Central projections of primary auditory fibres in Tettigoniidae (Orthoptera: Ensifera)

Abstract

1. Physiological responses of individual primary auditory fibres in the tettigoniidCaedida simplex were quantified for sound frequencies between 5 kHz and 35 kHz and the anatomical organisation of the central projections of several fibres was determined by staining individual fibres with Lucifer Yellow CH. 2. The most sensitive cells recorded were tuned to sound frequencies between 9 kHz and 25 kHz. These cells had roll-offs in sensitivity of 30 to 40 dB/octave for frequencies below and 40 to 50 dB/octave for sound frequencies above their centre frequency (Fig. 1). 3. All primary auditory fibres responded to a 50 ms sound pulse with a simple tonic discharge of spikes, that increased by 5 to 7 spikes/stimulus for a 20 dB increase in the sound intensity (Fig. 2A) and saturated at 15 spikes/stimulus. For each individual primary auditory fibre, the slope and the maximum response of the intensity-response characteristic were almost constant for different sound frequencies (Fig. 2B). 4. The central projections of identified primary auditory fibres ofC. simplex display a degree of tonotopic organisation. Cells tuned to sound frequencies lower than 16 kHz (Figs. 3, 4) project to the anterio-ventral region of the prothoracic ganglion while cells tuned to frequencies at or above 16 kHz project to the medio-ventral region of the ganglion (Figs. 5, 6). 5. In the prothoracic ganglion, the fibres of cells tuned to 16 kHz, the calling song frequency ofC. simplex bifurcate immediately prior to dividing into an extensive field of terminal arborisation (Fig. 5A). The extent of this arborisation is greater than that of primary auditory fibres tuned to other frequencies. This suggests that these cells synapse on a greater number of interneurons and/or have a more potent effect on specific central neurons, than primary fibres tuned to other frequencies.