Mesothoracic interneurons involved in flight steering in the locust

Abstract

1. 1. Twenty-eight interneurons with somata in the mesothoracic ganglion of the locust Locusta appear to be involved in steering in flight. All have their arborizations in the dorsal (flight) neuropil. All make directionally selective responses to visual, ocellar or wind stimuli corresponding to simulated deviations from course in flight. All are directly and/or indirectly postsynaptic to one or more sensory descending neurons (DNs) of the brain, which have been shown to produce steering behaviour when electrically stimulated. Convergence of DNs leads to complex responses by the interneurons. Some receive a variety of additional sensory information derived from thoracic receptors, especially wing proprioceptors and the ear. 2. 2. The membrane potentials of all these interneurons are modulated at flight frequency during fictive flight in deafferented preparations. Nineteen are depolarised in depressor phase (and/ or hyperpolarised in elevator phase); nine are depolarised in elevator phase (and/or hyperpolarised in depressor phase). The modulation derives in at least one case (neuron 016), and presumably in all, from synaptic drive from oscillator or oscillator-follower interneurons. None of the described cells, with the possible exception of IN 313, is thought to be part of the oscillator itself. 3. 3. The 28 ‘steering interneurons’ are characterised and named. Five (016, 035, 036, 302, 313) have been previously described in other contexts. Of these, the first 3 have been renamed; they were previously 116, 135 and 136 of Rowell and Pearson (1983). Four other previously named units are synonomized: 318 of Rowell and Pearson (1983) becomes 302 of Robertson and Pearson (1983), and 120 and 119 of Rowell and Pearson (1983), and 1AA of Elson (1987), all become 016. Two new modifications of the numbering system for locust interneurons are introduced. 4. 4. Five steering neurons are known to make monosynaptic, excitatory or inhibitory connections with flight motor neurons. Individual steering interneurons contribute between 25% and 10% of the total oscillatory drive to individual FMNs. 5. 5. The population of steering interneurons is the site of convergence of oscillator drive and sensory input prior to reaching the motor neuron: this allows sensory modulation of the strength and timing of drive to the individual motor neuron in the appropriate phase of the wing beat cycle, which is required for corrective steering. They are also a point at which the motor drive to the motor neuron is modified by proprioceptive reafference.