THE EFFECT OF BLOCKING INPUTS OF THE ACOUSTIC TRACHEA ON THE FREQUENCY TUNING OF PRIMARY AUDITORY RECEPTORS IN 2 SPECIES OF TETTIGONIIDS

The effect of plugging the input structures of the acoustic trachea of the forelegs of two tettigoniid species on the frequency tuning of th e auditory receptor cells of the intermediate organ and of the crista acoustica was investigated. This was performed by long-lasting single cell recordings by which the threshold curves of the cells were studie d before and after carrying out manipulations on the ipsilateral slits and/or the spiracles. In some cases the distal tibia was also cut off during the experiments. Plugging the ipsilateral slits and/or the spi racle with Vaseline has different effects on the frequency tuning of t he auditory receptor cells. Closing the slits has a relatively small d amping effect being uniform in the stimulus frequency range between 2 kHz and 40 kHz. The tuning curves are left basically unchanged apart f rom a low elevation of the threshold by a few dB, Occluding the ipsila teral spiracle leads to a frequency-dependent clamping on the activity of the receptors, becoming stronger with increase of the stimulus fre quency. Generally, the threshold curves are a few dB less sensitive at their characteristic frequencies and the tuning curves are shifted to lower frequencies, whereby the low frequency threshold cure sides of the receptors are more sensitive by about +2 dB to +10 dB and the high frequency sides of the neurons are less sensitive by -5 dB up to -30 dB, depending on the stimulus frequency. When calculating the threshol d difference from the threshold curves before and after blocking the i psilateral spiracle in Mygalopsis marki, three groups of receptor cell s can be determined. All low frequency receptors and some of the middl e range receptors have zero values (i.e., no difference before and aft er plugging the spiracle) of the threshold differences at 5 kHz. Many middle range receptors and some of the ultrasonic receptors possess ze ro values at 10 kHz, and some of the ultrasonic receptors even have th em at 20 kHz. The conditions in Mygalopsis marki are similar to those in Gampsocleis gratiosa (the species investigated), although, in the l atter, the whole effect is transformed to lower frequencies that may r esult from the different sensitivity of the organs at low frequency le vel. Gampsocleis gratiosa has a higher proportion of low frequency to high frequency receptor cells in the organs than Mygalopsis marki. The effect of plugging the inputs of the acoustic trachea on the tuning o f the auditory receptor cells is explained by a simple (acousto-mechan ical) model of frequency-induced movements at the receptor row inside the organ. (C) 1994 Wiley-Liss, Inc.