Interneurons in the suboesophageal ganglion of the locust associated with flight initiation

Abstract

1. In order to gain insight into the role of the suboesophageal ganglion ofLocusta migratoria in flight control, seven interneurons originating in the suboesophageal ganglion were recorded during flight. Their activity patterns and their contribution to flight initiation were examined. During flight, three interneurons (288, 388, 488) were tonically excited, three interneurons (397, 398, 399) were tonically inhibited and one interneuron (576) was rhythmically active. 2. The three interneurons 288, 388 and 488 had activity patterns similar to those described for the mesothoracic interneurons termed 404 by Pearson et al. (1985b). These interneurons discharged tonically during flight. Two of them (288, 388) received indirect excitatory input from interneurons 404, and 488 excited indirectly the neurons 404. Intracellular stimulation of 488 evoked several cycles of flight. 3. The interneurons 397 and 398 were phasically excited at the onset of flight but tonically inhibited during flight. 397 received direct excitatory input from a G-neuron and directly excited an elevator motoneuron as well as a local non-spiking interneuron. 398 was excited phasic-tonically by wind blown on the head and was indirectly inhibited by the neurons 404. Intracellular stimulation of 398 inhibited the mesothoracic and metathoracic subalar motoneurons, decreased the cycle frequency of flight and sometimes stopped flight altogether. 4. The interneuron 399 was inhibited prior to the onset of flight and remained tonically inhibited. It was inhibited by antennal stimulation, by the 404 neurons and by the rhythmically active suboesophageal ganglion interneuron 576. Intracellular stimulation of 399 inhibited a small unit in the subalar muscle (129), decreased the cycle frequency of flight and sometimes also stopped flight. In two cases flight was initiated in the quiescent locust following the cessation of 399 stimulation. At this time spikes were suppressed in 399. 5. Interactions between the interneurons inhibited during flight were found. Intracellular stimulation of 397 excited indirectly 398 and inhibited indirectly 399. 6. It is concluded that interneurons in the suboesophageal ganglion contribute to flight control. The interneurons described in this study contributed to three aspects of flight initiation. 488 influenced the activity of the flight initiating interneurons 404 (Pearson et al. 1985 b), 397 and 398 contributed to the initial response of flight motoneurons, and finally 398 and 399 contributed to flight initiation by disinhibiton of the flight system.