Anatomy of the Ear

1. External Ear

• Auricle: Stratified squamous epithelium over a framework of elastic cartilage. The lobule (earlobe) is the only cartilage-free portion, containing adipose tissue instead. Sebaceous glands and hair follicles are present in the subcutaneous layer.
• External auditory canal (EAC): Approximately 2.5-3 cm long in adults. The outer one-third is cartilaginous (contains hair follicles, sebaceous glands, and apocrine glands that produce cerumen). The inner two-thirds is bony (thin squamous epithelium tightly adherent to bone, no glands or hair follicles). The narrowest point is the isthmus, at the bony-cartilaginous junction.
• Cerumen: Hydrophobic, slightly acidic (pH 6.0-6.5), formed by glandular secretions and sloughed epithelium in the cartilaginous canal. The canal is self-cleaning due to outward centrifugal migration of keratin from the tympanic membrane.
• Fissures of Santorini: Transverse slits in the cartilaginous canal that can allow spread of infection or tumor to surrounding soft tissues.
• Foramen of Huschke: A defect in the anterior bony canal from incomplete ossification; can allow disease spread to the parotid gland.

2. Tympanic Membrane

• Semitransparent, thin (~0.1 mm), ~1 cm in diameter, average surface area ~65 mm²
• Tilted anteriorly and inferiorly (like a miniature satellite dish)
• Three layers from outside to inside:
  1. Stratified squamous keratinized epithelium (outer)
  2. Dense connective tissue core with radially arranged outer fibers and circumferentially arranged inner fibers
  3. Simple cuboidal epithelium of the middle ear mucosa (inner)
• Two functional regions:
  • Pars tensa: the large, taut central portion
  • Pars flaccida: the small, lax upper portion (Shrapnell's membrane), prone to retraction pockets
• The umbo is the most inward point of the TM where the tip of the malleus manubrium attaches
• On otoscopy, a cone of light (light reflex) radiates anteroinferiorly from the umbo

3. Middle Ear

• The long process of the incus is the most vulnerable ossicle - it has a single nutrient vessel and lacks collateral circulation
• Two muscles act on the chain: tensor tympani (dampens low-frequency, attaches to malleus) and stapedius (attaches to stapes neck, protective acoustic reflex)
• The facial nerve passes through the middle ear, making it surgically vulnerable. Key landmarks include the cochleariform process, oval window, and pyramidal eminence

• Epitympanum (attic): superior to TM, contains the malleus head and incus body
• Mesotympanum: opposite the TM
• Hypotympanum: inferior to TM
• Eustachian tube: connects middle ear to nasopharynx for pressure equalization

4. Inner Ear

Cochlea

• Reissner's membrane separates the scala vestibuli from the scala media (functionally thin, maintains endolymph composition)
• Basilar membrane separates the scala media from the scala tympani
• Organ of Corti sits on the basilar membrane - contains the mechanosensory hair cells
• Stria vascularis: vascular epithelium on the outer wall of the scala media; produces endolymph (high K⁺, like intracellular fluid)
• Tectorial membrane: overlies the hair cells; deflects stereocilia during basilar membrane movement
• The scala vestibuli and scala tympani communicate at the apex via the helicotrema
• Tonotopy: high frequencies activate the stiff, narrow base; low frequencies activate the wide, flexible apex

• Base (near oval window): ~0.04 mm fiber length, stiff - responds to high frequencies
• Apex (helicotrema): ~0.5 mm fiber length, flexible - responds to low frequencies

Vestibular Labyrinth

• Saccule and utricle: detect linear acceleration and head position relative to gravity (macular organs with otoliths)
• Three semicircular canals (superior, posterior, horizontal): detect rotational/angular acceleration; each has an ampulla containing a crista with cupula

Sound transmission pathway (summary):

Sound waves → pinna/EAC → TM vibrates → malleus → incus → stapes footplate → oval window → perilymph in scala vestibuli → basilar membrane displacement → hair cell stereocilia deflect against tectorial membrane → mechanoelectrical transduction → cochlear nerve (CN VIII) → brainstem auditory nuclei

Temporal Bone (Housing)

• Squamous part: lateral wall of middle fossa
• Mastoid part: posterior, air-cell system
• Petrous part: dense bone housing the inner ear
• Tympanic part: forms most of the bony EAC