Active signaling of leg loading and unloading in the cockroach.

The ability to detect changes in load is important for effective use of a leg in posture and locomotion. While a number of limb receptors have been shown to encode increases in load, few afferents have been demonstrated to signal leg unloading, which occurs cyclically during walking and is indicative of slipping or perturbations. We applied mechanical forces to the cockroach leg at controlled rates and recorded activities of the tibial group of campaniform sensilla, mechanoreceptors that encode forces through the strains they produce in the exoskeleton. Discrete responses were elicited from the group to decreasing as well as increasing levels of leg loading. Discharges 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 bursts could effectively encode the rate of force decreases. These discharges may be important in indicating leg unloading in the step cycle during walking and could rapidly signal force decreases during perturbations or loss of ground support.