Utricular Compound Action Potential Thresholds to Pulsed Interaural Vibration in Guinea Pig, and Mechanical Tuning Curve Estimates in Mouse, Rat, Sheep, and Human.

Abstract

Purpose: Determine thresholds of vestibular compound action potentials (vCAPs) evoked by pulsed interaural bone-conducted vibration (BCV) and estimate synchronization tuning curves for sinusoidal BCV and stapes vibration (SV) in multiple mammalian species.

Methods: Thresholds were determined from previously reported vCAP data in response to vibration pulses with acceleration amplitudes ranging from ~ 0.0003 to 0.04 m s^-2 (~ 0.3-4 mG) and rise times from ~ 0.17 to 0.6 ms in guinea pigs of either sex. A simple two-degrees-of-freedom model was applied to estimate mechanical tuning curves underlying frequency-dependent synchronization.

Results: Stimulus-evoked vCAPs increased linearly with peak acceleration on a dB scale, and intercepted zero at a synchronization threshold of -54 dB (re: 1 m s^-2). Combining the threshold with the model predicts a critical shear rate of - 34 dB (re: 1 rad s^-1) between otoconia and the sensory epithelium is sufficient to evoke synchronized action potentials. The interaural acceleration threshold required to reach the critical shear rate predicts a V-shaped tuning curve for sinusoidal BCV with thresholds near -55 dB (re: 1 m s^-2) across species evaluated. The characteristic frequency (CF) was estimated as 210 Hz for humans, 286 Hz for sheep, 526 Hz for guinea pigs, 600 Hz for rats, and 688 Hz for mice. The threshold estimated for SV was - 50 dB (re: 1 m s⁻²) with relatively broad tuning extending approximately 1 kHz above the BCV CF.

Conclusion: Results are consistent with the hypothesis that synchronized action potentials in the utricular nerve are driven by the mechanical shear rate acting on sensory hair bundles.