Timing of forewing elevator activity during flight in the locust

Abstract

1. The time interval between the onset of depressor activity and the onset of the subsequent elevator activity (the D-E interval) was measured in the forewing muscles of tethered flying locusts. The D-E interval was also measured following the removal of all sensory input from the wings. 2. During long flight episodes (up to 4 min) in intact animals the D-E interval varied slowly with time but this variation was not related in any consistent manner with slow changes in wingbeat frequency. Consequently the phase of the onset of elevator activity in the depressor cycle usually changed slowly during a long flight episode (Fig. 1). These changes in phase were not large. Typically phase values for a single animal remained within the range of 0.35 to 0.4. 3. For short periods of flight activity (ca. 15 s) the cycle-by-cycle values of the D-E interval were linearly related to the corresponding values of cycle period (Fig. 2). However, the slope of the linear regression lines varied for different short periods of flight selected from a prolonged flight episode (Fig. 3). Moreover, the slopes depended on whether the animals were flown in a wind stream or in the absence of a wind stimulus (Fig. 7). For animals flown in a wind stream the slopes were distributed around a value close to that expected for a phase-constant motor pattern. The absence of wind reduced the slopes of the regression lines relating the D-E interval to cycle period (Figs. 7, 8). In this condition the motor pattern was often close to latency-constant. 4. The characteristics of the timing of elevator activity were similar in animals flying in upright and inverted positions regardless of the wind conditions (Figs. 5, 6, 8). Intracellular recordings from elevator motoneurons also failed to reveal any changes in the pattern of synaptic input when the animal was inverted (Fig. 9). 5. Our previous finding that deafferentation results in a major change in the characteristics of the timing of forewing elevator activity (Pearson and Wolf 1987) was confirmed. Following deafferentation there is an increase in the D-E interval (from about 20 ms to 50 ms) and a corresponding increase in the phase of the onset of elevator activity (from about 0.4 to 0.6) (Figs. 10, 11, 12). From these and other data we conclude that phasic afferent signals are necessary for determining the timing of the onset of elevator activity in intact flying animals.