Force detection in cockroach walking reconsidered: discharges of proximal tibial campaniform sensilla when body load is altered.

We examined the mechanisms underlying force feedback in cockroach walking by recording sensory and motor activities in freely moving animals under varied load conditions. Tibial campaniform sensilla monitor forces in the leg via strains in the exoskeleton. A subgroup (proximal receptors) discharge in the stance phase of walking. This activity has been thought to result from leg loading derived from body mass. We compared sensory activities when animals walked freely in an arena or on an oiled glass plate with their body weight supported. The plate was oriented either horizontally (70-75% of body weight supported) or vertically (with the gravitational vector parallel to the substrate). Proximal sensilla discharged following the onset of stance in all load conditions. In addition, activity was decreased in the middle third of the stance phase when the effect of body weight was reduced. Our results suggest that sensory discharges early in stance result from forces generated by contractions of muscles that press the leg as a lever against the substrate. These forces can unload legs already in stance and assure the smooth transition of support among the limbs. Force feedback later in stance may adjust motor output to changes in leg loading.