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 during 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 initiated in the first 30 ms of platform movement. Responses of individual receptors 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 contralateral side of the body and from the head toward the abdomen. The distal sensilla, oriented parallel to the tibia, discharged at longer latency to displacements in opposite directions. Plots of receptor activity versus displacement direction showed that proximal and distal sensilla are activated in non-overlapping ranges of movement direction. Afferent responses also increased as the platform was displaced more rapidly. These results are consistent with 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 terrains.