6Mechanoelectrical transduction
At the heart of hearing is a single mechanical trick repeated fifteen thousand times: a hair bundle is bent, and a current flows. Mechanoelectrical transduction is how the deflection of stereocilia opens ion channels and turns sound's movement into the hair cell's electrical response. It is astonishingly fast — fast enough to follow individual cycles of a sound wave — because the channels are pulled open directly by tiny protein filaments, with no slow chemistry in between. This module is the molecular event a cochlear implant ultimately stands in for.
TCThe cochlear battery
Transduction is powered by an unusual electrical arrangement. The stria vascularis pumps potassium into the endolymph of scala media and, in doing so, holds that compartment at about +85 mV — the endocochlear potential, the most positive extracellular voltage in the body. The hair cell's interior, meanwhile, sits near −45 mV. Across the top of the cell, then, is a potential difference of roughly 130 mV — a standing battery, continuously maintained, waiting for a channel to open.[2012, 2006]
That battery is not a luxury; its size sets the gain of the whole system. Hearing threshold worsens by roughly 1 dB for every millivolt the endocochlear potential falls. This is why the stria vascularis is so metabolically active, and why conditions that flatten the battery — many genetic disorders, ageing, loop diuretics such as furosemide — cause hearing loss without killing a single hair cell.[2010]
TCTip links gate the channels
The switch that lets this battery drive a current is the tip link. Each fine filament connects the tip of a shorter stereocilium to the side of its taller neighbour and is mechanically in series with a transduction channel. When the bundle is deflected toward its tall edge, the tip links are stretched and physically pull the channels open; deflection the other way slackens them and the channels close. Drag the bundle below.[1985]
CThe tip link is not a generic thread. Each one is built from two cadherin proteins joined end to end — cadherin-23 forming its upper part and protocadherin-15 its lower part, anchored beside the transduction channel itself. Mutations in either gene cause Usher syndrome, in which the same molecules are needed in the eye, so congenital deafness arrives together with a progressive retinitis-pigmentosa blindness — one of the most important deafness-with-blindness syndromes to recognise in a cochlear-implant candidate. A separate motor protein, myosin-1c, sits at the top of the link and continuously adjusts its tension; it is the engine of the adaptation described below.[2010, 2002]
TCThe ionic current — an upside-down potassium current
What flows through the open channel is mainly potassium, and this is the counter-intuitive part. In most cells potassium leaves; here it enters the hair cell, because the channel opens onto the potassium-rich, strongly positive endolymph, so both the chemical and the electrical gradients push K⁺ inward. The influx depolarisesthe cell. That depolarisation then opens voltage-gated calcium channels at the cell's base, and in the inner hair cell the calcium triggers release of glutamate onto the auditory-nerve fibres — converting the mechanical signal into a neural one.[2012]
CThe stria vascularis and the potassium loop
If potassium pours into hair cells all day, why does the endolymph never run flat? Because the cochlea continuously recyclesit. Potassium that enters through the transduction channels leaves the hair cell's base through dedicated channels (the most important is KCNQ4), is taken up by the supporting cells and the fibrocytes of the spiral ligament, and is passed cell-to-cell through gap junctions built largely of connexin-26back to the stria vascularis. There the three strial cell layers — marginal, intermediate, and basal — pump it, against gradient, back into the endolymph, regenerating both the high potassium concentration and the +85 mV battery. It is less a closed loop than a continuously driven circuit, and it is the cochlea's single most metabolically demanding job.[2006, 2006]
The two cochlear fluids it maintains are near mirror images. Endolymph is intracellular-like — about 157 mM potassium and only 1 mM sodium, with an unusually low calcium — while the perilymph around the cell bodies is ordinary extracellular fluid (~4 mM potassium, ~148 mM sodium). That engineered contrast, bathing the two ends of one cell in opposite ionic worlds, is what makes the inward potassium current possible.[2007]
Because so much of hearing depends on this ionic plumbing, many inherited deafnesses are mutations in its parts. Connexin-26 (GJB2) is the commonest cause of non-syndromic congenital deafness worldwide — a gap-junction protein that, when broken, stalls potassium recycling. Mutations in the strial potassium channels KCNQ1 and KCNE1 cause Jervell–Lange-Nielsen syndrome, pairing deafness with a dangerous cardiac long-QT arrhythmia — a diagnosis worth making before anaesthesia. KCNQ4 causes a progressive dominant loss (DFNA2); the chloride-channel subunit barttin causes Bartter syndrome type 4, deafness with renal salt-wasting. For a cochlear-implant programme these genes do more than explain a cause: they predict a cochlea whose hair cells and battery have failed but whose nerve is typically intact — the ideal substrate for electric hearing.[2007, 2010]
CFast, graded, and adapting
Three properties matter. Transduction is fast: because the channels are gated directly by the tip links rather than through a chemical cascade, the hair cell's voltage can follow the individual pressure cycles of a sound up into the kilohertz range — the basis of the temporal coding in Module 11. It is graded: the receptor potential varies smoothly with the size of the deflection, not all-or-none. And it adapts in two stages — a fast component, thought to be calcium rebinding near the channel and reclosing it within a fraction of a millisecond, and a slow component, in which the myosin-1c motor lets the tip-link anchor slide down the stereocilium. Together they continuously reset tip-link tension, keeping the cell poised at its most sensitive operating point across changing background positions.[1985, 2002]
CA useful nonlinearity
Notice in the widget that the cell depolarises more than it hyperpolarises: the input–output relationship is asymmetric and compressive. This nonlinearity is not a flaw. It is the source of the distortion products the ear generates, of two-tone suppression (Module 9), and — combined with the outer-hair-cell amplifier — of the distortion-product otoacoustic emissions used clinically (Module 8). A perfectly linear ear would emit nothing and suppress nothing.[2012]
FTWhat the cochlear implant bypasses
Every step in this module — the endocochlear battery, the tip links, the transduction channels, the potassium current, the glutamate release — is what a cochlear implant replaces. The device does not bend a hair bundle or open a channel; it delivers electrical current that depolarises the auditory-nerve fibre directly, downstream of the entire transduction apparatus. A flat endocochlear potential, missing tip links, or absent hair cells are therefore no obstacle to electric hearing — the implant skips the molecular machinery altogether.[2009]
With transduction understood, the next module returns to the outer hair cells and the one job we have kept deferring: how they amplify.
What is the main functional consequence of direct mechanical gating of the transduction channels?
What generates the endocochlear potential of about +80 mV?
What physically opens the hair cell's transduction channels?
Why does potassium flow INTO the hair cell through the transduction channel, depolarising it?