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Anatomy of the Inner Ear + Physiology of Hearing

(For PG ENT Exam - Based on Cummings, Scott-Brown's, KJ Lee's & Guyton)

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PART 1: ANATOMY OF THE INNER EAR

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Overview and Location

1. Cochlea - for hearing (auditory)
2. Vestibular apparatus - for balance (semicircular canals + otolith organs)

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A. BONY LABYRINTH

1. Vestibule

• Central part of the bony labyrinth, ~4 mm in diameter
• Lies between the internal auditory meatus (anteromedially) and the middle ear cavity (laterally)
• Contains two recesses on its medial wall:
  • Spherical recess (inferior/anterior) - houses the saccule
  • Elliptical recess (superior/posterior) - houses the utricle
• The oval window (fenestra vestibuli) is on its lateral wall; covered by the footplate of stapes
• The cochlea sits anterior to the vestibule, connected by the ductus reuniens
• Posterior and lateral to the vestibule lie the mastoid air cells
• Medially: posterior cranial fossa, where the endolymphatic duct and sac lie beneath the dura

2. Semicircular Canals

• Three canals, each in a different plane, each opening at both ends into the vestibule
• Each canal has a dilated end - the ampulla - containing the crista ampullaris

• The anterior and posterior canals share a common crus before opening into the vestibule posterosuperiorly
• The lateral canal opens independently
• Total: 5 openings from 3 canals into the vestibule

3. Cochlea

• Spiral bony canal making 2.5 to 2.75 turns around a central bony pillar called the modiolus
• Total length when uncoiled: approximately 35 mm
• Base is widest (high frequency), apex (helicotrema) is narrowest (low frequency)
• The bony spiral lamina projects from the modiolus like a screw thread, partially dividing the cochlear canal

• Scala vestibuli and scala tympani communicate at the apex via the helicotrema
• Scala vestibuli opens at the oval window (covered by stapes footplate)
• Scala tympani ends at the round window (fenestra cochleae), covered by the secondary tympanic membrane
• The cochlear aqueduct (perilymphatic duct) connects scala tympani to subarachnoid space

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B. MEMBRANOUS LABYRINTH

1. Cochlear duct (scala media) - in cochlea
2. Utricle - in vestibule (elliptical recess)
3. Saccule - in vestibule (spherical recess); connected to cochlear duct via ductus reuniens
4. Three membranous semicircular ducts - within bony semicircular canals
5. Endolymphatic duct and sac - extends beneath dura in posterior cranial fossa

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C. COCHLEAR DUCT (SCALA MEDIA) - DETAILED ANATOMY

• Roof: Reissner's membrane (vestibular membrane) - 2 layers of epithelium; so thin it doesn't impede sound transmission; maintains ionic composition of endolymph
• Outer wall: Spiral ligament + Stria vascularis (the metabolic engine - secretes endolymph and maintains +80 mV endocochlear potential)
• Floor: Basilar membrane (bearing the organ of Corti) + osseous spiral lamina

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D. BASILAR MEMBRANE

• Extends from the spiral lamina (modiolus) to the spiral ligament on the outer wall
• Contains 20,000-30,000 basilar fibers
• Tonotopic organization - critical for frequency analysis:
  • Base (near oval window): short, stiff, narrow fibers → responds to HIGH frequencies (20,000 Hz)
  • Apex (helicotrema): long, wide, floppy fibers → responds to LOW frequencies (20 Hz)
• Stiffness decreases ~100-fold from base to apex

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E. ORGAN OF CORTI

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F. VESTIBULAR END ORGANS

1. Semicircular Canals - Crista Ampullaris

• Located in the ampulla of each semicircular canal
• Contains hair cells with stereocilia + one kinocilium embedded in cupula (gelatinous mass)
• The cupula has the same specific gravity as endolymph - responds to angular acceleration
• Deflection of cupula toward utricle (utriculopetal) = excitation in lateral canal
• Deflection away from utricle (utriculofugal) = excitation in vertical canals

2. Utricle and Saccule - Macula

• Utricular macula: roughly horizontal plane - detects horizontal linear acceleration and head tilt
• Saccular macula: roughly vertical plane - detects vertical linear acceleration and gravity
• Hair cells embedded in gelatinous otolithic membrane containing otoconia (calcium carbonate crystals)
• The striola divides each macula; hair cell polarity reverses across the striola

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G. FLUIDS OF THE INNER EAR

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H. INNERVATION

• Cochlear nerve (lower division of CN VIII): cell bodies in spiral (cochlear) ganglion within the modiolus; ~30,000 bipolar neurons
  • Type I neurons (90-95%): large, myelinated; supply IHCs (monosynaptic)
  • Type II neurons (5-10%): small, unmyelinated; supply OHCs
• Vestibular nerve (upper division of CN VIII): cell bodies in Scarpa's (vestibular) ganglion in the internal auditory canal
  • Superior vestibular ganglion: innervates utricle, anterior/lateral SCC cristae
  • Inferior vestibular ganglion: innervates saccule, posterior SCC crista
• Efferent innervation (Olivocochlear bundle of Rasmussen):
  • Lateral olivocochlear (LOC): to IHC afferent dendrites
  • Medial olivocochlear (MOC): directly to OHC soma; modulates OHC motility (feedback system)

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I. BLOOD SUPPLY

• Labyrinthine artery (internal auditory artery): branch of anterior inferior cerebellar artery (AICA) in 83%, or basilar artery
• Divides into:
  • Cochlear artery → spiral modiolar artery → radiating arterioles → stria vascularis
  • Anterior vestibular artery → utricle, anterior/lateral SCC
  • Posterior vestibular artery → saccule, posterior SCC
• No autoregulation - highly susceptible to ischemia; terminal circulation (no anastomoses)

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J. INTERNAL AUDITORY CANAL (IAC)

• Approximately 8-10 mm long
• Contains (Bill's bar divides superior from inferior compartment):
  • Superior anterior: facial nerve (VII)
  • Superior posterior: superior vestibular nerve
  • Inferior anterior: cochlear nerve
  • Inferior posterior: inferior vestibular nerve (singular nerve to posterior SCC)

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PART 2: PHYSIOLOGY OF HEARING

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Overview - The Pathway of Sound

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A. IMPEDANCE MATCHING BY THE MIDDLE EAR

1. Tympanic membrane to stapes footplate area ratio: Effective vibrating area of TM ~55 mm²; stapes footplate ~3.2 mm²; ratio = ~17:1 → pressure amplification of 17x
2. Ossicular lever action: malleus handle: incus long process ratio = ~1.3:1
3. Combined amplification: ~17 × 1.3 = 22-fold pressure gain ≈ 27 dB

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B. COCHLEAR HYDRODYNAMICS - TRAVELING WAVE THEORY (von Bekesy, 1960 Nobel Prize)

1. Inward movement of stapes footplate at oval window → fluid displacement in scala vestibuli
2. Reissner's membrane is so thin it does not obstruct; scala vestibuli + scala media act as one chamber
3. Pressure wave travels through cochlear fluid → causes traveling wave along the basilar membrane
4. The wave travels from base to apex, building up in amplitude to a maximal point, then rapidly dying away
5. The point of maximal displacement depends on sound frequency (Place Principle)

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C. PLACE PRINCIPLE (Tonotopy) - Frequency Coding

• High frequencies: maximal vibration at the base of basilar membrane (short, stiff fibers)
• Low frequencies: maximal vibration at the apex (long, floppy fibers)
• 20 Hz → apex; 20,000 Hz → base
• The basilar membrane acts as a mechanical frequency analyzer
• This tonotopic map is preserved all the way to the auditory cortex

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D. ACTIVE AMPLIFICATION - COCHLEAR AMPLIFIER (OHC Motility)

• Outer hair cells (OHC) contain prestin (motor protein in the lateral wall) - allows OHCs to change length in response to receptor potentials (electromotility)
• When the basilar membrane moves, OHCs amplify this movement by up to 100-fold (40 dB gain)
• This active process sharpens the frequency tuning curve, making the cochlea extremely frequency-selective
• OHC motility is regulated by the medial olivocochlear efferent system (feedback from brainstem)
• Otoacoustic emissions (OAEs) - sounds emitted by the cochlea - are a direct consequence of OHC motility and are used clinically to test OHC function (newborn hearing screening)

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E. MECHANOELECTRICAL TRANSDUCTION AT THE HAIR CELL

1. Basilar membrane moves upward (toward scala vestibuli)
2. This causes shearing motion between the basilar membrane and the overhanging tectorial membrane
3. Stereocilia of hair cells are deflected in the excitatory direction (toward the tallest stereocilium/kinocilium)
4. Tip links (extracellular filaments connecting adjacent stereocilia tips) are stretched
5. Mechanical stretching directly opens mechanically gated K+ channels (MET channels) at the tips of shorter stereocilia
6. K+ and Ca²+ flow into the hair cell from the endolymph (high K+ in endolymph, negative intracellular)
7. Hair cell depolarizes
8. Depolarization opens voltage-gated Ca²+ channels at the basolateral membrane
9. Ca²+ triggers exocytosis of neurotransmitter (glutamate) at the ribbon synapses
10. Glutamate activates AMPA receptors on cochlear nerve afferents → action potentials → cochlear nerve

• K+ exits through basolateral K+ channels into perilymph
• K+ is recycled back to endolymph via supporting cells and stria vascularis (ionic recycling via gap junctions - connexin 26 and 30)

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F. ENDOCOCHLEAR POTENTIAL (ECP)

• The stria vascularis maintains a +80 mV electrical potential in the endolymph (endocochlear potential, ECP)
• The intracellular potential of hair cells is -70 mV relative to perilymph
• Therefore, across the apical membrane (stereocilia tips in endolymph), the total potential = -70 - (+80) = -150 mV
• This enormous 150 mV driving force is what makes the cochlea so exquisitely sensitive - it maximizes K+ flow through MET channels with minimal mechanical force

• K+ secretion by the stria vascularis (requires Na-K-ATPase)
• A highly vascular, metabolically active lateral wall structure

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G. FREQUENCY PRINCIPLE (VOLLEY PRINCIPLE) - Low Frequency Coding

• Phase-locking: individual nerve fibers fire in synchrony with the sound wave
• Volley principle: groups of fibers together fire volleys of impulses at the stimulus frequency
• Applicable for frequencies 20-1500 Hz
• The cochlear nucleus decodes these temporal patterns

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H. LOUDNESS CODING

1. Rate coding: louder sounds cause higher firing rates of individual nerve fibers (more receptor potentials per unit time)
2. Population coding: louder sounds vibrate a wider area of basilar membrane, recruiting more nerve fibers
3. Threshold differences: high-threshold, high-spontaneous-rate fibers are activated only at high intensities

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I. AUDITORY NEURAL PATHWAY

Organ of Corti
     ↓
Spiral ganglion (cochlear nerve - CN VIII)
     ↓
Cochlear nuclei (dorsal & ventral) - PONTOMEDULLARY JUNCTION
     ↓ (bilateral projections)
Superior olivary complex (SOC) - first binaural integration; localization of sound; gives off olivocochlear efferents
     ↓
Lateral lemniscus → Nucleus of lateral lemniscus
     ↓
Inferior colliculus (MIDBRAIN) - integration center; acoustic reflexes
     ↓
Medial geniculate nucleus (MGN) of THALAMUS
     ↓
Primary auditory cortex (A1) - Heschl's gyrus, superior temporal gyrus (Brodmann areas 41, 42)

• First crossing occurs at the trapezoid body (within brainstem)
• Both ipsilateral and contralateral projections exist from cochlear nuclei onwards
• Unilateral cortical lesion does NOT cause complete deafness (bilateral representation)
• Bilateral cortical lesions → cortical deafness
• Wernicke's area (area 22) - speech comprehension

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J. ACOUSTIC REFLEXES

1. Stapedius Reflex (Acoustic Reflex)

• Afferent: cochlear nerve → cochlear nucleus
• Efferent: facial nerve (VII) → stapedius muscle
• Threshold: 70-90 dB above hearing threshold
• Protective - reduces ossicular chain mobility for loud sounds
• Ipsilateral and contralateral pathways
• Clinical use: tympanometry, localizing facial nerve lesions

2. Tensor Tympani Reflex

• Less reliable in humans; trigeminal nerve (V3)
• Responds to sudden non-auditory stimuli (tactile)

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K. OTOACOUSTIC EMISSIONS (OAEs)

• Spontaneous OAEs: in absence of stimulus; present in 50-70% of normal ears
• Transient evoked OAEs (TEOAEs): in response to click stimuli; used in newborn hearing screening
• Distortion product OAEs (DPOAEs): two pure tones generate distortion products; highly frequency-specific
• OAEs are absent when OHCs are damaged (noise trauma, ototoxicity); present when conductive loss is the issue

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QUICK REVISION SUMMARY TABLE

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• Cummings Otolaryngology Head and Neck Surgery (Chapter 129 - Vestibular System; Chapter 135 - Imaging)
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol 2 - Applied Physiology of Hearing
• KJ Lee's Essential Otolaryngology, 11th Ed
• Guyton & Hall Textbook of Medical Physiology (Chapter 53 - The Sense of Hearing)