Phase-dependent influences of wing stretch receptors on flight rhythm in the locust.

1. In restrained locusts we have reexamined the effects on the flight rhythm of stimulating wing stretch receptors. Contrary to earlier reports, we have found a strong phasedependent influence. Single stimulus trains delivered close to the onset of depressor motor activity reset the flight rhythm. When presented approximately midway between depressor bursts, they had no effect. 2. Stretch-receptor stimulation timelocked to the occurrence of depressor activity caused an increase in the frequency of the flight rhythm that began on the first cycle of stimulus presentation. It also caused a marked prolongation of the duration of flight activity. These effects did not occur when the stimulus trains were delayed relative to depressor activity. 3. Presentation of stimulus trains at a constant rate close to the wingbeat frequency resulted in the entrainment of the flight rhythm to the stimulus train rate. During entrainment, the onset of each stimulus train was close to the onset of depressor activity. 4. Intracellular recordings from flight interneurons revealed that many of those depolarizing in phase with depressor activity received a short-latency excitatory connection from one or both of the forewing stretch receptors. One of these interneurons is presumed to be an element in the rhythm generator, since the application of brief depolarizing currents to the interneuron reset the flight rhythm. 5. We conclude the forewing stretch receptors are important elements in the system generating the normal flight rhythm for the following reasons: a) they can reset the flight rhythm, b) they discharge rhythmically during flight, c) their removal decreases wingbeat frequency, d) when stimulated at the appropriate phase in the flight cycle they elevate the wingbeat frequency, e) their activity can entrain the flight rhythm, and f) they make excitatory connections to at least one interneuron considered to be in the flight oscillator. 6. None of our data support the concept that phasic activity in the stretch receptors has a tonic influence on the flight rhythm generator. We suggest this concept be abandoned for the flight system of the locust. Since there is no evidence in any other rhythmic motor system of phasic sensory input exerting a tonic influence on motor output, this concept must now be regarded as unproved.