AL

Chapter 10 – The Inner Ear: Key Vocabulary

Anatomy & Physiology of the Inner Ear

  • Inner ear = “labyrinth” (complex, multi-chambered, yet minute)

  • Converts middle-ear mechanical energy into electro-chemical (neural) impulses processed in the brain

  • Two functional divisions

    • Vestibular portion → equilibrium / spatial orientation

    • Cochlear portion → hearing

  • Key learning objectives

    • Describe anatomy & physiology (10.1)

    • Explain vestibular contribution to spatial orientation (10.2)

    • List prenatal / perinatal / postnatal disorders (10.3)

    • Predict audiologic test outcomes for inner-ear disorders (10.4)

Development & Embryology

  • Differentiation begins 3rd gestational week; adult size by ≈6 mo (25th week)

  • Cochlear turns begin wk 6 → complete wk 10

  • Cochlear partitions & adult-size vestibular canals by mid-wk 8

  • Vestibular system adult size by wk 18

  • Rapid fetal growth ⇒ any interruption can cause significant, permanent deficits

Equilibrium: Multi-System Integration

  • Balance relies on three information streams

    • Visual system → environmental orientation cues

    • Proprioceptive/somatosensory → muscles, tendons, joints

    • Vestibular organs → gravity & inertia (rest vs movement)

  • Motion sickness = conflict between visual and vestibular signals (e.g.
    reading in a car: eyes report “stationary,” ears report “moving” ⇒ nausea)

Vestibular System

  • Membranous sacs in vestibule

    • Utricle → detects horizontal linear acceleration

    • Saccule → detects vertical acceleration

    • Together = utriculosaccular mechanism (linear acceleration)

  • Semicircular canals (arise from utricle; perpendicular planes)

    • Superior (anterior) → pitch: head tilts “shoulder to shoulder”

    • Lateral (horizontal) → yaw: shake “no”

    • Posterior → roll: nod “yes”

    • Each returns to utricle via enlarged ampulla housing cristae

  • Fluids

    • Endolymph fills utricle, saccule, canals (high K^+, low Na^+, +ve potential)

    • Perilymph surrounds membranous labyrinth (high Na^+, low K^+, –ve)

  • Head movement → inertial lag of fluids → hair-cell deflection → neural firing

  • Disorders

    • Vertigo (spinning sensation) when vestibular system damaged

    • Nystagmus: rapid, involuntary eye movements (shared projections vestibular ↔ oculomotor)

Cochlear (Auditory) System

  • Bony snail-shaped cochlea (~1 cm wide, 5 mm long; 2.5 turns)

    • Promontory: basal turn bulge between oval & round windows

  • Scalae (longitudinal chambers)

    • Scala vestibuli (upper; perilymph; begins @ oval window)

    • Scala media / cochlear duct (middle; endolymph)

    • Scala tympani (lower; perilymph; begins @ round window)

    • Helicotrema: apical connection between scala vestibuli & tympani

  • Vital membranes/structures

    • Reissner’s membrane → separates media & vestibuli

    • Basilar membrane → supports Organ of Corti; tonotopically organized

    • Spiral ligament → lateral wall support of scala media

    • Stria vascularis → produces endolymph, supplies O₂/nutrients

    • Modiolus → central bony core for blood + nerve supply

  • Organ of Corti (on basilar membrane)

    • 1 row inner hair cells (≈3 000; robust; 20 afferent fibers per IHC)

    • 3–5 rows outer hair cells (≈12–15 k; fragile; 1 afferent / 10 OHC)

    • Stereocilia project into gelatinous tectorial membrane

Inner-Ear Fluids Summary

  • Endolymph → scala media + vestibular organs (high K^+)

  • Perilymph → scala vestibuli & tympani (high Na^+)

  • Ductus reuniens connects cochlear and vestibular endolymph systems

Mechanical-to-Neural Transduction

  • Stapes pushes oval window → perilymph wave (base → apex)

    • High-freq tones: peak displacement basal end

    • Low-freq tones: peak displacement apical end

  • Basilar membrane motion shears stereocilia → mechano-electric transduction

    • Shear magnitude ∝ electrical response amplitude

    • Neurotransmitter released at hair-cell base → action potentials (AP) in afferent fibers

  • OHC motility actively “sharpens” the traveling wave → enhances frequency selectivity & boosts IHC stimulation for soft sounds (≈40–60 dB SPL)

  • IHCs can respond directly to strong (>40–60 dB) fluid movement

Frequency (Tonotopic) Analysis

  • Nerve fibers tuned to specific places along basilar membrane

    • 20 k–2 k Hz mapped basal → midpoint

    • 2 k–20 Hz mapped midpoint → apex (20 Hz at tip)

  • Enables fine frequency discrimination in humans

Theories of Hearing

  • Many historical theories; Békésy’s Traveling Wave (1940s) foundational:

    • Each stapes vibration launches traveling wave along basilar membrane

    • Place of maximal displacement determined by stimulus frequency

    • AP frequency coding mirrors basilar membrane movement frequency

    • Intensity encoded in amplitude of displacement & resulting neural firing

Otoacoustic Emissions (OAEs)

  • Cochlea generates sound; measurable in ear canal (Kemp, 1978)

  • Require healthy OHCs → absent when OHCs damaged

  • Clinical uses: newborn screening, threshold estimation, site-of-lesion

  • Types

    • Spontaneous (SOAEs) → no stimulus; present in 40–60 % normals (↑right ear, ↑female)

    • Transient-evoked (TEOAEs) → click/tone-burst; 500–4 k Hz; absent if thresholds >20–30 dB HL

    • Distortion-product (DPOAEs) → two pure tones f1, f2 produce response at 2f1-f2; mirrors thresholds ≤40–50 dB HL, useful up to high frequencies, ototoxic/noise monitoring

Auditory Neural Pathways

  • Afferent ~30 000 fibers (IHC → cochlear nucleus)

  • Efferent ~1 800 fibers (SOC → hair cells) modulate sensitivity

  • Spiral ganglion cell bodies in modiolus → form cochlear branch of CN VIII (vestibulocochlear)

  • Action potential amplitude increases with stimulus intensity

Békésy’s Traveling Wave Details

  • Stapes in/out ⇒ basilar membrane up/down

  • f_{input} sets distance to peak, rate of motion

  • Amplitude ↑ with sound intensity

  • Pitch perception arises from place coding + temporal patterns

Inner-Ear Disorders & Etiologies

Sensorineural Hearing Loss (SNHL)

  • Largest category; sensory (hair-cell) and/or neural (CN VIII) origin

  • Common patient complaint: “I hear you, but don’t understand you.”

Classification by Timing

  • Congenital (prenatal), Perinatal (during birth), Acquired/Postnatal

Congenital/Prenatal Causes

  • Genetic

    • Autosomal dominant → 50 % risk

    • Autosomal recessive → 25 % risk; accounts for ≈80 % profound genetic HL

    • Syndromic associations (e.g., Usher, Waardenburg)

  • Maternal infections: Rubella, CMV, syphilis, HSV, toxoplasmosis

  • Inner-ear malformations; anoxia; Rh incompatibility; thalidomide exposure

Perinatal Causes

  • Anoxia during delivery

  • CMV exposure

  • Head trauma (forceps, violent contractions)

  • Prematurity

Acquired/Postnatal Causes

  • Infections: otitis media labyrinthitis, meningitis, measles, mumps, rubella, influenza, diabetes, kidney disease

  • Toxins/chemicals (see ototoxic list)

  • Tobacco smoke, barotrauma, radiation, head trauma

  • Aging (presbycusis), noise exposure, autoimmune, idiopathic sudden loss, Ménière’s, semicircular canal dehiscence

Viral & Bacterial Infections

  • Meningitis

    • Viral form mild/self-limited; bacterial form severe (brain damage, death)

    • Bacterial infection or ototoxic antibiotics (e.g., aminoglycosides) → profound SNHL

    • Steroids added to reduce neurologic injury; experimental intralabyrinthine steroids

  • Viral labyrinthitis: measles, mumps, chicken pox, influenza → bilateral/ unilateral SNHL ± vertigo

  • High fever can damage cochlea

Ototoxicity

  • Drugs/chemicals toxic to cochlea or vestibular nerve; often attack high Hz first

  • Temporary vs permanent effects

  • Major classes & examples

    • Aminoglycoside antibiotics (amikacin, gentamycin, kanamycin, neomycin, streptomycin, tobramycin, vancomycin, erythromycin)

    • Antimalarials (quinine)

    • Loop diuretics, nicotine, alcohol, aspirin (chronic use)

    • Chemotherapeutics (cisplatin etc.) – monitor serum levels

Noise-Induced Hearing Loss (NIHL)

  • Continuous or impulse noise (explosion)

  • Temporary Threshold Shift (TTS) vs Permanent Threshold Shift (PTS)

  • Typically bilateral, high-frequency “noise notch”; may be asymmetric (e.g., shooting sports)

  • Most common adult acquired SNHL after presbycusis

Presbycusis

  • Age-related deterioration of TM, ossicles, windows, OHCs, neural pathways

  • Onset: men early 60s, women late 60s; more common/severe in men

  • Hallmark: phonemic regression (speech understanding ↓ disproportionate to pure-tone loss)

Ménière’s Disease (Endolymphatic Hydrops)

  • Episodic vertigo, fluctuating low-frequency SNHL, tinnitus, aural fullness ± vomiting

  • Usually unilateral (20–60 yrs, mean ≈40)

  • Etiologies: idiopathic, viral, trauma, degenerative, tumor

Sudden Idiopathic SNHL (SISNHL)

  • ≥30 dB drop across ≥3 octaves within 72 h, usually unilateral

  • Otologic emergency; early intervention improves prognosis

Autoimmune Inner-Ear Disease (AIED)

  • Body’s immune system attacks inner ear → bilateral, fluctuating progressive SNHL ± tinnitus, fullness, vertigo

Semicircular Canal Dehiscence Syndrome (SCDS)

  • Thinning/absence of bone over superior SCC → “third window”

  • Symptoms: vertigo, oscillopsia, autophony, disequilibrium

Audiologic Testing Implications

  • SNHL → air & bone thresholds elevated with no air-bone gap; speech reception thresholds ≈ PTA; word-recognition often reduced

  • OAEs absent/present depending on OHC status

  • Tympanometry usually normal (type A) unless comorbid middle-ear pathology

  • Reflexes: may be elevated or absent with cochlear loss >~60 dB; may show recruitment patterns

  • Example audiogram (Pt. 3540403) shows mild SNHL (PTA ≈25–28 dB HL) with high QuickSIN SNR loss, good WRS; tymps type A

    • \text{PTA} = \frac{500\,Hz + 1k\,Hz + 2k\,Hz\,\text{thresholds}}{3}

    • Speech Intelligibility Index (SII) indicates functional audibility (R = 0.73, L = 0.85)

Summary of Key Numbers / Equations

  • Adult cochlea length ≈35 mm; basilar width 0.1–0.5 mm (base → apex)

  • Endolymph vs perilymph ionic composition: [K^+]{endo} \gg [K^+]{peri}; opposite for Na^+

  • Definition SISNHL: \ge 30\,\text{dB} at \ge 3 consecutive octaves within 72\,h

  • Traveling-wave: place of max displacement x(f) monotonically decreases with f (higher f → basal)

  • AC PTA formula (above) for clinical summaries

Practical / Ethical Considerations

  • Rapid fetal development underscores importance of maternal health & teratogen avoidance

  • Ototoxic monitoring critical when life-saving medications used

  • Noise conservation programs needed occupationally & recreationally

  • Early detection (UNHS with OAEs/ABRs) & intervention (hearing aids, CI) mitigate language delays

  • Informed consent & counseling essential when explaining inner-ear disorders to patients