The Role of An Identified Brain Neurone in Mediating Optomotor Movements in A Moth

1. Completely unrestrained moths show an optomotor turning response to horizontal movement during pre-flight warm up or flight. 2. As the moth warms up there is a sequential recruitment of first neck, then abdominal, leg and finally some wing muscles, into the optomotor turning response. 3. Extracellular motoneurone spikes were recorded from neck muscles during optomotor stimulation. As the stimulus is oscillated from side to side, motoneurones on the ipsilateral side are excited. The latency of this response increases greatly in dim light. 4. Flight motoneurones were not observed to spike in response to movements of the optomotor stimulus, but subthreshold oscillations of 5–10 mV, in phase with the response of the identified optomotor interneurone D1, were observed. Three motoneurones, the second pleuroaxillary, the subalar and the dorsal longitudinal to the more medial, ventral fibre bundle, showed depolarizations in phase with the response of the ipsilateral D1 interneurone. Synaptic potentials in these motoneurones followed action potentials in Dl, suggesting that D1 provides a direct, excitatory input to them. 5. An excitatory postsynaptic potential (EPSP) from D1 sums with a steady depolarization of all three directly postsynaptic motoneurones to produce an action potential.