (Fig. 4). This suggests a differential effect of VM-evoked inhibition on two homosynaptic processes, depression and facilitation. 4. Stimulation of VM causes primary afferent depolarization (PAD) in mechanosensory afferents (MSAs) (Fig. 5). 5. Statements 1–4 are consistent with our view that presynaptic inhibition of MSAs arises in part from activity in central interneurons driven by input from thoracic walking legs as well as other segmental appendages. 6. Stable, short latency PAD, is correlated one-for-one with action potentials in the interganglionic connective. This suggests that some final inhibitory interneurons may be multisegmental (Fig. 6). 7. Occlusion experiments (Fig. 8) suggest that at least part of the PAD-producing network activated during walking may also be activated by the giant axons. Convergence of excitatory input from giant fibers and VM is also shown in one identified inhibitory neuron (Fig. 9).

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