Entrainment of the locust central flight oscillator by wing stretch receptor stimulation

Summary

• 1. Experiments were designed to examine phase-dependent influences of the wing stretch receptor (SR) afferents on the central oscillator in the flight system of the locust. Reasons were also sought for the failure of earlier workers to find phase-dependent influences of SR activity on the flight oscillator.

• 2. In preparations with the hindwing sensory nerves left intact, electrical stimulation of the two forewing SRs caused an immediate increase in oscillator frequency reaching a maximum of 16–20 Hz as described by Pearson et al. (1983). After cutting the hindwing sensory nerves, the same stimulation increased the frequency from 6–8 Hz to 12–14 Hz. The absolute reduction in cycle period caused by the stimulation was reduced from 15–25 ms to 10–15 ms as a result of cutting the hindwing sensory nerves.

• 3. Stimulation of two forewing SRs in completely deafferented preparations in bursts occurring at a constant rate could entrain the flight oscillator. During entrainment, depressor spikes occurred close to the time of the stimulus. The flight oscillator could follow changes in the entrainment frequency, usually only up to 1 Hz (10% cycle period) compared to 4–6 Hz (40–50% cycle period) seen by Pearson et al. (1983). Entrainment could still be elicited when the chordotonal organ afferents were co-stimulated.

• 4. Bilateral stimulation of the hindwing SRs could also entrain the central flight oscillator over a similar range of frequencies as was observed for forewing SR stimulation.

• 5. Stimulation of a lateral pair of SRs (one forewing and the ipsilateral hindwing SR) was observed to produce 1∶1 entrainment in only one out of fifteen animals. However, a phase-dependent influence on the oscillator rhythm could be demonstrated by stimulation time-locked to the oscillator output (depressor EMG). SR stimulation close to the time of the depressor spike increased the oscillator frequency and prolonged the duration of rhythmic activity. Stimulation occurring approximately midway between depressor bursts had no obvious effect on the frequency or duration of the oscillator rhythm.

• 6. The only conditions under which a slow increase in oscillator frequency could be produced by stimulation of a lateral pair of SRs was when the SR stimulus frequency was set much higher than the central oscillator frequency. It is concluded that the failure of earlier workers to observe phase-dependent effects of SR stimulation on the oscillator frequency was due to stimulation of a lateral rather than segmental pair of SRs and the method they used in their attempt to demonstrate phase-dependence. Their observation of a slow phase-independent increase in flight frequency possibly resulted from the high SR stimulus frequencies employed.