NEURAL MECHANISMS OF ADAPTIVE GAIN-CONTROL IN A JOINT CONTROL LOOP - MUSCLE FORCE AND MOTONEURONAL ACTIVITY

An adaptive gain control system of a proprioceptive feedback system, t he femur-tibia control loop, is investigated. It enables the joint con trol loop to work with a high gain but it prevents instability oscilla tions. In the inactive stick insect, the realisation of specific chang es in gain is described for tibial torque, for extensor tibiae muscle force and for motoneuronal activity. In open-loop experiments, sinusoi dal stimuli are applied to the femoral chordotonal organ (fCO). Change s in gain that depend on fCO stimulus parameters (such as amplitude, f requency and repetition rate), are investigated. Furthermore, spontane ous and touch-induced changes in gain that resemble the behavioural st ate of the animal are described. Changes in gain in motoneurones are a lways realised as changes in the amplitude of modulation of their disc harge frequency. Nevertheless, depending on the stimulus situation, tw o different mechanisms underlie gain changes in motoneurones. (i) Chan ges in gain can be based on changes in the strength of the sensorimoto r pathways that transmit stimulus-modulated information from the fCO t o the motoneurones. (ii) Changes in gain can be based on changes in th e mean activity of a motoneurone by means of its spike threshold: when , during the modulation, the discharge of a motoneurone is inhibited f or part of the stimulus cycle, then a change in mean activity subseque ntly causes a change in modulation amplitude and gain. A new neuronal mechanism is described that helps to compensate the low-pass filter ch aracteristics of the muscles by an increased activation, especially by a sharper distribution of spikes in the stimulus cycle at high fCO st imulus frequencies.