JN-00625-2006.R1, accepted Load signals assist the generation of movement dependent reflex reversal in the femur-tibia joint of stick insects

Reinforcement of movement is an important mechanism by which sensory feedback contributes to motor control for walking. We investigate how sensory signals from movement and load sensors interact in controlling the motor output of the stick insect femur-tibia (FT-) joint. In stick insects, flexion signals from the femoral chordotonal organ (fCO) at the FTjoint and load signals from the femoral campaniform sensilla (fCS) are known to individually reinforce stance phase motor output of the FT-joint by promoting flexor and inhibiting extensor motoneuron activity. We quantitatively compared the time course of inactivation in extensor tibiae motoneurons in response to selective stimulation of fCS and fCO. Stimulation of either sensor effects extensor activity in a qualitatively similar manner but with a significantly different time course and frequency of occurrence. Inactivation of extensor motoneurons due to fCS stimulation was more reliable but more than three-fold slower as compared to the extensor inactivation in response to flexion signals from the fCO. In contrast, simultaneous stimulation of both sense organs produced inactivation in motoneurons with a time course typical for fCO stimulation alone, but with a frequency of occurrence characteristic for fCS stimulation. This increase in probability of occurrence was also accompanied by a delayed re-activation of the extensor motoneurons. Our results indicate for the first time that fCS load signals from the leg affect the processing of movement related feedback from the fCO in controlling motor output. Page 2 of 22