Fig. 3. Feedback loop controlling femur–tibia joint position in insects. (A) Anatomical situation, semi-schematic diagram of a locust middle leg. The means of experimental access to elements of the joint control loop are indicated: a clamp attached to a transducer (TD) allows stimulation of the femoral chordotonal organ (fCO) via its apodeme (tendon); a suction electrode (SE) records activity of fCO sensillae; a hook electrode (HE) monitors discharges of motoneurons supplying the extensor (or flexor) tibiae muscle; and movement of the tibia is monitored optically. (B) Cybernetic diagram of the feedback loop for joint control. Anatomical correlates of the elements of the control circuit are indicated (cybernetic terms in brackets). Numbers in A and B indicate corresponding structures (e.g. 1, sensor, fCO). (C) Resistance reflex generated in the quiescent animal. A ramp-and-hold stimulus delivered to the fCO (bottom trace; arrow indicates imposed fCO elongation, mimicking tibia flexion) elicits a spike discharge in the nerve supplying the extensor muscle (top trace; spikes of the three innervating motoneurons are marked: FETi, fast; SETi, slow extensor motoneuron; CI, common inhibitor). Note the pronounced response to movement and the smaller maintained discharge due to altered fCO position. Thresholds and bins for evaluation are indicated; for details, see text.