• Pinna (auricle) - the visible external structure
• External auditory canal - channels sound inward

• Tympanic membrane (eardrum) - vibrates in response to sound
• Ossicles: malleus, incus, and stapes - the three smallest bones in the body, transmitting vibrations
• Eustachian tube - equalizes pressure between the middle ear and throat

• Cochlea - snail-shaped structure that converts vibrations into nerve signals
• Semicircular canals - three fluid-filled loops responsible for balance and spatial orientation
• Vestibule - connects the cochlea and semicircular canals
• Vestibulocochlear nerve (CN VIII) - carries auditory and balance signals to the brain

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Ear Anatomy - Detailed Study Guide

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Overview

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1. The Temporal Bone

• Temporal line: ~5 mm inferior to the lowest level of the middle fossa floor
• MacEwen triangle (suprameatal triangle): depressed cribriform area posterior to the spine of Henle; marks the mastoid antrum
• Stylomastoid foramen: anterior limit of the digastric groove; facial nerve exits here
• Arcuate eminence: prominence of the superior semicircular canal on the middle fossa floor
• Cummings Otolaryngology, Chapter 126

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2. The External (Outer) Ear

2a. Auricle (Pinna)

• Keratinizing squamous epithelium over an elastic cartilage framework
• Perichondrium is tightly bound to the lateral surface, loosely bound medially
• Sebaceous glands and hair follicles are in the subcutaneous layer
• The lobule (earlobe) is cartilage-free and contains adipose tissue

2b. External Auditory Canal (EAC)

• ~2.5 cm in adults; runs from the tympanic membrane to the external meatus
• Lateral 1/3: cartilaginous skeleton - contains hair follicles, sebaceous and apocrine glands
• Medial 2/3: bony wall (formed by tympanic part of temporal bone) - thin squamous epithelium, no glands, no hair follicles
• Isthmus: the bony-cartilaginous junction - narrowest part of the canal

• Produced in the cartilaginous canal from glandular secretions + sloughed epithelium
• Hydrophobic, slightly acidic (pH 6.0 - 6.5)
• Normal flora of the EAC: predominantly gram-positive (S. auricularis, S. epidermidis)

• Fissures of Santorini: transverse slits in cartilaginous canal - allow spread of infection/tumors to surrounding soft tissues
• Foramen of Huschke: defect in anterior bony canal (incomplete ossification) - allows disease spread to parotid gland's deep lobe

• Cummings Otolaryngology, Chapter 130

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3. The Middle Ear

3a. Spatial Divisions of the Middle Ear

3b. Walls of the Middle Ear

• Promontory: rounded bulge caused by the basal coil of the cochlea
• Oval window (fenestra vestibuli): posterosuperior to promontory; sealed by the footplate of the stapes
• Round window (fenestra cochleae): inferior to oval window; sealed by the secondary tympanic membrane; allows fluid pressure waves in the cochlea to dissipate
• Tympanic plexus: on the promontory, formed primarily by the tympanic branch of CN IX (Jacobson's nerve)

3c. The Tympanic Membrane (TM)

• Separates the EAC from the middle ear
• Divided into the pars flaccida (superiorly - slack, lacking fibrous layer) and pars tensa (the main body - has a fibrous layer)
• The umbo is the lowest point, attached to the tip of the malleus handle
• The TM vibrates in response to sound pressure waves and transmits motion to the ossicular chain

3d. The Ossicular Chain

• Malleus-incus: synovial joint
• Incus-stapes: synovial joint
• The ossicles amplify sound by ~25-30 dB through the surface area ratio of the TM to the oval window plus the lever arm of the ossicular chain

• Tensor tympani: attached to the malleus handle; innervated by CN V3 (medial pterygoid nerve); dampens loud sounds
• Stapedius: attached to the neck of the stapes; innervated by CN VII (facial nerve); the acoustic reflex - protective against intense noise

3e. The Eustachian (Pharyngotympanic) Tube

• Connects the middle ear to the nasopharynx; angled ~45° from middle ear to nasopharyngeal opening at the torus tubarius
• Proximal 1/3: bony (in petrous bone) - widest segment
• Distal 2/3: fibrocartilaginous, normally collapsed at rest; shaped like an inverted J in cross-section
• Bony-cartilaginous junction: narrowest point
• Opens during swallowing and yawning via contraction of tensor veli palatini - equalizes pressure between middle ear and atmosphere
• Internal carotid artery is closely associated with the medial wall at its tympanic opening (may be dehiscent)

3f. The Mastoid

• Air cells develop from outgrowth of the middle ear space; degree of pneumatization varies greatly between individuals
• Mastoid antrum: directly posterior to the epitympanum via the aditus
• Important surgical landmark: posterior to the pyramid of the mastoid

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4. The Inner Ear

• Bony labyrinth: outer shell of dense bone filled with perilymph
• Membranous labyrinth: inner fluid-filled sacs and ducts filled with endolymph (K+-rich, Na+-poor - similar in composition to intracellular fluid)

4a. The Cochlea (Hearing)

• Scala vestibuli and scala tympani communicate at the helicotrema (apex)
• Scala tympani ends at the round window

4b. The Organ of Corti (Auditory Transducer)

• Inner hair cells (IHCs): ~3,500; single row; flask-shaped; primary sensory receptors - 95% of afferent innervation
• Outer hair cells (OHCs): ~12,000; three rows; cylindrical; mainly efferent innervation; provide cochlear amplification (active electromotility)
• Tectorial membrane: acellular; overlies the hair cell bundles; OHC stereocilia are embedded in it
• Tunnel of Corti: delineated by inner and outer pillar cells
• Supporting cells: Deiters cells, Hensen cells, Claudius cells, Boettcher cells

• Base of cochlea → detects high frequencies (basilar membrane narrow, stiff)
• Apex of cochlea → detects low frequencies (basilar membrane wide, floppy)

1. Stapes footplate movement displaces perilymph in scala vestibuli
2. Pressure wave creates a traveling wave along the basilar membrane
3. The wave peaks at the region tuned to that frequency
4. Basilar membrane deflection moves hair cell stereocilia
5. Stereocilia bending opens mechanically gated K⁺ channels
6. K⁺ influx depolarizes the hair cell → neurotransmitter (glutamate) release
7. Action potentials travel via CN VIII to the auditory cortex

4c. Vestibular Apparatus (Balance)

Semicircular Canals - Angular Acceleration

• Anterior (superior) semicircular canal
• Posterior semicircular canal
• Lateral (horizontal) semicircular canal

Otolith Organs - Linear Acceleration

4d. Vestibular Ganglia and Nerve Pathways

• Superior vestibular ganglion: receives input from utricle, anterior saccule, anterior and lateral semicircular canals
• Inferior vestibular ganglion: receives input from posterior saccule and posterior semicircular canal
• Both join the vestibulocochlear nerve (CN VIII), which enters the brainstem at the pontomedullary junction
• Central projections: thalamus, cortex, cerebellum, descending spinal cord, extraocular motor nuclei (for the vestibulo-ocular reflex, VOR)

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5. Embryology of the Ear

• Middle and outer ear structures derived from 1st and 2nd branchial arches/grooves
• K.J. Lee's Essential Otolaryngology

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6. The Facial Nerve in the Ear

• Cummings Otolaryngology, Chapter 126

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7. Clinical Correlations

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8. Quick Review: Key Numbers to Remember

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