Tonotopic organization of the auditory neuropile in the bushcricket Tettigonia viridissima

Abstract

Invertebrate central neural structures, especially sensory or motor neuropiles, have proved rather intractable to conventional physiological analysis as they are considered to have a diffuse or unordered structure. This is also true for the auditory neuropiles in the ventral nerve cord of orthopterans, where receptor fibres synapse to interneurones. In the bushcricket, Tettigonia viridissima, this neuropile constitutes the terminations of about 40 receptor fibres, which derive from chordotonal receptor cells in the tibia of the foreleg. Zhantiev¹ and Rheinlaender² have shown that each receptor cell is tuned to a ‘best frequency’. Oldfield³ has further shown that the sensilla are organized tonotopically within the organ, the proximal and distal receptors being tuned to low and high frequencies respectively. Besides the finding that auditory receptor axons form a diffuse neuropile on each side of the prothoracic ganglion^4,5, there are no data on the projection of single receptor fibres within the neuropile. The results presented here show that the auditory neuropile of T. viridissima is tonotopically organized, extending to within the central nervous system the tonotopic organization of the peripheral sense organ.