Sensitivity to ultrasound in an identified auditory interneuron in the cricket: a possible neural link to phonotactic behavior

Summary

• 1. In the cricket,Teleogryllus oceanicns, an identified auditory interneuron, interneuron-1 (int-1), is described morphologically and physiologically (Figs. 1,2). There is one such neuron in each hemiganglion of the prothoracic ganglion. The medial dendrites of int-1 overlap part of the terminal field formed by the auditory afferent axons from the ear and int-1's axon ascends to the brain, terminating on the same (ipsilateral) side (Fig- 2).

• 2. The neuron has a two-part frequency response characteristic: (1) its spontaneous activity is suppressed by low frequencies (3 to 8 kHz) at threshold-to-moderate intensities (Fig. 9 B), and (2) it is strongly excited at high frequencies, especially ultrasonic, from 15–100 kHz (Fig. 3).

• 3. Int-1 produces more spikes per tone pulse (Fig. 4) and its reponse latency decreases (Fig. 5), with increasing levels of intensity when stimulated by ultrasound.

• 4. Two-tone inhibition occurs in int-1. When a 30 kHz (normally excitatory) tone is combined with a 5 kHz tone (which suppresses spontaneous activity), the combination tone results in a diminished response, compared to the response to the excitatory tone alone (Fig. 6).

• 5. The excitation of int-1 shows lateralization. Excitation is stronger in the neuron ipsilateral to the sound source, than in the contralateral int-1 (Fig. 9).

• 6. Int-1 responds to electronically-generated pulse trains that simulate bat-echolocation signals. The neuron is responsive to a range of ultrasonic frequencies that are contained in the echolocation signals of insectivorous bats (Fig. 11).

• 7. In light of its response characteristics, we speculate that int-1 plays a role in the detection of ultrasonic signals emitted in the cricket's normal environment by hunting bats.