Anatomy of the Cochlea and the Vestibular System

Vocabulary flashcards covering the functional anatomy, structural specialisations, and cell biology of the mammalian auditory and vestibular systems.

Organ of Corti

The mechanosensitive sensory epithelium of the mammalian cochlea that detects fluid vibrations produced by sound waves, consisting of one row of inner hair cells and three rows of outer hair cells.

Maculae

Vestibular sensory epithelia located in the utricle and saccule that detect gravity as well as linear acceleration and deceleration.

Cristae

Vestibular sensory patches positioned inside the ampulla of each of the three semicircular canals that detect rotational acceleration.

Stereocilia

Rigid, actin-rich projections on the apical surface of hair cells organized in a staircase-like arrangement of ascending heights.

Kinocilium

A single tubulin-rich true cilium located behind the tallest row of stereocilia; it persists throughout life in vestibular hair cells, is present only during development in organ of Corti hair cells, and is absent in mature auditory hair cells.

Tip links

Fine filamentous connections extending from the tips of shorter stereocilia to the sides of adjacent taller stereocilia that mechanically gate mechano-electrical transduction ($\text{MET}$) channels.

Hair Bundle Polarity

The functional directional sensitivity where deflection towards the tallest stereocilia activates the hair cell by opening $\text{MET}$ channels for $\text{K}^+$ influx, whereas deflection towards the shortest stereocilia closes channels and deactivates the cell.

Cupula

A gelatinous structure overlying the crista ampullaris into which hair bundles—all oriented in a single direction—are embedded to detect fluid displacement.

Otoconia

Calcium carbonate crystals structured around a protein-rich matrix that reside on top of the fibrous otoconial (otolithic) membrane to provide inertial mass.

Striola

A dividing boundary across the utricular and saccular maculae along which hair bundles reverse their morphological polarization direction.

Type I vestibular hair cell

A flask-shaped vestibular hair cell enveloped by a single, large afferent calyx nerve ending.

Type II vestibular hair cell

A cylindrical vestibular hair cell innervated by classic bouton-type afferent and efferent synaptic terminals.

Tonotopy

The spatial frequency-place coding along the cochlea where high-frequency sounds are detected at the narrow, stiff base and successively lower frequencies are detected towards the wide, flexible apex.

Basilar membrane

The extracellular matrix supporting the organ of Corti, composed of radially oriented fibrils made of type IV collagens ($\text{COL4A1--COL4A5}$) and collagen IX, which systematically widens and decreases in stiffness from base to apex.

Alport syndrome

A hereditary condition characterised by sensorineural deafness and kidney failure, caused by mutations in the $COL4A5$ gene that encodes collagen type IV alpha-5.

Tectorial membrane

An extracellular matrix overlying the organ of Corti containing type II, V, and IX collagens, $\alpha$-tectorin, $\beta$-tectorin, and otogelin; the tips of the tallest outer hair cell stereocilia are embedded in its underside.

Inner hair cells (IHCs)

The primary auditory sensory receptor cells of the cochlea, arranged in a single row over the inner bony lip with straight, fence-like stereociliary bundles that do not contact the tectorial membrane.

Outer hair cells (OHCs)

Cochlear amplifier cells arranged in three rows over the flexible portion of the basilar membrane, featuring V- or W-shaped hair bundles embedded in the tectorial membrane that exert mechanical force to enhance sensitivity and frequency selectivity.

Prestin

A motor protein densely packed at approximately $6000\,\text{particles/mm}^2$ in the lateral plasma membrane of mammalian outer hair cells that powers somatic electromotility.

Deiters' cells

Supporting cells in the organ of Corti that cup the bases of outer hair cells and extend phalangeal processes up to form the apical reticular lamina.

Pillar cells

Supporting cells (inner and outer) that frame the tunnel of Corti, providing structural rigidity to translate basilar membrane vibration into apical shearing forces.

IHC afferent innervation

The sensory wiring pattern where each inner hair cell is innervated by approximately $20$ afferent nerve fibers divided across roughly $3$ sub-populations with different sound sensitivity thresholds to code loudness.

Aminoglycoside ototoxicity

Selective damage and loss of outer hair cells leading to elevated sound intensity thresholds and degraded frequency selectivity, while inner hair cells and afferents remain functional.

Human organ of Corti dimensions

In humans, this sensory tissue spans $28\text{--}44\,\text{mm}$ in length (averaging $\sim 35\,\text{mm}$), contains approximately $15{,}000$ total hair cells, and is acoustically sensitive across $8\text{--}9$ octaves.