Hair Cell Mechanotransduction

Mechanotransduction is the step at which hair cells convert mechanical movement into an electrical signal. Each hair cell carries a bundle of stereocilia linked at their tips; when sound deflects the bundle, tip links pull open mechanically gated ion channels, letting current flow and changing the cell's membrane potential. This conversion, completed within microseconds, is what makes hearing fast and sensitive.

Definition

Hair-cell mechanotransduction is the process by which deflection of the stereociliary bundle tensions tip links that gate mechanosensitive ion channels, converting mechanical stimulation into a receptor potential.

Scope

This topic covers the molecular and biophysical machinery of hair-cell mechanoelectrical transduction: the stereociliary bundle, tip links, the mechanotransduction channel, gating and adaptation, and how transduction feeds the cochlear amplifier. Although the MeSH descriptor names vestibular hair cells, the transduction machinery described here is shared by auditory (cochlear) and vestibular hair cells and is presented for its role in hearing. The entry is reference-educational and not a guide to diagnosing or treating hair-cell pathology.

Core questions

• How does deflection of the stereociliary bundle open transduction channels?
• What is the role of tip links in gating?
• How does adaptation reset sensitivity and extend dynamic range?
• How does transduction drive the outer-hair-cell-based cochlear amplifier?

Key concepts

• Stereociliary bundle
• Tip links
• Mechanotransduction (MET) channel
• Gating spring model
• Receptor (transduction) potential
• Fast and slow adaptation
• Calcium dependence of adaptation
• Coupling to outer-hair-cell electromotility (prestin)

Mechanisms

Stereocilia within a bundle are graded in height and joined near their tips by fine tip links. Sound-driven motion of the cochlear partition deflects the bundle toward its tall edge, increasing tension in the tip links and opening mechanotransduction channels at their lower ends; cations, including calcium, enter and depolarize the cell, producing a receptor potential within microseconds (Vollrath, Kwan, & Corey, 2007). Calcium entering the channel drives adaptation, a fast and a slower process that resets channel sensitivity, keeps the transducer in its operating range, and contributes to frequency tuning (Fettiplace & Fuchs, 1999). In outer hair cells the resulting voltage change drives prestin-based length changes, feeding mechanical energy back into the travelling wave as the cochlear amplifier (Zheng et al., 2000; Pickles, 2012).

Clinical relevance

Because transduction depends on intact stereociliary bundles and tip links, damage to these structures from noise or other insults can impair hearing, and the shared machinery links auditory and vestibular sensory function. This entry describes normal transduction for reference and education and is not a basis for individual diagnosis or treatment.

History

Biophysical work from the late twentieth century established that hair-cell transduction is mechanically gated and fast, leading to the tip-link and gating-spring models, while molecular studies in the 2000s began to identify the components of the transduction apparatus and adaptation, a synthesis reviewed by Vollrath, Kwan, and Corey (2007).

Key figures

• David P. Corey
• Robert Fettiplace
• Melissa A. Vollrath
• Peter Dallos

Seminal works

• vollrath-kwan-corey-2007
• fettiplace-fuchs-1999

Frequently asked questions

What opens the hair cell's transduction channels?

Deflection of the stereociliary bundle increases tension in the tip links connecting adjacent stereocilia, and this tension mechanically pulls the mechanotransduction channels open.

Why is mechanotransduction important for hearing?

It is the step that turns the mechanical vibration of sound into the electrical receptor potential of the hair cell, and it does so fast enough to follow rapidly changing sounds.