The wind-sensitive cercal receptor/giant interneurone system of the locust,Locusta migratoria

Summary

• 1. Wind directed at the cerci of tethered (Fig. 1 A) or dissected (Fig. 1B) locusts initiates rhythmical activity in elevator and depressor flight muscles.

• 2. Electrical stimulation of the cercal nerve (200 Hz) also evokes flight motor activity which then continues in the absence of further stimulation (Fig. 2A). Rhythmical activity in DL (depressor) flight muscles begins 168.2±21.4 ms (mean±s.d.) after cercal stimulation. Wind directed at the cerci during such activity increases the frequency of rhythmical bursts in DL motor neurones (Fig. 2B), and reinitiates rhythmical activity in flight interneurones and muscles following a previous flight sequence (Fig. 2C).

• 3. Depolarizing current injected into either GIN 2 or 4 in the terminal ganglion evokes rhythmical activity in thoracic flight muscles (Fig. 3). This evoked activity begins during the current pulse but only rarely continues beyond the termination of current injection.

• 4. The rhythmical depolarizations in an elevator flight interneurone (301) are identical in shape whether evoked by wind on the head (Fig. 4A) or wind stimulation of the cerci (Fig. 4 B). Both pathways activate the same thoracic oscillator. The membrane potential of one of the 4 GINs in the terminal ganglion (GIN 2) also oscillates during flight motor activity (Fig. 5A). The oscillations remain as tightly in antiphase to the activity of depressor flight muscles as those in a thoracic interneurone of the central flight oscillator (Fig. 5 B, C).

• 5. Wind and electrical stimulation of the cerci evoke EPSPs, IPSPs, or both, in a number of thoracic interneurones and motor neurones (Figs. 6, 7; Table 1). In some neurones both the polarity and latency of responses depend on which cercus is stimulated. A group of thoracic interneurones and one motor neurone respond with a latency sufficiently short so as to suggest a direct input from cercal GINs (Fig. 8).

• 6. Simultaneous intracellular penetrations of cercal GINs and interneurones of the flight motor pathway demonstrate short latency connections between GIN 1 and FIN 302 (dep), GIN 1 and FIN 301 (ele), GIN 4 and FIN 710 (ele), GIN 4 and FIN 320 (ele) (Figs. 9, 10, 11).

• 7. In the jump motor pathway, four cercal interneurones are found to evoke EPSPs in thoracic premotor interneurone 714 (Fig. 12). These EPSPs could then be identified in an actual response of neurone 714 to cercal stimulation (Fig. 13). Interneurone 714 in turn makes an excitatory connection with the FETi jump motor neurone in the metathoracic ganglion (Fig. 14).

• 8. The connectivities established between cercal GINs and neurones of thoracic flight (Fig. 15), and jump (Fig. 16), motor pathways are summarized.