NONSPIKING LOCAL INTERNEURONS IN INSECT LEG MOTOR CONTROL .2. ROLE OFNONSPIKING LOCAL INTERNEURONS IN THE CONTROL OF LEG SWING DURING WALKING

1. Nonspiking local interneurons (NSIs) were recorded intracellularly in the mesothoracic ganglion of semi-intact locusts walking on a tread wheel. Interneurons were characterized by their connectivity to motone urons. Their activity patterns in the step cycle and the effect curren t injection had on the leg movement were analyzed. We examined interne urons that provided excitatory or inhibitory synaptic drive to a subse t of motoneurons active during the swing movement of walking. 2. Inter neuron activity was observed to support or oppose the actual leg movem ent. Both supporting and opposing interneurons were active simultaneou sly, lending support to the idea that the actual motor output of walki ng is generated by the adjustment of parallel antagonistic pathways of signal processing. 3. The examined interneurons showed qualitatively the same patterns of activity during forward and backward walking. Thi s indicates that swing movement in both situations may be generated by similar neuronal networks (although the mechanism of movement reversa l remains unclear). 4. At least two functional types of NSIs could be distinguished. First, there were interneurons whose depolarization pat terns showed distinct variability, often correlated with duration or a mplitude of the swing movement. As a rule, current injection had minor , if any, effects on leg movement. Populations of these interneurons a ppear to be involved in the control of a coordinated swing movement by driving appropriate sets of muscle groups. The second type of NSIs sh owed more stereotyped activity patterns that varied relatively little with changes in the swing movement. Current injection had strong effec ts on the leg movement and could, for example, arrest the leg in the s tance phase. These interneurons appear to be primarily involved in the trigger mechanism of leg swing.