Harmonics of outer hair cell motility

Area of Science:

• Auditory Neuroscience
• Cellular Electrophysiology
• Bioacoustics

Background:

• Outer hair cells (OHC) in the organ of Corti are crucial for auditory sensitivity and frequency analysis.
• Nonlinear processes within the cochlea are believed to underlie the auditory system's enhanced sound detection capabilities.
• The voltage-dependent mechanical activity of OHCs contributes significantly to cochlear amplification.

Purpose of the Study:

• To analyze harmonic distortion in the mechanical response of OHCs under voltage clamp conditions.
• To investigate the relationship between transmembrane voltage and OHC length changes, focusing on nonlinearities.
• To differentiate the contributions of OHC mechanical nonlinearity versus stereociliar transduction to in vivo mechanical distortion.

Main Methods:

• Whole-cell voltage clamp technique applied to OHCs.
• Sinusoidal transmembrane voltage stimulation to evoke mechanical responses.
• Analysis of DC, fundamental, and second harmonic components of OHC length changes.
• Modeling of OHC electromechanical behavior and voltage-to-length (V-delta L) function.

Main Results:

• OHCs produced DC, fundamental, and second harmonic length changes in response to voltage stimulation.
• Mechanical second harmonic distortion decreased with increasing frequency, while predicted transmembrane second harmonic voltage increased.
• The phase of mechanical second harmonic distortion did not match the phase of the predicted transmembrane voltage.
• In vivo modeling indicated minimal contribution of OHC mechanical nonlinearity to mechanical distortion at moderate sound levels.

Conclusions:

• OHC length changes are consistent with a nonlinear voltage-to-length (V-delta L) function.
• In vivo, the dominant source of mechanical response nonlinearity in OHCs is the nonlinear stereociliar transduction process.
• Understanding these nonlinearities is key to elucidating cochlear amplification mechanisms.