Summary

1. Picrotoxin, eserin, butyrylcholine and acetylcholine bring about increase of impulse discharges on the T large fibre in the cord. Picrotoxin gives conspicuous increase of impulse discharges in the response of the T large fibres to sound, while the excitatory effects of the latter three agents are not so conspicuous.

2. Such effects can be explained on the assumption that picrotoxin inhibits the inhibitory synapses, so that the T large fibre is fully activated by both tympanic nerves, while the latter three agents activate both the excitatory and inhibitory fibres.

3. GABA, γ-aminobutyrylcholine and D-tubocurarine act reversibly as inhibitors of the activity of the T large fibre. The response evoked in the T large fibre may be suppressed by the activities of the inhibitory interneurons activated by the tympanic nerve fibres.

4. After the application of picrotoxin solution to the prothoracic ganglion the threshold of the T large fibre near to a sound source rises while that of the opposite side falls. The inhibitory effect seems to be eliminated by the drug action and the T large fibre is activated by both the ipsilateral and the contralateral excitatory fibres.

5. The increased information about a source of sound which arises from the central inhibitory interaction is disturbed by the application of picrotoxin.

6. The conclusion that the T large fibre has excitatory synapses with the tympanic nerves, and inhibitory synapses with the inhibitory interneurons activated by the tympanic neurons, has been confirmed pharmacologically.

7. The inhibitory interneurons are activated not only by the natural activity of the tympanic nerve, but also by activity elicited electrically with square pulses.