E.3
What is hearing?
§E · Bioelectricity and mechanotransduction

E.3 The endocochlear potential

The cochlea powers its own sensitivity. Rather than relying on the sound to supply the energy that drives the sensory current, it maintains a standing battery — a compartment held at a large positive voltage by active pumping — so that the faint mechanical signal only has to gate a current the cochlea has already energised. That battery is the endocochlear potential.

A compartment at +80 mV

The cochlear duct is divided into fluid-filled compartments. The scala media, containing endolymph, is held at about +80 mV+80\,\text{mV} relative to the surrounding perilymph — the endocochlear potential — maintained by continuous active pumping of K⁺ by the stria vascularis. Hair cells straddle the boundary: their apical (top) surfaces face the positive endolymph, their basolateral (side) surfaces face perilymph near the ordinary cell resting potential of about −60 mV-60\,\text{mV}.

SCALA MEDIA — endolymph[K⁺] = 150 mM, [Na⁺] = 1 mMV = +80 mV (endocochlear potential)HAIR CELL[K⁺]_in = 140 mMV_cell = −60 mVMET channelSCALA TYMPANI — perilymph[K⁺] = 5 mM, V = 0 mVK⁺ (driven by 140 mV)K⁺ leakSTRIAvascularisNa/K ATPase+ K channelspump K⁺ upDriving force across MET = +80 mV − (−60 mV) = +140 mV → K⁺ rushes in when MET opens

The endocochlear potential is the unique feature of cochlear physiology. The stria vascularis actively pumps K+ from perilymph back to endolymph, maintaining the +80 mV endolymph-side voltage. Combined with the −60 mV resting voltage of the hair-cell soma, the *electrochemical* driving force across an open MET channel is +140 mV — a battery powerful enough to drive ~14 pA of K+ current per channel. The energy comes from ATP burned in the stria, not from the acoustic signal itself.

The driving voltage across the transduction channel

The mechanically-gated (MET) channels sit in the apical membrane. The voltage driving current through an open one is the difference between the endolymph outside and the cytoplasm inside,

ΔVMET  =  Vendolymph−Vcytoplasm  =  (+80)−(−60)  =  +140 mV.\Delta V_\text{MET} \;=\; V_\text{endolymph} - V_\text{cytoplasm} \;=\; (+80) - (-60) \;=\; +140\,\text{mV}.
where
ΔVMET\Delta V_\text{MET}
driving voltage across an open MET channel mV
VendolymphV_\text{endolymph}
endocochlear potential mV
VcytoplasmV_\text{cytoplasm}
hair-cell resting potential mV

The key point is where the energy comes from. Endolymph and cytoplasm both hold roughly 150 mM150\,\text{mM} K⁺, so the potassium Nernst potential across the apical membrane is near zero — the 140 mV140\,\text{mV} is almost entirely electrical, supplied by ATP burned in the stria vascularis, not by the acoustic stimulus. A channel of conductance ∼100 pS\sim 100\,\text{pS} across 140 mV140\,\text{mV} passes about 14 pA14\,\text{pA} when open, matching the measured single-channel current.

By pre-charging this battery, the cochlea decouples the energy of transduction from the information. The sound does no work against the channel; it only opens and closes the gate, modulating a current the endocochlear potential drives. How that gate is opened by mechanical deflection is the mechanotransduction of the next lesson.