Evolutionary adaptation of a reflex system: sensory hysteresis counters muscle 'catch' tension.

The metathoracic femoral chordotonal organ is a receptor of the locust, Schistocerca, hindleg that encodes the angle of the femoro-tibial joint. However, the discharge of the organ shows considerable hysteresis, in that there is a substantial decline in the level of afferent firing when the tibia is moved and then returned to its initial position. Similar hysteresis is also seen in some joint receptors and interneurons of other invertebrates and vertebrates. When the chordotonal organ is stimulated in freely moving locusts, mimicking sudden changes in joint angle, reflex discharges can be elicited in the tibial extensor muscle that resist apparent joint movement and also show similar hysteresis. This pattern of motoneuron activity is demonstrated to potentially function to eliminate residual, 'catch' muscle tensions that result from increases in motoneuron firing frequency. This adaptation could also serve to produce accurate load compensation.