Rhythmic modulation of the responsiveness of locust sensory local interneurons by walking pattern generating networks.

1. Spiking local interneurons in the thoracic ganglia of the locust are known to process mechanosensory information from leg receptors and to contribute to postural reflexes. It is not clear, however, whether these interneurons are also involved in the control of leg movement during walking or whether their role in mediating local reflexes is phasically modulated by the walking pattern generating networks. To address these questions, intracellular recordings from spiking local interneurons in a midline population were made both in a "fictive walking" preparation reduced to isolated thoracic ganglia and in semi-intact locusts walking on a treadwheel. 2. In fictive walking (and therefore deafferented) preparations, rhythmic synaptic inputs were observed in 72% of the sampled local interneurons. These inputs resulted in a modulation of the interneurons' firing rate in phase with the centrally generated rhythm. A similar rhythmic drive was seen in spiking local interneurons recorded from semi-intact walking locusts. This drive persisted after amputation of the leg from which the interneurons received their sensory inputs. These results indicate that this population of interneurons receives inputs from central networks that participate in the generation of walking. 3. In fictive walking preparations where a hindleg was left attached to its segmental ganglion, individual local interneurons could be characterized physiologically by their receptive field properties. The central inputs were found to modulate the activity of members of all subclasses of interneurons (i.e., purely exteroceptive, purely proprioceptive, and with mixed receptive fields. The mode of modulation observed could oppose or assist the effect of sensory input expected to occur during normal walking.(ABSTRACT TRUNCATED AT 250 WORDS)