Ear anatomy for exam point of view
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Ear Anatomy - Exam Guide
Here is a structured, high-yield breakdown of ear anatomy from authoritative sources (Cummings Otolaryngology, KJ Lee's Essential Otolaryngology, Costanzo Physiology, Gray's Anatomy).
Overview: Three Divisions
The ear divides into external ear, middle ear, and inner ear. All three housed within the temporal bone, which has four parts: squamous, mastoid, petrous, and tympanic.
1. Temporal Bone - High Yield Facts
| Part | Key Features |
|---|---|
| Squamous | Forms lateral middle fossa floor; temporal line ~5 mm inferior to middle fossa floor |
| Mastoid | Contains mastoid antrum; MacEwen triangle (cribriform area) marks it; SCM pulls mastoid tip inferiorly |
| Petrous | Contains inner ear structures; arcuate eminence = prominence of superior SCC on middle fossa floor |
| Tympanic | Forms floor and anterior wall of external auditory canal |
- Temporal bone articulates with sphenoid, parietal, occipital, and zygomatic bones
- Stylomastoid foramen = anterior limit of digastric groove (where facial nerve exits)
- Mastoid antrum = largest mastoid air cell; located deep to MacEwen triangle
2. External Ear
Auricle
- Composed of elastic fibrocartilage (except lobule = fat only)
- Sensory supply: Great auricular nerve (C2/C3), Auriculotemporal nerve (V3), Arnold's nerve (CN X branch - important in ear-cough reflex)
External Auditory Canal (EAC)
- Outer 1/3 - cartilaginous (skin has hair follicles + ceruminous glands; fissures of Santorini allow infection spread)
- Inner 2/3 - bony (skin is thin, no adnexa; directly on bone)
- Total length ~24 mm
- Foramen of Huschke - potential defect in bony EAC (tumor spread pathway)
- Bony-cartilaginous junction - narrowest point; pathway for tumor spread
Blood Supply of EAC
- Posterior auricular artery, deep auricular artery (from maxillary artery), superficial temporal artery
3. Tympanic Membrane (TM)
Layers (4 layers - classic exam question)
- Lateral - squamous epithelium (continuous with EAC skin)
- Radiate fibrous layer (outer fibrous)
- Circular fibrous layer (inner fibrous)
- Medial - mucosal layer (middle ear mucosa)
The annular ligament anchors TM peripherally to the tympanic sulcus (bony ring).
Two Parts
| Region | Features |
|---|---|
| Pars tensa | Lower 4/5; 4-layer; attached to annulus; site of most perforations |
| Pars flaccida (Shrapnell membrane) | Upper 1/5; 2-layer (no fibrous); superior to anterior/posterior mallear folds; site of cholesteatoma origin |
Surface Landmarks (otoscopy)
- Umbo = tip of malleus handle (most depressed central point)
- Mallear stria = handle visible through TM
- Lateral process = most prominent point at upper end of handle
- Cone of light = seen anteroinferiorly (5 o'clock right ear, 7 o'clock left ear)
- Anterior and posterior mallear folds = define upper limit of pars tensa
Dimensions
- Total area: 70-80 mm²
- Effective vibrating area: ~55 mm²
Prussak Space (important for cholesteatoma)
- Anterior: lateral malleal fold
- Posterior: lateral malleal fold
- Superior: lateral malleal fold
- Inferior: lateral process of malleus
- Medial: neck of malleus
- Lateral: Shrapnell membrane
4. Middle Ear
Divisions (relative to tympanic annulus)
| Division | Location |
|---|---|
| Hypotympanum | Below level of TM |
| Mesotympanum | Level of TM; contains most ossicles |
| Epitympanum (attic) | Above TM; contains head of malleus + body of incus |
| Mastoid antrum | Posterior extension |
Ossicles - Embryology (Very High Yield)
| Ossicle | Branchial Arch Origin |
|---|---|
| Head of malleus, body + short process of incus | 1st arch (Meckel's cartilage) |
| Manubrium of malleus, long process of incus, stapes superstructure | 2nd arch (Reichert's cartilage) |
| Stapes footplate | Otic capsule (not branchial arch) |
Ossicles are adult size at birth - formed by 8 weeks gestation.
Ossicle Parts
Malleus: Head → Neck → Manubrium (handle) → Anterior process + Lateral process
Incus: Body → Short process → Long process → Lenticular process (articulates with stapes)
Stapes: Head → Neck → Anterior crus + Posterior crus → Footplate (1.41 × 2.99 mm)
Incus: Body → Short process → Long process → Lenticular process (articulates with stapes)
Stapes: Head → Neck → Anterior crus + Posterior crus → Footplate (1.41 × 2.99 mm)
Ossicular Joints
- Malleoincudal joint = diarthrodial
- Incudostapedial joint = diarthrodial
- Stapediovestibular (labyrinthine) joint = syndesmotic (annular ligament)
Most Vulnerable Part of Ossicular Chain
Long process of the incus - single nutrient vessel, no collateral circulation; most commonly eroded in chronic otitis media.
Middle Ear Muscles
| Muscle | Nerve Supply | Action |
|---|---|---|
| Tensor tympani | CN V3 (medial pterygoid nerve) | Pulls malleus medially, tenses TM |
| Stapedius | CN VII (facial nerve) | Pulls stapes posteriorly, dampens loud sounds (acoustic reflex) |
Stapedius = smallest muscle in the body. Paralysis in Bell's palsy causes hyperacusis.
Eustachian Tube (ET)
- Connects middle ear to nasopharynx
- Angle: ~45° from horizontal in adults (more horizontal in children - explains frequent OM in children)
- Proximal 1/3: bony; distal 2/3: fibrocartilaginous (collapsed at rest)
- Opens during swallowing/yawning via tensor veli palatini (CN V3)
- Bony-cartilaginous junction = narrowest point
Important Middle Ear Relations
- Roof (tegmen tympani) - separates middle ear from middle fossa
- Floor - jugular bulb below
- Anterior wall - internal carotid artery (may be dehiscent)
- Posterior wall - facial nerve, aditus ad antrum
- Medial wall - promontory (cochlear basal turn), oval window, round window, facial nerve canal
Windows
- Oval window - accepts stapes footplate; lies in sagittal plane; transmits vibrations to scala vestibuli
- Round window - covered by secondary tympanic membrane; allows pressure relief
5. Facial Nerve in the Temporal Bone
The facial nerve traverses the fallopian canal with three segments:
| Segment | Details | Clinical Relevance |
|---|---|---|
| Labyrinthine | Shortest, narrowest; from fundus IAC to geniculate ganglion | Most vulnerable to swelling (herpes zoster/Ramsay Hunt) |
| Tympanic (horizontal) | Above oval window; most often dehiscent | Injured in cholesteatoma surgery |
| Mastoid (vertical) | From 2nd genu to stylomastoid foramen | Exits at stylomastoid foramen |
- Geniculate ganglion: gives off Greater Superficial Petrosal Nerve (GSPN) → lacrimal gland
- Chorda tympani: branches from mastoid segment → taste ant. 2/3 tongue, submandibular/sublingual glands
6. Inner Ear
Cochlea - Three Scalae
| Chamber | Fluid | Boundaries |
|---|---|---|
| Scala vestibuli | Perilymph | Communicates with oval window |
| Scala media (cochlear duct) | Endolymph | Between Reissner's and basilar membranes |
| Scala tympani | Perilymph | Communicates with round window |
- Scala vestibuli and tympani communicate at the apex via helicotrema
- Reissner's membrane separates scala vestibuli from scala media
- Basilar membrane separates scala media from scala tympani
- Stria vascularis produces endolymph (high K+, low Na+)
Organ of Corti (on basilar membrane)
- Contains inner hair cells (1 row, ~3500) and outer hair cells (3 rows, ~12,000)
- Hair cells (cilia) embedded in tectorial membrane
- Inner hair cells = primary auditory transducers (90% of afferent fibers)
- Outer hair cells = amplifiers (active motility, otoacoustic emissions)
- Cell bodies in spiral ganglion → cochlear nerve → CN VIII
Tonotopy (Exam Favorite)
- Base of cochlea = high frequency (20,000 Hz)
- Apex of cochlea = low frequency (20 Hz)
Vestibular Apparatus
- 3 semicircular canals (superior/anterior, posterior, horizontal/lateral) - detect angular acceleration
- Utricle - macula oriented horizontally; detects linear acceleration and head tilt
- Saccule - macula oriented vertically; detects vertical linear acceleration
Fluids
- Perilymph - similar to CSF (high Na+, low K+); in scala vestibuli and tympani
- Endolymph - similar to intracellular fluid (high K+ ~150 mM, low Na+); in scala media; produced by stria vascularis
- Endolymphatic hydrops = excessive endolymph → Meniere's disease
7. Embryology Summary
| Structure | Origin |
|---|---|
| Auricle | 1st and 2nd branchial arch hillocks (6 hillocks of His) |
| EAC | 1st branchial groove |
| Middle ear cavity, ET | 1st pharyngeal pouch (tubotympanic recess) |
| Malleus head, incus body/short process | 1st arch (Meckel's) |
| Malleus handle, incus long process, stapes superstructure | 2nd arch (Reichert's) |
| Stapes footplate | Otic capsule |
| Inner ear (cochlea, vestibule, SCCs) | Otic vesicle (otocyst) from ectoderm |
- Mastoid antrum appears at 21 weeks gestation
- Mastoid tip poorly developed at birth → facial nerve more superficial and prone to surgical injury in neonates
- Eustachian tube doubles in length from birth to adulthood
8. Blood Supply
- EAC: Posterior auricular + superficial temporal arteries (external ear); deep auricular artery (bony EAC)
- Middle ear: Anterior tympanic (from maxillary), inferior tympanic (from ascending pharyngeal), superior tympanic (from middle meningeal), stylomastoid (from posterior auricular)
- Inner ear: Labyrinthine artery (from AICA or basilar artery) - end artery, no collaterals → labyrinthine infarction is devastating
9. Nerve Supply Summary (Exam High Yield)
| Region | Nerves |
|---|---|
| Auricle outer surface | Auriculotemporal (V3), greater auricular (C2/C3), lesser occipital (C2), Arnold's nerve (CN X) |
| EAC skin | Arnold's nerve (CN X) - stimulation causes ear-cough reflex + vagal syncope |
| Middle ear mucosa | Jacobson's nerve (CN IX, tympanic branch) → tympanic plexus |
| Stapedius | CN VII |
| Tensor tympani | CN V3 |
| Cochlea (afferent) | CN VIII (cochlear division), spiral ganglion |
| Vestibule (afferent) | CN VIII (vestibular division), Scarpa's ganglion |
10. High-Yield Exam Quick-Recall
| Fact | Answer |
|---|---|
| Smallest bone in body | Stapes |
| Smallest muscle in body | Stapedius |
| Most vulnerable ossicle to erosion | Long process of incus |
| Pars flaccida is also called | Shrapnell membrane |
| Cholesteatoma originates from | Pars flaccida |
| Scala with endolymph | Scala media |
| Endolymph produced by | Stria vascularis |
| High K+ fluid in ear | Endolymph |
| Prussak space - lateral wall | Shrapnell membrane |
| ET opened by | Tensor veli palatini (CN V3) |
| Hyperacusis in Bell's palsy - due to | Stapedius paralysis |
| Arnold's nerve stimulation causes | Ear-cough reflex, vagal syncope |
| Facial nerve most dehiscent in | Tympanic (horizontal) segment above oval window |
| Arcuate eminence = prominence of | Superior semicircular canal |
| Helicotrema connects | Scala vestibuli and scala tympani at cochlear apex |
| Base vs apex of cochlea - frequency | Base = high freq; Apex = low freq |
Sources: Cummings Otolaryngology Head and Neck Surgery, Ch. 126 | KJ Lee's Essential Otolaryngology | Costanzo Physiology 7th Ed, Ch. 3 | Langman's Medical Embryology
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