Acoustic tuning mechanisms in reptilian hair cells

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 (CFa’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 CFa’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 CFa 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 Cfa in the turtle cochlea, suggesting a micromechanical contribution. Hair cells dissociated from the liquid cochlea resonate electrically, but at frequencies tenfold lower than their CFa’s [R. A. Eatock et al., J. Neurosci. 13, 1767–1783 (1993)].