The tritocerebral commissure giant (TCG) wind-sensitive interneurone in the locust
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The TCG interneurone of the locust (Schistocerca gregaria, Locusta migratoria) integrates information from the aerodynamic sense organs on the head, the wind hairs and the antennae. Its physiological characteristics and activity in the flying animal are analyzed using intracellular and extracellular methods.
Intracellular recording shows that TCG receives excitatory inputs from ipsilateral wind hairs in fields 2, 4 and 5 and from those in the posterior portion of field 1. Inhibitory inputs to the TCG originate from hairs in the anterior portion of field 1 and from all hairs of field 3. There are no contralateral inputs to the TCG (Figs. 3, 4). In addition, the ipsilateral antenna provides a weak, directionally selective input (Fig. 5).
The cell is rhythmically active in flight, firing in bursts in approximate synchrony with depressor muscle activity (Fig. 7). The bursting activity originates from rhythmic wind turbulences in front of the head, caused by the beating of the wings and by the rhythmic head movements during flight. Excitatory and inhibitory inputs from the wind hairs and the antennae act in concert to produce the TCG's rhythm with its particular relationship to the flight-muscle activity (Fig. 10).
The wind hairs and the antenna — as measured by TCG activity — are maximally sensitive to turbulences at frequencies between 20 and 25 Hz. This corresponds to the natural flight frequency (Fig. 11).
Waxing over all the wind hairs and the antennae causes a reduction of the flight frequency. This is achieved largely by an increase of the depressor to elevator latency, whereas the elevator to depressor latency remains relatively unchanged (Fig. 12).
Electrical stimulation of the wind-hair sensory cells during flight, so as to mimic their natural stimulation, demonstrates the fast phasic influence of these sense organs on the flight motor. A single electrical stimulus to the wind hairs generally excites elevator motor neurones, causing their activity to be slightly advanced in time (compared to an unstimulated wing-beat interval). This effect varies quantitatively as a function of the stimulus timing within the wing-beat cycle. In contrast, depressor motor neurone activity is advanced when wind-hair stimulation occurs 6–15 ms before their normal activity and is retarded when stimuli are presented at 15–40 ms latency (Fig. 13). Single stimulus pulses are sufficient to reset the original wing beat rhythm by approximately 0.6 ms. This suggests that the wind-sensory system is a component of the flight-rhythm generator (Fig. 14).
KeywordsInhibitory Input Motor Neurone Wing Beat Schistocerca Gregaria Flight Motor
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