Invariance of Oscillator Interneurone Activity During Variable Motor Output by Locusts

Abstract

ABSTRACT Simultaneous intracellular recordings were made in locusts from (a) flight motor neurones and (b) output interneurones of the flight oscillator. The insects were mounted with the head at the centre of rotation of an artificial horizon. During fictive flight, these animals responded to simulated deviations from course with the changes in motor output appropriate to course-correction manoeuvres, as previously described. In the motor neurone of depressor muscle MN98 (meso-thoracic second basalar) these changes take the form of systematic variation in amplitude in the cyclical depolarization seen in the neurone in flight which, in turn, leads to variation in the number of action potentials per cycle (from 0·3) and in the latency of the first spike (up to 19 ms difference). These changes are closely related to the perceived movement of the horizon. The oscillator output, as recorded in metathoracic interneurone 511, shows, in contrast, very little change. The fraction of its variation which is correlated with horizon movement is vanishingly small (e.g. for number of action potentials per burst r2 = 0·008). The exteroceptive sensory inputs which modify motor output during steering do not, therefore, affect the oscillator appreciably. Thus, by exclusion, the motor patterns of compensatory steering are due exclusively to summation of the oscillator drive with the sensory inputs. This takes place in the motor neurones and especially in the premotor interneurones, as previously described.