Dynamic responses of tibial campaniform sensilla studied by substrate displacement in freely moving cockroaches

Responses of the tibial campaniform sensilla, receptors that encode strains in the exoskeleton, were characterized by recording sensory activities dur ing perturbations in freely standing cockroaches. The substrate upon which the animal stood was displaced horizontally using ramp and hold stimuli at varied rates. The sensilla showed short latency responses that were initiat ed in the first 30 ms of platform movement Responses of individual receptor s depended upon the direction of displacement and the orientation of their cuticular cap. Proximal receptors, whose caps are perpendicular to the long axis of the tibia, responded to displacements directed from the contralate ral side of the body and from the head toward the abdomen. The distal sensi lla, oriented parallel to the tibia, discharged at longer latency to displa cements in opposite directions. Plots of receptor activity versus displacem ent direction showed that proximal and distal sensilla are activated in non -overlapping ranges of movement direction. Afferent responses also increase d as the platform was displace more rapidly. These results are consistent w ith a model in which displacements produce forces that result in bending of the tibia. This information could be utilized to detect the direction and rate of forces that occur during leg slipping or in walking on unstable ter rains.