Bioelectricity and mechanotransduction

The electrochemical physics the hair cell runs on: Nernst and GHK potentials, the endocochlear battery, tip-link gating, and the prestin motor.

The cochlea is a mechanical device, but it works because of electrochemical gradients. This reference chapter collects the bioelectric physics that the cochlea and auditory-nerve chapters rely on, building on the electromagnetism of the physics book. It runs from the equilibrium of a single ion to the standing battery that powers transduction, the mechanical gate that controls it, and the molecular motor that amplifies the result.

• E.1 The electrochemical potential and the Nernst equation — chemical plus electrical potential, and the voltage that balances a concentration gradient.
• E.2 The Goldman–Hodgkin–Katz equation and the resting potential — the permeability-weighted compromise voltage of a multi-ion membrane.
• E.3 The endocochlear potential — the $+80\,\text{mV}$ battery maintained by the stria vascularis, and the $140\,\text{mV}$ that drives transduction.
• E.4 Mechanotransduction: tip-link gating — the gating spring, and the sigmoidal open probability of the MET channel.
• E.5 Prestin and the cochlear amplifier — piezoelectricity, the electromotile motor, and feeding energy back into the travelling wave.