Encoding of forces by cockroach tibial campaniform sensilla: implications in dynamic control of posture and locomotion

Forces exerted by a leg in support and propulsion call vary considerably wh en animals stand upon or traverse irregular terrains. We characterized the responses of the cockroach tibial campaniform sensilla, mechanoreceptors wh ich encode force via strains produced in the exoskeleton, by applying force s to the leg at controlled magnitudes and rates. We also examined how senso ry responses are altered in the presence of different levels of static load . All receptors exhibit phasico-tonic discharges that reflect the level and rate of force application. Our studies show that: (1) tonic discharges of sensilla can signal the level of force, but accurate encoding of static len ds may be affected by substantial receptor adaptation and hysteresis; (2) t he absolute tonic sensitivities of receptors decrease when incremental forc es are applied at different initial load levels; (3) phasic discharges of s ensilla accurately encode the rate of force application; and (4) sensitivit ies to changing rates of force are strictly preserved in the presence of st atic loads. These findings imply that discharges of the sensilla are partic ularly tuned to the rate of change of force at all levels of leg loading. T his information could be utilized to adapt posture and walking to varying t errains and unexpected perturbations.