Proprioceptive gating of inhibitory pathways to hindleg flexor motoneurons in the locust

Summary

• 1. The response of hindleg flexor tibiae motoneurons to visual, auditory and proprioceptive stimuli, and to combinations of these stimuli, was recorded intracellularly in the locustLocusta migratoria.

• 2. Visual and auditory stimuli when presented alone or together rarely evoked any postsynaptic potentials in the flexor motoneurons. However, the combination of proprioceptive input with either visual or auditory stimuli resulted in a high probability of large IPSPs occurring in response to either the visual or the auditory stimulus. From these data we conclude there are strong polysynaptic inhibitory pathways from the auditory and visual systems to the hindleg flexor tibiae motoneurons and that transmission through these pathways is gated by input from hindleg proprioceptors.

• 3. The features of the IPSPs evoked by visual and auditory stimuli in the presence of proprioceptive input were identical suggesting that all IPSPs were produced by activation of a single interneuron. Intracellular recording from an identified interneuron (the M-neuron) known to make strong inhibitory connections to flexor motoneurons, and known to receive visual, auditory and proprioceptive input, showed that the probabilities of evoking spikes in this interneuron and of evoking IPSPs in flexor motoneurons were strongly correlated. From these and other data we conclude that the M-neuron is the site of proprioceptive gating of inhibitory transmission from both the visual and auditory systems to flexor motoneurons.

• 4. We speculate that the function of proprioceptive gating is to enable visual and auditory stimuli to trigger a jump, and to ensure that external stimuli do not inhibit flexor activity at times when the animal is not prepared to jump.