Sound detection and localization are important for crickets. Interneurons located in the prothoracic ganglion play a crucial role in the initial processing of the auditory inputs. Two of the most readily recorded and dye-marked auditory interneurons in the prothoracic ganglion of the cricket are the omega cells (ON/1) and the ascenders (AN/2). By using a new photoinactivation technique to selectively inactivate these cells, the synaptic relationship between them could be studied. Our results indicate that the ON/1 cells are connected to each other with reciprocal inhibitory synapses. An ON/1 cell responds to contralateral stimulation with strong inhibition mediated by the other ON/1 cell. When one cell is killed, this inhibition is removed, and a weak excitatory response is unmasked. Unlike the ON/1 cell, AN/2 produces an inhibitory response when stimulated ipsilaterally to the cell body side, and this response is also removed when the ipsilateral ON/1 cell is killed, providing strong evidence that ON/1 is the source of the inhibition. As with the ON/1, the inhibition is replaced by weak excitation.
Physiological recordings were obtained from identified receptors in the tympanal organ of Gryllus bimaculatus. By immersing the prothoracic leg in Ringer solution and removing the anterior tympanic membrane the auditory receptors were exposed without significantly altering the frequency response of the auditory organ (Fig. 1). Each receptor was tuned to a specific sound frequency. For sound frequencies below this characteristic frequency the roll-off in sensitivity decreased from 20-30 dB/octave to 10-15 dB/octave as the characteristic frequency of receptors increased from 3-11 kHz (Fig. 4A). For each individual receptor the slope, dynamic range and maximum spike response were similar for different sound frequencies (Fig. 9A). The receptors were tonotopically organized with the characteristic frequency of the receptors increasing from the proximal to the distal end of the array (Figs. 5, 6). Several receptors had characteristic frequencies of 5 kHz. These receptors were divided into two groups on the basis of their maximum spike response produced in response to pure tones of increasing intensity (Fig. 7). Independent of the tuning of the receptor no two-tone inhibition was observed in the periphery, thus confirming that such interactions are a property of central integration.
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