Proprioceptive input on the locust flight motor revealed by muscle stimulation

Abstract

1. The locust is suspended in the laminar air stream of an open jet wind tunnel. During flight the 1st basalar muscle of the hindwing (M127) or of the forewing (M97) is stimulated every 6th or 8th wing beat with a single electric pulse triggered by simultaneously recorded action potentials of another downstroke muscle. The exact time relationship of the stimulus relative to the wing beat cycle is varied systematically (Fig. 1). 2. In response to the electric stimulus the time of occurrence of other downstroke muscles changes slightly depending on the phase of the stimulus respective to the wing beat period. Stimulation of M127 (hindwing) leads to the following results: all recorded muscles react qualitatively in the same manner. If the stimulus occurs during the downstroke phase (first half of wing beat period) up to 3 subsequent wing beat intervals in the direct downstroke muscles are slightly elongated (Fig. 2). If the stimulus occurs during the upstroke phase the next following interval is decreased and the subsequent interval slightly increased (Fig. 3). 3. If the stimulated muscle is cut near its apodeme at the wing hinge region the influence of the stimulus on the other muscles vanishes (Fig. 6). Thus, the effect depends on the mechanical action of the stimulated muscle on the wing hinge. Therefore, the possibility is excluded that the antidromic spike evoked in the motor axon of the stimulated muscle is responsible for the described reactions. 4. The latency between the stimulus and the next influenced muscle action potential may be as short as 20 ms (Fig. 7). 5. Stimulation of the forewing 1st basalar muscle shows reactions which are different from animal to animal. In contrast to the “hindwing stimulation” different muscles may react differently to a particular stimulus (Fig. 8). 6. The general conclusion which can be drawn is that the sensory equipment of the wing and wing hinge region monitors wing movement quite specifically and exerts subtle influences on motor elements of the flight system.