Connections of Hindwing Tegulae with Flight Neurones in the Locust, Locusta Migratoria

1. 1. The connections of afferents from the hindwing tegulae to flight motoneurones and interneurones in the locust, Locusta migratoria , have been determined by selectively stimulating the tegula afferents while recording intracellularly from identified neurones in the meso- and metathoracic ganglia. 2. 2. Electrical stimulation of the hindwing tegula nerve (nerve lCla) revealed two groups of afferents distinguished by a difference in their conduction velocities. Both groups of afferents made excitatory connections to hindwing elevator motoneurones in the ipsilateral half of the metathoracic ganglion. Latency measurements indicated that these connections were monosynaptic. Stimulation of the hindwing tegula nerve also evoked excitatory postsynaptic potentials (EPSPs) in elevator motoneurones in the mesothoracic ganglion and in the contralateral half of the metathoracic ganglion, and inhibitory postsynaptic potentials (IPSPs) in forewing and hindwing depressor motoneurones. The latencies of these evoked EPSPs and IPSPs indicated that the initial responses were produced via interneuronal pathways. 3. 3. None of the recordings revealed EPSPs in depressor motoneurones or IPSPs in elevator motoneurones in response to hindwing tegula stimulation. This observation differs from that in Schistocerca gregaria where it has been reported that the large tegula afferents produce EPSPs in depressors and IPSPs in elevators (Kien & Altman, 1979). 4. 4. Some of the interneurones in disynaptic excitatory and inhibitory pathways to motoneurones were identified. These interneurones received input from both hindwing tegulae and were readily excited beyond threshold by mechanical stimulation of the tegulae or by electrical stimulation of the tegula afferents. The contribution of one excitatory interneurone to the electrically evoked EPSPs was assessed by blocking spike initiation in the interneurone while recording simultaneously from a flight motoneurone. 5. 5. Based on our observations of the central connections of tegula afferents to flight motoneurones and the previously reported discharge patterns of these afferents during tethered flight (Neumann, 1985), we propose that a major function of the hindwing tegulae in L. migratoria is to generate the initial depolarizations in forewing and hindwing elevator motoneurones during flight. Consistent with this proposal was our finding that ablation of the hindwing tegulae delayed the onset of elevator activity relative to the onset of the preceding depressor activity.