Abstract:

Tuning by hair cells can be mechanical or electrical. The best-studied examples of each class are from reptilian ears. In the free-standing region of the alligator lizard's cochlea, where hair cells have acoustic characteristic frequencies (CF[inf a]'s) that range systematically from 1 to 4 kHz, acoustic tuning can be explained by micromechanical resonances of the hair bundles [D. M. Freeman and T. F. Weiss, Hear. Res. 48, 37--68 (1990)]. In the turtle cochlea, hair cells have CF[inf a]'s from 30 to 700 Hz and the acoustic tuning derives from electrical resonance of the hair cell membrane, reflecting the interplay of capacitive current and several ionic currents [Y.-C. Wu et al., Prog. Biophys. Mol. Biol. 63, 131--158 (1995)]. Tonotopic variation in CF[inf a] arises from variation in hair bundle height in the lizard and from variation in ion channel kinetics and number in the turtle. Coupled mechanical and electrical tuning [T. F. Weiss, Hear. Res. 7, 353--360 (1982)] seems likely but has not been demonstrated. Hair bundle height varies with Cf[inf a] in the turtle cochlea, suggesting a micromechanical contribution. Hair cells dissociated from the liquid cochlea resonate electrically, but at frequencies tenfold lower than their CF[inf a]'s [R. A. Eatock et al., J. Neurosci. 3, 1767--1783 (1993)].