On the Function of a Locust Flight Steering Muscle and its Inhibitory Innervation

Abstract

ABSTRACT In tethered flying locusts, the pleuroalar (or pleuroaxillary) muscle of the forewing (M85) was stimulated via its efferent nerve. The effect on the angular setting of the wing was observed using photogrammetry. Maximal tetanic contraction of the muscle reduced downstroke pronation and upstroke supination by more than 25°. A more physiological stimulus regime resulted in angular changes of about 7°, which is near the range observed during steering manoeuvres. This confirms that the pleuroalar muscle plays an important role in adjusting the wing’s aerodynamic angle of attack, as proposed in anatomical studies by Pfau (1978). Unit a of the pleuroalar muscle was found to be innervated by the common inhibitor neurone 1 (CI) of the segmental ganglion. IJPs with amplitudes between 2 and 10mV were elicited by action potentials in CI. A basic tonus was observed in the pleuroalar muscle in the absence of activity in excitatory motoneurones. CI input reduced this basic contracture but did not affect EJPs or muscle twitches elicited by excitatory input. Activity of the common inhibitor was recorded intracellularly and with nerve electrodes in tethered flying locusts. Tonic discharges were observed with spike frequencies ranging from 5 to 35 Hz, 25 Hz being a typical value. EMG recordings from the two units of the pleuroalar muscle showed that only unit a was active during most horizontal flight sequences. While its discharge was modulated in response to imposed roll movements, unit b was recruited primarily during ipsilateral roll. These results indicate functional specialization between pleuroalar muscle units a and b and suggest that the inhibitory innervation of unit a functions in the fine adjustment of wing pronation.