Connections of the locust wing tegulae with metathoracic flight motoneurons

Abstract

1. Electrical stimulation of the hindwing tegula inSchistocerca gregaria can elicit synchronous firing in tegula afferents. Recordings from the metathoracic wing sensory nerve, 1C1, showed that both the exact position of the stimulus wires in the tegula and the stimulus voltage determined which of two groups of tegula afferents were predominantly activated (Fig. 2). 2. Recordings were made from metathoracic nerve 1 and flight motoneuron (MN) cell bodies during tegula stimulation. It was found that the faster conducting fibre group from the tegula elicited an EPSP in a wing depressor motoneuron (127) and an IPSP in a wing elevator motoneuron (113). The slightly slower conducting fibre group produced the opposite effects, i.e., an IPSP in the depressor (127) and an EPSP in the elevator motoneuron (113). Latency measurements and responses to high frequency stimulation indicated that the excitatory connections are monosynaptic whereas the inhibitory connections are probably mediated by nonspiking interneurons (Figs. 4–7). 3. Stimulation of the hindwing tegula produced PSP's of the same sign in a contralateral motoneuron as in its ipsilateral homologue. The contralaterally evoked EPSP's were mediated by a polysynaptic pathway. These EPSP's often potentiated with repetitive stimulation whereas the ipsilateral responses did not (Fig. 9). 4. Stimulating both hindwing tegulae caused interactions at the motoneurons. Stimulation of the contralateral hindwing tegula potentiated the response of MN 113 to an ipsilateral stimulus given up to 30–40 ms later. Ipsilateral stimulation did not potentiate the contralateral response (Fig. 10). This interaction between the two inputs probably occurs at the tegula-to-motoneuron synapse. 5. Simultaneous stimulation of the fore- and hindwing tegulae at flight frequency could evoke a larger EPSP in MN 113 than when the hindwing alone was stimulated (Fig. 11). 6. These interactions between tegulae result in a considerable amplification of their individual inputs to motoneurons during flight.