5Organ of Corti & hair cells

TThe organ of Corti

The organ of Corti is the sensory epithelium sitting on the basilar membrane along the whole length of the cochlea. It carries the hair cells, a scaffold of supporting cells (pillar cells framing the fluid-filled tunnel of Corti, and Deiters cells cradling the outer hair cells), and above it a gelatinous shelf, the tectorial membrane. Crucially, the hair cells' bodies sit in ordinary perilymph while their tops — the hair bundles — project up into the potassium-rich endolymph, a voltage and chemical arrangement the next module turns into a transducer.[2012]

Select the parts below. The single most important relationship to see is the shearing motion: when the basilar membrane moves up and down, the tectorial membrane slides across the top of the organ, bending the stereocilia sideways. Vertical membrane motion becomes sideways bundle deflection — the stimulus the hair cell actually feels.

CThe tectorial membrane is not a passive lid. It is a precisely engineered gel of collagen and special proteins called tectorins, and only the tallest stereocilia of the outer hair cells are actually embedded in its underside. Deleting a single tectorin gene in mice does not abolish the shearing motion but detunes the cochlea — it changes the gain and the timing of the amplifier — which is why the tectorial membrane is now thought of as a resonant structure that helps sharpen frequency selectivity, not merely a lever. In humans, tectorin mutations cause an inherited mid-frequency hearing loss.[2000, 2010]

TCTwo kinds of hair cell

There are roughly 15,000 hair cells per cochlea at birth, and they fall into two populations with different shapes, positions, innervation, and roles:

[1972]

TCInner hair cells — the true sensors

Despite being outnumbered three-to-one, the inner hair cells are the cells that hear. Almost the entire output of the cochlea to the brain comes from them: about 95% of auditory-nerve fibresare afferents contacting inner hair cells, with roughly ten fibres dedicated to each cell. When an inner hair cell's bundle is deflected, it depolarises and releases glutamate onto those fibres, generating the nerve impulses that carry sound centrally. Everything the rest of the auditory system works with starts here.[1972, 2009]

CThe handoff itself is built for speed and range. Each inner hair cell carries 5–30 ribbon synapses, specialised release sites where a presynaptic dense body tethers a queue of glutamate-filled vesicles right over voltage-gated calcium channels. The amount of transmitter released tracks the calcium current almost instantly and in finely graded steps, so a single synapse can signal with microsecond timing across a five-order-of-magnitude range of sound level. This ribbon synapse is the exact junction a cochlear implant bypasses — and the site that fails in many forms of auditory neuropathy, where hair cells transduce normally but the message never cleanly reaches the nerve.[2010]

TCOuter hair cells — the amplifiers

The outer hair cells barely signal to the brain at all — their few afferents (type II) are so thin their function is still debated. Instead they are motor cells: they receive efferent control from the brainstem and they change lengthin time with the travelling wave, feeding mechanical energy back into it. This active process sharpens and boosts the cochlea's response — the cochlear amplifier of Module 7 — and it is the same machinery that produces the otoacoustic emissions used in newborn screening.[2012]

TCThe hair bundle

The business end of every hair cell is its hair bundle: a few rows of stiff stereociliaarranged in a graded “staircase” from short to tall. Fine filaments called tip links — fine molecular cables of cadherin-23 and protocadherin-15 — connect the tip of each shorter stereocilium to the side of its taller neighbour. Deflecting the bundle toward its tall edge stretches the tip links; deflecting the other way slackens them. That mechanical tug is what opens and closes the transduction channels — and mutations in those two cadherins cause Usher syndrome, deafness with progressive blindness. The molecular detail is the subject of the next module.[2012, 2010]

FTWhat the cochlear implant replaces

Locate the implant precisely in this picture. A cochlear implant does not restore the organ of Corti, the tectorial shearing, the hair bundles, or the outer-hair-cell amplifier — all of that is bypassed. What it substitutes for is the inner hair cell's output: the job of converting sound into nerve activity. Where a healthy inner hair cell releases transmitter onto the nerve, the implant delivers a pulse of current to the same nerve fibres directly.[2009]

That is why the survival of the spiral-ganglion neurons matters and the survival of the hair cells does not: the implant steps in exactly at the hair-cell-to-nerve handoff. The next two modules open up the two halves of what it bypasses — how a hair cell transduces sound (Module 6), and how the outer hair cells amplify it (Module 7).