Auditory interneurones in the prothoracic ganglion of the cricket,Gryllus bimaculatus

Summary

• 1. A high-frequency auditory neurone (HF₁AN) was identified morphologically and physiologically in the prothoracic ganglion of the cricket,Gryllus bimaculatus DeGeer.

• 2. The main inputs of the HF₁AN are from the lateral and medial parts of the auditory neuropile ipsilateral to the axon and to a lesser extent, the medial part of the contralateral auditory neuropile. The single axon ascends to the brain and the soma is located either near the base of the anterior connective or close to the base of nerve N₁ contralateral to the axon (Fig. 1).

• 3. The HF₁AN is sensitive to sound stimulation in the whole range of tested (from 2 to 100 kHz) frequencies. The neurone is not tuned to a particular frequency and has greatest sensitivity in the range 13–25 kHz (Fig. 2). At frequencies below 10–12 kHz the neurone receives additional inhibitory inputs which greatly modify its response pattern from phasic-tonic with prolonged discharge to phasic with an inhibitory pause (Figs. 3–4). The inhibitory effect is intensity dependent and its frequency range varies between animals. In the range of frequencies where the inhibitory effect is pronounced, sound stimulation evokes suppression of activity during the sound pulse and increase of activity between the pulses. On the contrary, at high frequencies the cell gives a burst of spikes to each sound pulse and background activity (if present) is suppressed after each burst for at least 1–2 s (Figs. 5, 6).

• 4. The HF₁AN responds in a phasic-tonic manner to pure high-frequency tones of long duration (1,300 ms) throughout the whole intensity range (Fig. 7).

• 5. The HF₁AN responds strongly to high-frequency ticks of the courtship song of conspecific males and to the location sounds of sympatric bats (Fig. 8).

• 6. Both anatomical and physiological properties indicate that the HF₁AN may belong to the neuronal circuits controlling evasive behavior and/or arousal of the cricket.