---

Clinical Anatomy of the Ear — Physiology & Research Methods of Auditory and Vestibular Analyzers

---

I. CLINICAL ANATOMY OF THE EAR

A. Temporal Bone

• Squamous part — forms the lateral wall of the middle fossa; bears the temporal line, a surface landmark aligned with the zygomatic process.
• Mastoid part — contains the mastoid antrum (deep to the MacEwen triangle); communicates with the middle ear via the aditus ad antrum.
• Petrous part — contains the inner ear and internal auditory canal; the arcuate eminence on its superior surface marks the position of the superior semicircular canal.
• Tympanic part — forms most of the osseous external auditory canal (EAC) and the floor/anterior wall of the middle ear.

---

B. External Ear

• Helix, antihelix, scapha, triangular fossa
• Concha (resonant frequency ~5300 Hz), tragus, antitragus
• Intertragal notch, lobule (the only cartilage-free part)

• ~2.5 cm in adults; lateral third is cartilaginous/membranous, medial two-thirds is bony.
• The cartilaginous portion contains hair follicles, sebaceous glands, and apocrine (ceruminous) glands that produce cerumen — a hydrophobic, slightly acidic (pH 6.0–6.5) substance.
• The isthmus (bony-cartilaginous junction) is the narrowest point; a site of osteomyelitis granulation in malignant otitis externa.
• Fissures of Santorini — natural defects in the cartilaginous canal that allow spread of infection or neoplasm to the parotid gland.
• Foramen of Huschke — a defect from incomplete ossification in the anterior bony canal; allows spread to the deep parotid lobe.
• The canal has a unique self-cleansing mechanism: keratinous epithelium migrates centrifugally from the TM toward the meatus.

---

C. Tympanic Membrane (TM)

• Peripherally, both fibrous layers merge into the annular ligament, anchoring the TM in the bony tympanic sulcus.
• Pars tensa (thicker, inferior) — has all four layers; the majority of the TM.
• Pars flaccida / Shrapnell membrane (thinner, superior) — lacks the fibrous layer; bounded by the notch of Rivinus; vulnerable to retraction cholesteatoma.
• Otoscopic landmarks: manubrium/mallear stria, umbo (tip of malleus), lateral process, light reflex (anterior-inferior quadrant), pars flaccida, annulus.

---

D. Middle Ear

• Hypotympanum — below the TM annulus
• Mesotympanum — medial to the TM; contains the oval window (occupied by the stapes footplate) and round window
• Epitympanum (attic) — superior compartment; contains the head of malleus and body of incus; the Prussak space lies lateral to the malleus neck and is the origin of pars flaccida cholesteatomas

---

E. Inner Ear (Labyrinth)

• Scala vestibuli — perilymph; communicates with vestibule
• Scala media (cochlear duct) — endolymph; contains the organ of Corti
• Scala tympani — perilymph; ends at the round window membrane

• Contains inner hair cells (IHC) — one row; primary afferent transducers (~3500 cells)
• Contains outer hair cells (OHC) — three rows; electromotile amplifiers (~12,000 cells)
• Stereocilia are connected by tip links

• 3 semicircular canals (SCC) — superior (anterior), posterior, lateral — detect angular acceleration
• Otolith organs: utricle (horizontal acceleration, head tilt) and saccule (vertical acceleration)
• Each SCC ends in an ampulla containing a crista ampullaris with hair cells embedded in a cupula

---

II. PHYSIOLOGY OF THE AUDITORY ANALYZER

A. Sound Transmission — Outer and Middle Ear

1. Outer ear: The pinna and EAC funnel sound and amplify specific frequencies. The concha resonates at ~5300 Hz; the EAC at ~3000 Hz. The head shadow effect assists binaural sound localization:
   • Interaural time difference — critical for low frequencies
   • Interaural amplitude difference — critical for high frequencies
2. Middle ear impedance matching: Air-to-fluid impedance mismatch (~30 dB loss without compensation) is overcome by three mechanisms:
   • Area ratio of TM (~55 mm²) to oval window (~3.2 mm²) — ~17:1 pressure amplification
   • Lever ratio of the ossicular chain (~1.3:1)
   • Conical shape of the TM (buckling effect)

   Acoustic impedance has three components: stiffness (dominates low frequencies), mass (dominates high frequencies), and resistance (damping). Resonance occurs when stiffness and mass cancel out.

3. Sound conduction pathways:
   • Ossicular (air) conduction — dominant pathway: TM → malleus → incus → stapes footplate → oval window → perilymph
   • Bone conduction — vibration of temporal bone directly stimulates cochlear fluids

B. Cochlear Transduction

• High frequencies → base (narrow, stiff)
• Low frequencies → apex (wide, compliant)

1. Basilar membrane displacement deflects hair cell stereocilia via the tectorial membrane
2. Tip links open mechanically gated K⁺ channels (TMC1, TMC2 proteins)
3. K⁺ influx from endolymph (high [K⁺], +80 mV endocochlear potential maintained by the stria vascularis) depolarizes the hair cell
4. Depolarization opens voltage-gated Ca²⁺ channels on the basolateral surface
5. Ca²⁺ triggers exocytosis of neurotransmitter (glutamate) at the ribbon synapse → activates primary afferent auditory neurons (CN VIII)
6. K⁺ is recycled back to the stria vascularis via connexin gap junctions in supporting cells

C. Auditory Neural Pathway

D. Efferent Auditory System

• Triggered by loud sounds (~70–90 dB above threshold)
• Sound → cochlea → CN VIII → cochlear nucleus → interneurons → facial nerve nucleus → stapedius muscle contraction
• Increases ossicular impedance → protects cochlea from loud, low-frequency sounds
• Bilateral and consensual (like the pupillary light reflex)

• Medial OC (MOC) fibers → synapse directly on OHCs → reduce OHC electromotility → suppress basilar membrane response
• Function: improves speech perception in noise by suppressing background

---

III. PHYSIOLOGY OF THE VESTIBULAR ANALYZER

• Endolymph deflects the cupula in each ampulla
• Hair cell stereocilia are displaced → depolarization or hyperpolarization depending on direction
• The lateral SCC pair operates as a push-pull system: head rotation to one side increases firing rate in the ipsilateral canal and decreases it contralaterally

• Hair cells are embedded in the macula, covered by an otolithic membrane containing calcium carbonate crystals (otoliths/otoconia)
• Inertia of otoconia deflects stereocilia → detects head tilt and linear motion

• 1st order: Scarpa's ganglion → superior and inferior vestibular nerves → CN VIII
• 2nd order: Vestibular nuclei (medulla) → cerebellum, spinal cord (vestibulospinal reflex), extraocular motor nuclei (vestibuloocular reflex, VOR), cortex

---

IV. RESEARCH METHODS / AUDIOLOGICAL AND VESTIBULAR TESTING

A. Auditory Analyzer

B. Vestibular Analyzer

---