Open loop analysis of a feedback mechanism controlling the leg position in the stick insect Carausius morosus: Comparison between experiment and simulation

In the legs of the stick insect Carausius morosus a feedback mechanism exists to control the value of the angle between femur and tibia. It is possible to investigate the open loop system by moving as input experimentally the receptor apodeme of the femoral chordotonal organ, which acts as feedback transducer measuring the angle between femur and tibia (Bässler, 1965). As output the forces are measured separately which are developed by the two antagonistic muscles moving the femur-tibia joint. The response of this system to different step-, sine-, ramp-and -functions are measured. An electronic analog model is constructed to simulate the biological system (Fig. 1). Although a number of different nonlinearities arise in the biological system, as a first-order approximation the model shows a sufficient fit to the experimental results (Figs. 2–9). The main characteristics of the model are as follows. It consists of two independent subsystems, the flexor system and the extensor system. Each subsystem again consists of two parallel branches with high-pass properties of different time constants. In each subsystem one branch is only excitable by input functions of a slope smaller than a certain degree. It is remarkable, that no mutual inhibitory influence between the sub-systems controlling the antagonistic muscles is necessary in the model.Supported by the Deutsche Forschungsgemeinschaft (Nr. Ba 578/1)