Frequency selectivity of hair cells and nerve fibres in the alligator lizard cochlea.

Publisher Website
Google Scholar
Abstract
Receptor potentials of hair cells and spike discharges of cochlear nerve fibers were recorded with micropipettes from the free-standing region of the basilar papilla of anesthetized alligator lizards in response to tones. In this region the hair-cell stereocilia are free-standing, i.e., they protrude directly into endolymph and are not in contact with a tectorial membrane. The frequency selectivity of hair-cell responses was measured by means of isovoltage contours of the DC (V0) and fundamental AC (V1) component of the receptor potential, i.e., iso-V0 and iso-V1 contours. The frequency selectivity of the nerve-fiber discharge was measured by isorate (iso-.LAMBDA.0) contours. Iso-V0, iso-V1 and iso-.LAMBDA.0 contours are basically V-shaped with a characteristic frequency (cf) defined as the frequency at which minimum sound pressure (Pmin) is required to evoke the criterion value of the response. Receptor potential iso-V0 contours and neural iso-.LAMBDA.0 contours have similar slopes: the mean slopes of the low-frequency sides (dB/decade) are -43.0 and -44.3; the slopes of the high-frequency sides are 85.0 and 80.2. The band widths of iso-V0 and iso-.LAMBDA.0 contours away from cf are similar (mean values of Q30dB are 0.40 and 0.53, respectively). The band widths of iso-.LAMBDA.0 contours near cf are narrower than those of iso-V0 contours (mean values of Q10dB are 2.34 and 1.20, respectively). The shapes of the contours near cf depend on the isoresponse criteria. Whether or not iso-V0 and iso-.LAMBDA.0 contours are similar near cf was not determined. The shapes of iso-V1 contours differ from those of iso-V0 and iso-.LAMBDA.0 contours. Nerve fiber cf are tonotopically organized in the nerve, with lowest cf recorded from fibers innervating the border of free standing and tectorial regions, a region in which hair-cell stereocilia are longest, and the highest cf recorded from fibers innervating the end of the free-standing region in which hair-cell stereocilia are shortest. The cf of nerve-fiber response (and by implication hair-cell response) is correlated with the height of the stereociliary tuft. The shapes of iso-.LAMBDA.0 contours vary systematically with cf and tonotopically with nerve position. Both Pmin and the positions of the sides of iso-.LAMBDA.0 contours (i.e., portions of the contours, respectively, above and below cf) vary systematically with cf. Since measurements of the displacement of the basilar membrane show no systematic variation with longitudinal position, an additional frequency-selective mechanism may exist, localized between the motion of the basilar membrane and the generation of the receptor potential. This data can be used to specify characteristics of this additional frequency-selective mechanism. This additional mechanism may result from a mechanical resonance of the stereociliary tuft of a hair cell with the resonant frequency depending on tuft height.