Auditory input to the flight system of the locust

Abstract

1. Sound can initiate sequences of rhythmic activity in indirect flight muscles, and flight inter- and motorneurones in the locust,Locusta migratoria (Fig. 1). The initiation of rhythmic motor activity was found to be influenced by both the intensity and the carrier frequency of the stimulus (Table 1, Fig. 2) with 5 kHz and 16 kHz being most effective. 2. Presentation of sound during rhythmic motor activity, in the absence of wind, has a transient excitatory effect on the frequency of the motor rhythm (Fig. 3). A phase-dependent effect on the motor rhythm could be demonstrated when short tones were presented (Fig. 4). 3. Sound which was previously ineffective was found to reliably reinitiate motor activity immediately following sequences of rhythmic motor activity (Fig. 5A). This activation of the flight motor by sound occurred over a definite time course of about 0.6 s after a sequence of motor activity (Fig. 5B). 4. Correlated with this post-flight excitatory phase is an increased responsiveness to sound by elevator flight interneurones and motorneurones after a sequence of rhythmic motor activity, compared with the response to the same stimulus immediately prior (Fig. 6). The time course of this elevated response to sound in one flight interneurone, (207), was found to match the period of the post-flight excitatory phase (Fig. 7). 5. On severing the ventral nerve cord anterior to the prothoracic ganglion, rhythmic motor activity could still be initiated by either a wind stream directed onto the head (Fig. 8A), or by sound (Fig. 8B). The wind-sensitive input to the FM, possibly from mechanoreceptors on the body, adapts rapidly and remains subthreshold for initiation of RMA despite the presence of further wind (Fig. 8C). However, concomitant presentation of sound and wind raises the excitatory input to above threshold and regularly elicits cycles of activity in DL muscles (Fig. 8D). 6. Simultaneous recording of activity in DL muscles on both sides of the body, shows the relative timing of motor activity to be the same when the neck connectives have been severed as in intact preparations, and whether wind or sound inititiates motor activity (Fig. 9). 7. Examination of the timing of events in identified elements of the auditory and flight systems following sound stimulation (Fig. 10), shows auditory interneurones to respond first, followed by flight interneurones and then flight motorneurones. Responses to sound are seen earlier in elevator flight motorneurones than in DL muscles. Some flight motorneurones receive auditory input before such information reaches the brain via a well-known auditory interneurone [B1].