Active signaling of leg loading and unloading in the cockroach

The ability to detect changes in load is important for effective use of a l eg in posture and locomotion. While a number of limb receptors have been sh own to encode increases in road, few afferents have been demonstrated to si gnal leg unloading, which occurs cyclically during walking and is indicativ e of slipping or perturbations. We applied mechanical forces to the cockroa ch leg at controlled rates and recorded activities of the tibial group of c ampaniform sensilla, mechanoreceptors that encode forces through the strain s they produce in the exoskeleton, Discrete responses were elicited from th e group to decreasing as well as increasing le, els of leg loading. Dischar ges of individual afferents depended on the direction of force application, and unit responses were correlated morphologically with the orientation of the receptor's cuticular cap. No units responded bidirectionally. Although discharges to decreasing levels of load were phasic, we found that these b ursts could effectively encode the rate of force decreases. These discharge s may be important in indicating leg unloading in the step cycle during wal king and could rapidly signal force decreases during perturbations or loss of ground support.