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

Receptor potentials of hair cells and spike discharges of cochlear nerve fibres were recorded with micropipettes from the free-standing region of the basilar papilla of anaesthetized 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 d.c. (V0) and fundamental-a.c. (V1) component of the receptor potential, i.e. iso-V0 and iso-V1 contours. The frequency selectivity of the nerve-fibre discharge was measured by iso-rate (iso-V0) contours. Iso-V0, iso-V1 and iso-V0 contours are basically V-shaped with a characteristic frequency (c.f.) 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-V0 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-V0 contours away from c.f. are similar (mean values of Q30dB are 0.40 and 0.53, respectively). The band widths of iso-V0 contours near c.f. are narrower than those of iso-V0 contours (mean values of Q10dB are 2.34 and 1.20, respectively). However, the shapes of the contours near c.f. depend on the iso-response criteria, and we have not determined whether or not iso-V0 and iso-V0 contours are similar near c.f. The shapes of iso-V1 contours differ from those of iso-V0 and iso-V0 contours. Nerve fibre c.f.s are tonotopically organized in the nerve, with lowest c.f.s recorded from fibres innervating the border of free-standing and tectorial regions, a region in which hair-cell stereocilia are longest, and the highest c.f.s recorded from fibres innervating the end of the free-standing region in which hair-cell stereocilia are shortest. The c.f. of nerve-fibre response (and by implication hair-cell response) is, therefore, correlated with the height of the stereociliary tuft. The shapes of iso-V0 contours vary systematically with c.f. and, therefore, tonotopically with nerve position.(ABSTRACT TRUNCATED AT 400 WORDS)