Intra- and intersegmental pathways active during walking in the locust

Electrical stimulation of femoral chordotonal organs, trochanteral campaniform sensilla, trochanteral hairplates and tibial muscles was used to reveal neuronal pathways active in the standing and walking locust. Responses evoked by campaniform sensilla stimulation were also recorded intracellularly from flexor motoneurons in fixed animals. The trochanteral campaniform sensilla have a direct short-latency connection to tibial extensor motoneurons and more labile, longer-latency, excitatory and inhibitory connections to the tibial flexors of the same leg. Trains of stimuli to the trochanteral campaniform sensilla initiated an early swing only if the stimulation was timed to occur during late stance. The importance of this type of load afference in step-timing was demonstrated by amputating the mesothoracic leg: the stump oscillated at a higher than normal frequency. Addition of a prosthetic leg restored normal stepping. Stimulation of the femoral chordotonal organ revealed short latency, excitatory pathways to both extensor and flexor motoneurons of the same leg. Trains of stimuli to the organ initiated early swing of this leg if applied late in stance. Stimulation of either the flexor or the extensor muscle evoked a response in the antagonist group of the same leg which was abolished by amputation distal to the muscles. The flexor-evoked response functioned only in the presence of load afference. The same was found for the pathway to the walking-pattern generator activated by stimulating the flexor muscle. Stimulation of the posterior trochanteral hairplates often evoked a swing but the latency could be several hundred milliseconds. Deafferentation showed that sensory input is critical for interganglionic coordination. There are labile polysynaptic excitatory and inhibitory pathways from the trochanteral campaniform senilla to the flexor motoneurons of the adjacent leg. Trains could evoke an early swing in the adjacent leg if time to occur during late stance and if the homonymous leg itself was not in late stance. Stimulation of the chordotonal organ revealedfast-conducting stable pathways to the flexors and extensors of all the ipsilateral legs. Trains could induce an early swing if timed late in the stance of the adjacent leg and if the homonymous leg itself was not in late stance. Amputation of the adjacent leg had no effect on the direct evoked responses but swing could not be evoked unless a prosthesis was added. Load afference is necessary for the effectiveness of the intersegmental chordotonal input to the walkingpattern generator. Stimulation of the trochanteral hairplate revealed no intersegmental pathway. The intra- and intersegmental pathways revealed by our experiments are summarized diagrammatically. The results suggest that an important function of load afference is to modulate the flow of proprioceptive and motor information within the walking-pattern generator.