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

I. OVERVIEW

II. EXTERNAL EAR

Auricle (Pinna)

• Helix — large outer rim, ending inferiorly at the fleshy lobule (only part without cartilage)
• Antihelix — curved ridge parallel and anterior to the helix
• Concha — deep hollow from which the external acoustic meatus opens
• Tragus — elevation anterior to the meatal opening
• Antitragus — elevation above the lobule, opposite the tragus

• Great auricular nerve (C2–C3) — outer and posterior surfaces
• Auriculotemporal nerve (V3) — anterior and superior surfaces
• Lesser occipital nerve (C2) — superior part of the posterior surface
• Auricular branch of vagus [X] — small contribution

External Acoustic Meatus

III. TYMPANIC MEMBRANE

• Umbo — the central concavity produced by attachment of the handle (manubrium) of the malleus
• Cone of light — anteroinferior bright light reflex visible on otoscopy
• Pars flaccida (Shrapnell's membrane) — thin, superior part above the malleolar folds, clinically significant in cholesteatoma
• Pars tensa — thick, taut remainder of the membrane
• Fibrocartilaginous ring — attaches the membrane peripherally to the tympanic sulcus

IV. MIDDLE EAR (TYMPANIC CAVITY)

Walls

Ossicular Chain

1. Malleus (hammer) — handle (manubrium) attached to tympanic membrane; head in the epitympanic recess
2. Incus (anvil) — articulates with malleus (incudomalleolar joint) and stapes (incudostapedial joint)
3. Stapes (stirrup) — footplate seals the oval window via the annular ligament

Middle Ear Muscles

• Tensor tympani — arises from cartilaginous part of pharyngotympanic tube; inserts on handle of malleus; pulls malleus medially, tensing the tympanic membrane; innervated by medial pterygoid nerve (V3)
• Stapedius — smallest skeletal muscle in the body; arises from pyramidal eminence; inserts on neck of stapes; pulls stapes posteriorly, dampening oscillations; innervated by facial nerve [VII]. The acoustic reflex (contraction of stapedius to loud sounds) protects the cochlea.

Pharyngotympanic (Eustachian/Auditory) Tube

• Connects the middle ear to the nasopharynx, equalizing air pressure on both sides of the tympanic membrane
• Bony part (1/3): near the middle ear
• Cartilaginous part (2/3): toward the nasopharynx
• Opens during swallowing and yawning; dysfunctional in otitis media with effusion ("glue ear")

V. INTERNAL EAR (LABYRINTH)

Bony Labyrinth

1. Vestibule — central chamber; contains the oval window in its lateral wall; communicates anteriorly with the cochlea and posterosuperiorly with the semicircular canals
2. Three semicircular canals — anterior (superior), posterior, and lateral (horizontal); each oriented at right angles to the others; each has one dilated ampulla; detect rotational (angular) acceleration
3. Cochlea — anterior spiral structure, 2½–2¾ turns around a central bony column (the modiolus); the spiral lamina extends from the modiolus and partially divides the cochlear canal

Membranous Labyrinth

• Utricle and saccule (otolithic organs) — in the vestibule; contain the macula
  • Utricle: detects linear acceleration in the horizontal plane and sideways head tilts
  • Saccule: detects linear acceleration in the vertical plane (up-down, forward-back)
• Three semicircular ducts — within the bony canals; each ampulla contains a crista ampullaris (responds to angular/rotational acceleration)
• Cochlear duct (scala media) — contains the spiral organ of Corti (organ of hearing); bounded by Reissner's membrane (above), basilar membrane (below), and spiral ligament (laterally)

Blood Supply to the Internal Ear

• Membranous labyrinth: supplied by the labyrinthine artery (usually a branch of the AICA or basilar artery), dividing into the common cochlear artery and anterior vestibular artery
• The blood–labyrinth barrier (analogous to BBB) maintains the unique ionic composition of endolymph

VI. VESTIBULOCOCHLEAR NERVE [VIII]

• Enters the internal acoustic meatus (IAC), accompanied by the facial nerve [VII]
• Divides into:
  • Cochlear nerve — cell bodies in the spiral ganglion within the modiolus
  • Vestibular nerve — cell bodies in the vestibular (Scarpa's) ganglion in the IAC; upper and lower divisions innervate the cristae and maculae

VII. PHYSIOLOGY OF THE AUDITORY ANALYZER

Sound Transmission (Conduction)

1. Sound waves are collected by the auricle and travel through the external meatus to vibrate the tympanic membrane
2. Vibrations are amplified and transmitted via the ossicular chain (malleus → incus → stapes) to the oval window
3. The middle ear achieves a ~25 dB pressure gain via two mechanisms:
   • Area ratio — tympanic membrane (~55 mm²) is ~17× larger than the oval window (~3.2 mm²)
   • Lever action of the ossicular chain (~1.3:1)
4. Stapes footplate vibration creates pressure waves in the perilymph of the scala vestibuli
5. Waves travel up the scala vestibuli, around the helicotrema (apex), and down the scala tympani to the round window (which bulges to accommodate fluid displacement)

Cochlear Transduction (Sensory)

1. Pressure differences across the basilar membrane cause it to deflect; the basilar membrane is tonotopically organized — high-frequency sounds (20 kHz) maximally stimulate the base, low-frequency sounds (20 Hz) the apex
2. The spiral organ of Corti on the basilar membrane contains:
   • Inner hair cells (IHCs) — single row; primary afferent sensory cells (~3,500); 95% of cochlear nerve fibers synapse here
   • Outer hair cells (OHCs) — three rows (~12,000); electromotile amplifiers that sharpen frequency tuning via prestin-driven somatic motility
3. Hair cell stereocilia are embedded in (OHC) or contact (IHC) the tectorial membrane; basilar membrane deflection shears the stereocilia
4. Tip links between stereocilia open mechanically-gated MET channels, allowing K⁺ and Ca²⁺ influx from endolymph → depolarization → neurotransmitter (glutamate) release at the IHC base → activation of cochlear nerve fibers

Central Auditory Pathway

VIII. PHYSIOLOGY OF THE VESTIBULAR ANALYZER

Otolithic Organs (Utricle & Saccule — Linear Acceleration)

• The macula contains hair cells embedded in a gelatinous membrane topped by otoconia (calcium carbonate crystals)
• When the head tilts or accelerates linearly, otoconia shift due to gravity/inertia → shear force on stereocilia → hair cell depolarization or hyperpolarization (depending on direction of deflection relative to the kinocilium)

Semicircular Canals (Angular Acceleration)

• The crista ampullaris in each ampulla bears hair cells embedded in the gelatinous cupula (which spans the full width of the ampullary lumen)
• Angular head rotation causes inertia-based endolymph lag → cupula deflection → hair cell transduction
• The three canals are orthogonally arranged; each canal works in a push-pull pair with its contralateral counterpart (lateral–lateral; anterior–contralateral posterior)

Vestibulo-Ocular Reflex (VOR)

Central Vestibular Pathway

IX. RESEARCH AND CLINICAL METHODS

Auditory Assessment

Vestibular Assessment

X. KEY CLINICAL CORRELATIONS