ROLE OF LOCAL NONSPIKING INTERNEURONS IN THE GENERATION OF RHYTHMIC MOTOR-ACTIVITY IN THE STICK INSECT

Local nonspiking interneurons in the thoracic ganglia of insects are i mportant premotor elements in posture control and locomotion. It was i nvestigated whether these interneurons are involved in the central neu ronal circuits generating the oscillatory motor output of the leg musc le system during rhythmic motor activity. Intracellular recordings fro m premotor nonspiking interneurons were made in the isolated and compl etely deafferented mesothoracic ganglion of the stick insect in prepar ations exhibiting rhythmic motor activity induced by the muscarinic ag onist pilocarpine. All interneurons investigated provided synaptic dri ve to one or more motoneuron pools supplying the three proximal leg jo ints, that is, the thoraco-coxal joint, the coxa-trochanteral joint an d the femur-tibia joint. During rhythmicity in 83% (n = 67) of the rec orded interneurons, three different kinds of synaptic oscillations in membrane potential were observed: (1) Oscillations were closely correl ated with the activity of motoneuron pools affected; (2) membrane pote ntial oscillations reflected only certain aspects of motoneuronal rhyt hmicity; and (3) membrane potential oscillations were correlated mainl y with the occurrence of spontaneous recurrent patterns (SRP) of activ ity in the motoneuron pools. In individual interneurons membrane poten tial oscillations were associated with phase-dependent changes in the neuron's membrane conductance. Artificial changes in the interneurons' membrane potential strongly influenced motor activity. Injecting curr ent pulses into individual interneurons caused a reset of rhythmicity in motoneurons. Furthermore, current injection into interneurons influ enced shape and probability of occurrence for SRPs. Among others, iden tified nonspiking interneurons that are involved in posture control of leg joints were found to exhibit the above properties. From these res ults, the following conclusions on the role of nonspiking interneurons in the generation of rhythmic motor activity, and thus potentially al so during locomotion, emerge: (1) During rhythmic motor activity most nonspiking interneurons receive strong synaptic drive from central rhy thm-generating networks; and (2) individual nonspiking interneurons so me of which underlie sensory-motor pathways in posture control, are el ements of central neuronal networks that generate alternating activity in antagonistic leg motoneuron pools. (C) 1995 John Wiley and Sons, I nc.