THE NEURAL BASIS OF CATALEPSY IN THE STICK INSECT .4. PROPERTIES OF NONSPIKING INTERNEURONS

The known nonlinearities of the femur-tibia control loop of the stick insect Carausius morosus (enabling the system to produce catalepsy) ar e already present in the nonspiking interneuron E4: (1) The decay of d epolarizations in interneuron E4 following slow elongation movements o f the femoral chordotonal organ apodeme could be described by a single exponential function, whereas the decay following faster movements ha d to be characterized by a double exponential function. (2) Each of th e two corresponding time constants was independent of stimulus velocit y. (3) The relative contribution of each function to the total amount of depolarization changed with stimulus velocity. (4) The characterist ics described in (1)-(3) were also found in the slow extensor tibiae m otoneuron. (5) Single electrode voltage clamp studies on interneuron E 4 indicated that no voltage dependent membrane properties were involve d in the generation of the observed time course of decay. Thus, we can trace back a certain behavior (catalepsy) to the properties of an ide ntified, nonspiking inter neuron.