May 22, 2026 PDF Available

Topic Overview

ANATOMY OF THE EAR

Physiology of Hearing & Balance

MedMentor EDU | ENT Study Notes

Contents at a Glance

1. Introduction & Divisions of the Ear

2. Embryology of the Ear

3. External Ear — Auricle & EAC

4. Tympanic Membrane

5. Middle Ear — Tympanic Cavity, Ossicles, Eustachian Tube, Mastoid

6. Facial Nerve in Temporal Bone

7. Inner Ear — Cochlea & Vestibular Apparatus

8. Blood Supply & Nerve Supply of Inner Ear

9. Physiology of Hearing

10. Physiology of Balance

11. Master Comparison Tables

12. High-Yield Exam Pearls

SECTION 1 | Introduction & Divisions of the Ear

1.1 Definition & Functions

The ear is the organ responsible for hearing (auditory function) and maintenance of equilibrium and balance (vestibular function).

Function	Structure Involved	Role
Hearing	External + Middle + Inner ear	Collection, conduction and perception of sound
Equilibrium	Vestibular apparatus	Maintenance of posture and balance

1.2 Divisions of the Ear

Division	Components	Function
External Ear	Auricle (pinna), EAC, Tympanic membrane	Collection and conduction of sound
Middle Ear	Tympanic cavity, Ossicles, Eustachian tube, Mastoid air cells	Transmission and amplification of sound
Inner Ear	Cochlea, Vestibule, Semicircular canals	Hearing and balance

1.3 Clinical Importance of Ear Anatomy

Otoscopic examination
Myringotomy and tympanoplasty
Mastoid surgery
Cochlear implant surgery
Facial nerve surgery

Clinical Correlation	Significance
Facial nerve traverses middle ear	At risk during ear surgery
Sigmoid sinus & dura related to mastoid	Route for intracranial spread of infection
Inner ear damage	Causes vertigo and sensorineural deafness
Eustachian tube dysfunction	Predisposes to otitis media

1.4 Flowchart — Divisions of the Ear

EAR — Structural Overview

EAR

↓

├── EXTERNAL EAR: Pinna | External Auditory Canal | Tympanic Membrane

↓

├── MIDDLE EAR: Tympanic Cavity | Ossicles | Eustachian Tube | Mastoid

↓

└── INNER EAR: Cochlea | Vestibule | Semicircular Canals

SECTION 2 | Embryology of the Ear

2.1 Development of the External Ear

Pinna — Hillocks of His

Six mesenchymal hillocks appear around the first branchial groove. Three arise from the first branchial arch and three from the second.

Hillock	Derivative	Arch
1	Tragus	First arch
2	Helix	Second arch
3	Helix	Second arch
4	Antihelix	Second arch
5	Antitragus	Second arch
6	Lobule	Second arch

Pinna initially located in lower neck; ascends during mandibular growth. Fully developed by 20th week of gestation.

External Auditory Canal

Develops from the first branchial cleft (ectodermal groove)
A solid epithelial meatal plug forms initially
Canalization occurs by the 7th month

2.2 Development of the Tympanic Membrane

Layer	Germ Layer Origin
Outer epithelial layer	Ectoderm (from first branchial groove)
Middle fibrous layer	Mesoderm
Inner mucosal layer	Endoderm (from first pharyngeal pouch)

2.3 Development of the Middle Ear

Middle ear develops from the first pharyngeal pouch (endoderm) — the tubotympanic recess.

Structure	Derivative
Tubotympanic recess	Middle ear cavity + Eustachian tube
Proximal part of recess	Eustachian tube
Distal part of recess	Tympanic cavity

Ossicular Origins

Ossicle	Branchial Arch of Origin
Malleus	First branchial arch
Incus	First branchial arch
Stapes suprastructure	Second branchial arch
Stapes footplate	Otic capsule

Important: Ossicles are FULLY OSSIFIED AT BIRTH.

Mastoid Development

Mastoid antrum present at birth
Mastoid air cells develop postnatally
Pneumatization continues until puberty
Depends on Eustachian tube function and middle ear ventilation

2.4 Development of the Inner Ear

Inner Ear Embryological Sequence

Otic placode (ectodermal thickening near hindbrain)

↓

Otic pit (invagination)

↓

Otic vesicle / Otocyst (detached ectodermal sac)

↓

Membranous labyrinth (utricle, saccule, semicircular ducts, cochlear duct)

↓

Cochlear duct coils → 2.5 turns

2.5 Germ Layer Derivation — Summary

Structure	Germ Layer
External auditory canal	Ectoderm
Middle ear mucosa	Endoderm
Fibrous layer of TM	Mesoderm
Inner ear	Ectoderm (otic placode)

2.6 Congenital Anomalies

Anomaly	Description	Clinical Significance
Microtia (Grade I–III)	Underdeveloped pinna (Grade I=small; Grade III=peanut ear)	May be associated with canal atresia
Accessory auricle	Extra auricular appendage near tragus	Persistence of accessory hillocks
Preauricular sinus	Congenital epithelial tract near ascending helix	Recurrent infection and abscess
Congenital aural atresia	Failure of EAC canalization	Causes conductive deafness
Congenital cholesteatoma	Keratinizing epithelial cyst behind intact TM	Found in middle ear

SECTION 3 | External Ear

3.1 Auricle (Pinna) — Surface Anatomy

Structure	Description
Helix	Outer curved margin of pinna
Antihelix	Inner curved ridge parallel to helix
Tragus	Small cartilaginous projection anterior to EAC
Antitragus	Projection opposite tragus
Lobule	Soft lower non-cartilaginous part
Concha	Deep cavity leading into EAC
Scaphoid fossa	Depression between helix and antihelix
Triangular fossa	Depression between limbs of antihelix
Darwin tubercle	Small prominence on posterior superior helix (evolutionary remnant)

�� Pinna / Auricle Anatomy — Surface Landmarks

[ Diagram to be drawn here ]

Cartilaginous Framework

Made of elastic fibrocartilage — flexible, covered by perichondrium
Lobule lacks cartilage
Ear length increases with age; male pinna generally larger

3.2 Muscles, Blood Supply, Lymphatics & Nerve Supply of Pinna

Extrinsic Muscles

Muscle	Action
Auricularis anterior	Pulls ear forward
Auricularis superior	Elevates ear
Auricularis posterior	Pulls ear backward

Blood Supply

Artery	Source
Posterior auricular artery	External carotid artery
Superficial temporal artery	External carotid artery

Lymphatic Drainage

Part of Pinna	Drains to
Upper lateral surface	Superficial parotid nodes
Medial surface	Mastoid nodes
Lobule	Upper deep cervical nodes

Nerve Supply

Nerve	Area Supplied
Great auricular nerve	Majority of pinna
Auriculotemporal nerve	Anterosuperior part
Lesser occipital nerve	Upper medial surface
Vagus nerve (Arnold nerve)	Concha

3.3 Applied Anatomy of Pinna

Condition	Description
Othematoma	Blood between cartilage and perichondrium; common in wrestlers; complication = cauliflower ear
Perichondritis	Infection of perichondrium; common organism: Pseudomonas aeruginosa
Bat ear	Prominent protruding ear due to underdeveloped antihelix
Frostbite injury	Cold-induced ischemic injury of pinna

3.4 External Auditory Canal (EAC)

General Anatomy

Feature	Details
Length	Approximately 24 mm in adults
Shape	S-shaped canal
Direction	Directed medially, forward and upward
Isthmus	Narrowest portion of bony EAC — important surgical landmark

Otoscopy technique: pinna pulled upward and backward in adults; downward and backward in children.

Parts of EAC

Part	Extent	Key Features
Cartilaginous	Outer 1/3	Hair follicles, sebaceous & ceruminous glands, Fissures of Santorini
Bony	Inner 2/3	Temporal bone, thin tightly adherent skin, Foramen of Huschke

�� External Auditory Canal — Cartilaginous vs Bony Parts

[ Diagram to be drawn here ]

Ceruminous glands: modified apocrine sweat glands that produce ear wax (functions: lubrication, antibacterial action, dust trapping).

Fissures of Santorini: small defects in cartilage that allow spread of infection to the parotid region.

Self-cleansing mechanism: epithelial migration carries debris outward from umbo.

Resonance: EAC amplifies sounds between 2000–4000 Hz.

Relations of EAC

Wall	Relation
Anterior wall	Temporomandibular joint
Posterior wall	Mastoid
Inferior wall	Parotid gland
Medial end	Tympanic membrane

Blood & Nerve Supply of EAC

Nerve	Supply
Auriculotemporal nerve	Anterior wall and roof
Vagus nerve (Arnold reflex)	Posterior wall and floor — stimulation may cause cough, syncope, vomiting
Facial nerve	Small contribution

3.5 Applied Anatomy of EAC

Condition	Key Points
Wax impaction	Canal blockage; predisposed by narrow canal, hearing aids; symptoms: deafness, tinnitus, cough
Foreign body	Common in children; vegetable matter swells with water; complications: TM perforation
Furunculosis	Staph aureus infection of hair follicle in cartilaginous EAC; severe pain, tender tragus
Otitis externa — Diffuse	Pseudomonas / Staph; pain, discharge, itching, hearing loss
Otitis externa — Malignant	Necrotizing; Pseudomonas; skull base osteomyelitis; in diabetics
Keratosis obturans	Keratin accumulation; severe pain; widened canal
Exostosis	Multiple bilateral bony swellings; cold water swimmers; conductive HL

Referred Otalgia Source	Nerve
Tonsil	Glossopharyngeal nerve
Teeth	Trigeminal nerve
TMJ	Auriculotemporal nerve
Larynx	Vagus nerve
Cervical spine	C2, C3

SECTION 4 | Tympanic Membrane

4.1 General Anatomy

Parameter	Detail
Shape	Oval, concave laterally
Position	Obliquely placed at medial end of EAC; faces downward, forward, laterally
Obliquity	Approximately 55° with floor of canal
Vertical diameter	9–10 mm
Horizontal diameter	8–9 mm
Thickness	0.1 mm
Umbo	Central depressed point — attachment of malleus handle
Tympanic annulus	Fibrocartilaginous ring; superiorly deficient at Notch of Rivinus

�� Tympanic Membrane — Anatomy and Quadrants

[ Diagram to be drawn here ]

4.2 Parts of the Tympanic Membrane

Part	Characteristics	Clinical Importance
Pars tensa	Large, tense lower part; contains all 3 layers	Main sound-conducting portion
Pars flaccida (Shrapnell membrane)	Small, lax upper part; lacks fibrous layer; above lateral process of malleus	Common site for cholesteatoma formation

4.3 Layers of the Tympanic Membrane

Layer	Origin	Features
Outer epithelial layer	Ectoderm	Continuous with EAC skin
Middle fibrous layer	Mesoderm	Contains radial and circular fibres; ABSENT in pars flaccida
Inner mucosal layer	Endoderm	Continuous with middle ear mucosa

4.4 Nerve Supply of Tympanic Membrane

Surface	Nerve Supply
Outer surface	Auriculotemporal nerve + Vagus nerve
Inner surface	Glossopharyngeal nerve (tympanic branch)

4.5 Quadrants of Tympanic Membrane

Quadrant	Clinical Importance
Anterosuperior	Close to ossicles; opening of Eustachian tube nearby
Anteroinferior	SAFE SITE for myringotomy — avoids ossicles and chorda tympani
Posterosuperior	Cholesteatoma common; dangerous area
Posteroinferior	Middle ear fluid collection; common perforation site in CSOM

4.6 Applied Anatomy of Tympanic Membrane

Feature	Safe CSOM	Unsafe CSOM
Site	Pars tensa	Pars flaccida
Perforation type	Central	Marginal / Attic
Cholesteatoma	Absent	Present
Complications	Rare	Common

Cone of Light

Triangular light reflex seen in the anteroinferior quadrant of normal TM

Distorted or absent in middle ear disease — important otoscopic sign

SECTION 5 | Middle Ear

5.1 Tympanic Cavity — General Anatomy

The middle ear cleft is a continuous air-filled space within the temporal bone lined by mucosa, consisting of the Eustachian tube, tympanic cavity, and mastoid air cells.

Division	Position
Epitympanum (attic)	Above tympanic membrane
Mesotympanum	Opposite tympanic membrane
Hypotympanum	Below tympanic membrane

5.2 Six Walls of the Middle Ear

Wall	Structure / Content	Important Relation
Roof (Tegmental wall)	Tegmen tympani — thin plate of bone	Middle cranial fossa / temporal lobe dura; route for intracranial spread
Floor (Jugular wall)	Thin plate	Jugular bulb — may protrude into middle ear
Lateral wall	Tympanic membrane + lateral attic wall	—
Medial wall	Promontory, oval window, round window, facial nerve prominence, lateral SCC	Contains cochlea and inner ear structures
Anterior wall	Opening of ET, canal for tensor tympani	Internal carotid artery
Posterior wall	Aditus ad antrum, pyramid, facial recess	Mastoid air cells

�� Tympanic Cavity — Six Walls and Key Relations

[ Diagram to be drawn here ]

5.3 Important Spaces and Recesses

Space / Recess	Boundaries	Clinical Importance
Prussak space	Between pars flaccida and neck of malleus	Most common site of origin of cholesteatoma
Facial recess	Medial: facial nerve; Lateral: chorda tympani; Superior: fossa incudis	Surgical access route for cochlear implants
Sinus tympani	Deep recess posterior to promontory	Hidden site for cholesteatoma; difficult surgical access
Tympanic isthmus	Narrow communication between mesotympanum and epitympanum	Ventilation pathway
Aditus ad antrum	Communication between epitympanum and mastoid antrum	Route for spread of infection to mastoid

�� Prussak Space, Facial Recess & Sinus Tympani

[ Diagram to be drawn here ]

5.4 Contents of Middle Ear — Summary

Structure	Components
Ossicles	Malleus, Incus, Stapes
Muscles	Tensor tympani, Stapedius
Nerves	Chorda tympani, Tympanic plexus (Jacobson nerve)
Ligaments	Superior & anterior malleolar, posterior incudal, annular
Vessels	Anterior tympanic artery, stylomastoid artery

5.5 Ossicles

The three auditory ossicles form a chain connecting the tympanic membrane to the oval window. They are the smallest bones in the human body.

Feature	Malleus	Incus	Stapes
Shape	Hammer	Anvil	Stirrup
Weight	23 mg	27 mg	2.5 mg
Largest part	Head	Body	Footplate
Attached to TM	Yes	No	No
Muscle attached	Tensor tympani	None	Stapedius
Articulates with	Incus	Malleus & Stapes	Incus
Arch of origin	First	First	Second (suprastructure) / Otic capsule (footplate)

�� Ossicles and Ossicular Chain — Malleus, Incus, Stapes

[ Diagram to be drawn here ]

Ossicular Muscles

Feature	Tensor Tympani	Stapedius
Nerve supply	Mandibular nerve (V3)	Facial nerve (VII)
Origin	Cartilaginous ET	Pyramid of posterior wall
Insertion	Upper handle of malleus	Neck of stapes
Function	Tenses TM; reduces excessive vibration	Prevents excessive stapes movement; protective reflex
Paralysis causes	Hypermobile TM	Hyperacusis

Ossicular Joints

Joint	Type	Between
Incudomalleolar joint	Saddle synovial joint	Head of malleus and body of incus
Incudostapedial joint	Ball-and-socket synovial joint	Lenticular process of incus and head of stapes

Transformer Mechanism of the Middle Ear (Impedance Matching)

Mechanism	Effect
Area ratio — TM (~55 mm²) : Oval window (~3.2 mm²)	~17:1 pressure amplification
Ossicular lever action — malleus longer than incus	~1.3:1 mechanical advantage
Curved membrane (buckling) effect	Additional force transmission
Total amplification	Approximately 22–25 dB

�� Transformer Mechanism of the Middle Ear

[ Diagram to be drawn here ]

5.6 Eustachian Tube

Feature	Details
Length	~36 mm in adults
Course	Directed downward, forward, medially
Bony part	Lateral 1/3 — in petrous temporal bone; rigid
Cartilaginous part	Medial 2/3 — elastic fibrocartilage; mobile; opens into nasopharynx
Normal state	Closed; opens during swallowing, yawning, sneezing
Chief muscle opening ET	Tensor veli palatini
Ostmann fat pad	Fatty tissue around cartilaginous ET; maintains tube closure

Function	Role
Ventilation	Aerates middle ear
Pressure equalization	Equalizes atmospheric and middle ear pressure
Drainage	Removes secretions via mucociliary transport
Protection	Prevents nasopharyngeal reflux and infection

Feature	Child	Adult
Length	Short	Long (~36 mm)
Direction	Horizontal	Oblique (~45°)
Width	Wider	Narrower
Infection risk	Higher	Lower

�� Eustachian Tube — Anatomy, Bony & Cartilaginous Parts, Child vs Adult

[ Diagram to be drawn here ]

ET Dysfunction → Clinical Consequences

ET dysfunction caused by: URTI, allergy, adenoid hypertrophy

Effects: negative middle ear pressure → TM retraction → otitis media / OME

Barotrauma: failure of pressure equalization during flying or diving

5.7 Mastoid Antrum and Air Cell System

Mastoid antrum is present at birth. Mastoid air cells develop postnatally and pneumatization continues until puberty.

Relations of Mastoid Antrum

Wall	Relation
Roof	Middle cranial fossa
Posterior wall	Sigmoid sinus
Medial wall	Lateral semicircular canal
Inferior wall	Mastoid air cells

Pneumatization Types

Type	Features
Pneumatic	Well-developed air cells — normal
Diploic	Marrow-containing spaces — less pneumatized
Sclerotic	Poor air cells — due to recurrent infection

Surgical Landmarks

Landmark	Significance
MacEwen (Suprameatal) triangle	Surface landmark for mastoid antrum; bounded by supramastoid crest, posterior EAC wall, vertical tangent line
Trautmann triangle	Surgical approach to posterior cranial fossa; bounded by posterior SCC, sigmoid sinus, superior petrosal sinus
Korner septum	Petrosquamous lamina — may obstruct mastoid surgery
Mastoid emissary vein	Connects sigmoid sinus to extracranial veins — route for spread of infection

�� Mastoid Air Cell System & MacEwen Triangle

[ Diagram to be drawn here ]

Abscess	Direction of Spread
Bezold abscess	Into neck (through mastoid tip)
Citelli abscess	Posteriorly (toward occiput)
Luc abscess	Subperiosteally over mastoid cortex

SECTION 6 | Facial Nerve in the Temporal Bone

6.1 Introduction

The facial nerve (CN VII) carries motor, taste, parasympathetic, and sensory fibres. Its intratemporal course is clinically important in all ear surgery.

Total intratemporal length: approximately 30 mm.

6.2 Intratemporal Course — Segment by Segment

Segment	Location	Length	Key Features
Meatal	Internal auditory canal	—	Relations: facial (superior), cochlear (inferior), vestibular (posterior)
Labyrinthine	Fundus of IAC → Geniculate ganglion	~4 mm	NARROWEST segment; above cochlea; POOREST blood supply → most vulnerable in Bell palsy
Geniculate ganglion	First genu (sharp bend)	—	Contains sensory cell bodies; gives rise to GSPN
Tympanic	Medial wall of middle ear	~11 mm	Below lateral SCC; above oval window; dehiscence may occur
Second genu	Bend at posterior wall	—	Junction of tympanic and mastoid segments
Mastoid	Vertical descending segment	~13 mm	Gives nerve to stapedius and chorda tympani
Stylomastoid foramen	Exit point	—	Enters parotid gland; divides into temporofacial and cervicofacial divisions

�� Facial Nerve Course in Temporal Bone — All Segments

[ Diagram to be drawn here ]

6.3 Branches Inside the Temporal Bone

Branch	Origin	Fibres	Function
Greater superficial petrosal nerve (GSPN)	Geniculate ganglion	Preganglionic parasympathetic	Lacrimal gland secretion via pterygopalatine ganglion
Nerve to stapedius	Mastoid segment	Motor	Supplies stapedius muscle
Chorda tympani	Mastoid segment	Taste + parasympathetic	Taste from anterior 2/3 tongue; secretion from submandibular & sublingual glands

Chorda tympani course: arises in mastoid → enters middle ear → passes MEDIAL to handle of malleus → exits through petrotympanic fissure → joins lingual nerve.

6.4 Blood Supply of Intratemporal Facial Nerve

Segment	Blood Supply
Labyrinthine segment	Internal auditory artery (branch of AICA); POOREST vascularity
Tympanic segment	Petrosal artery (branch of middle meningeal)
Mastoid segment	Stylomastoid artery (branch of posterior auricular)

6.5 Surgical Landmarks for Facial Nerve

Landmark	Identifies
Lateral semicircular canal	Facial nerve in tympanic segment
Short process of incus	Facial recess region
Digastric ridge	Facial nerve near stylomastoid foramen
Cog (bony ridge near anterior epitympanum)	Landmark during cholesteatoma surgery

6.6 Applied Anatomy

Condition	Details
Bell palsy	Idiopathic LMN facial paralysis; labyrinthine segment most vulnerable due to poor blood supply; features: facial asymmetry, inability to close eye, forehead involvement
Hyperacusis	Increased sound sensitivity due to stapedius paralysis (facial nerve palsy)
Bell phenomenon	Upward rolling of eyeball on attempted eye closure — seen in facial palsy
Iatrogenic injury	During mastoidectomy or cholesteatoma surgery; risk increased by dehiscent canal or anatomical variations

SECTION 7 | Inner Ear

7.1 Overview — Bony Labyrinth

The inner ear is located within the petrous temporal bone. The bony labyrinth is a system of cavities filled with perilymph; within it lies the membranous labyrinth filled with endolymph.

Component	Contents	Function
Vestibule	Utricle and saccule	Central part of bony labyrinth; connects cochlea anteriorly and SCCs posteriorly
Semicircular canals (3)	Semicircular ducts	Mutually perpendicular; detect angular acceleration
Cochlea	Cochlear duct (scala media)	Hearing; ~2.5 turns around modiolus

�� Bony and Membranous Labyrinth — Overview

[ Diagram to be drawn here ]

7.2 Inner Ear Fluids

Feature	Endolymph	Perilymph
Location	Membranous labyrinth	Between bony and membranous labyrinth
Potassium	HIGH	Low
Sodium	Low	HIGH
Production	Stria vascularis (mainly)	Filtration from blood / CSF
Resembles	Intracellular fluid	CSF / extracellular fluid
Function	Hair cell stimulation; maintains endocochlear potential	Mechanical conduction

Endocochlear potential: +80 mV inside cochlear duct; essential for hair cell depolarization.

Blood-labyrinth barrier: physiological barrier regulating passage of substances into inner ear fluids (analogous to blood-brain barrier).

7.3 Cochlea — Detailed Anatomy

Scalae of the Cochlea

Scala	Fluid	Position	Connected to
Scala vestibuli	Perilymph	Upper	Oval window
Scala media (cochlear duct)	Endolymph	Middle	Contains Organ of Corti
Scala tympani	Perilymph	Lower	Round window

Helicotrema: opening at the apex connecting scala vestibuli to scala tympani.

Modiolus: central conical bony core containing the spiral ganglion and cochlear nerve fibres.

Membranes of the Cochlea

Membrane	Position	Function
Reissner membrane	Between scala vestibuli and scala media	Separates perilymph from endolymph
Basilar membrane	Supports Organ of Corti	Tonotopic — base: high frequency (narrow, stiff); apex: low frequency (wide, flexible)
Tectorial membrane	Overlies hair cells	Gelatinous; stimulates hair cells on deflection

�� Cochlea Cross-Section — Scalae, Organ of Corti, Basilar Membrane

[ Diagram to be drawn here ]

Organ of Corti

Component	Function
Inner hair cells (1 row)	Primary sensory transduction; main hearing receptor
Outer hair cells (3 rows)	Cochlear amplification and frequency selectivity; source of OAEs
Pillar cells	Form the tunnel of Corti; structural support
Tunnel of Corti	Triangular space between pillar cells
Stria vascularis	Produces endolymph; maintains endocochlear potential
Spiral ganglion	Contains bipolar neurons; first-order auditory neurons

�� Organ of Corti — Hair Cells, Tectorial Membrane, Pillar Cells

[ Diagram to be drawn here ]

Tonotopic Organization of Cochlea

Cochlear Region	Frequency Detected
Base (narrow, stiff basilar membrane)	High frequency (4000 Hz+)
Apex (wide, flexible basilar membrane)	Low frequency (<1000 Hz)

Otoacoustic Emissions (OAE): sounds generated by outer hair cell activity; used for neonatal screening and ototoxicity monitoring.

7.4 Vestibular Apparatus

Components and Functions

Structure	Location	Detects	Receptor
Utricle (larger)	Upper posterior vestibule	Horizontal linear acceleration; head tilt	Macula (horizontal orientation)
Saccule (smaller)	Anteroinferior to utricle	Vertical linear acceleration	Macula (vertical orientation)
Lateral SCC	Horizontal	Horizontal rotation	Crista ampullaris
Superior SCC	Vertical	Nodding movements	Crista ampullaris
Posterior SCC	Vertical	Side tilting	Crista ampullaris

�� Vestibular Apparatus — Utricle, Saccule, Semicircular Canals

[ Diagram to be drawn here ]

Crista Ampullaris and Cupula

Crista ampullaris: sensory ridge in the ampulla of each semicircular canal; detects rotational acceleration
Cupula: gelatinous structure covering crista; deflects with endolymph movement to stimulate hair cells

Otoconia

Calcium carbonate crystals embedded in the otolithic membrane of utricle and saccule
Function: increase membrane weight, enhance response to gravity and linear acceleration
Clinical importance: dislodged otoconia → BPPV (Benign Paroxysmal Positional Vertigo)

�� Crista Ampullaris, Cupula and Otolithic Membrane

[ Diagram to be drawn here ]

Vestibular Nuclei and Connections

Nucleus	Function
Superior nucleus	Eye movements (vestibulo-ocular reflex)
Medial nucleus	Vestibulo-ocular reflex (VOR)
Lateral nucleus	Postural control (Deiters nucleus)
Inferior nucleus	Vestibulocerebellar coordination

Tract / Connection	Function
Lateral vestibulospinal tract	Maintains extensor muscle tone
Medial vestibulospinal tract	Controls head and neck posture
Vestibulocerebellar connections	Coordination of balance via flocculonodular lobe
Vestibulo-ocular reflex (VOR)	Stabilizes vision during head movement — eyes move opposite to head

SECTION 8 | Blood Supply & Nerve Supply of the Inner Ear

8.1 Blood Supply

The labyrinthine (internal auditory) artery is usually a branch of the Anterior Inferior Cerebellar Artery (AICA), sometimes directly from the basilar artery.

Critical Feature — End Artery

The labyrinthine artery is an END ARTERY with NO significant collateral circulation

Occlusion → sudden sensorineural hearing loss + vertigo

Inner ear hair cells are extremely sensitive to ischemia

Branch	Supply
Anterior vestibular artery	Utricle, superior and lateral semicircular canals
Common cochlear artery → proper cochlear artery	Cochlea (all turns)
Common cochlear artery → vestibulocochlear artery	Part of cochlea + posterior SCC + saccule

8.2 Nerve Supply — CN VIII Vestibulocochlear Nerve

Feature	Cochlear Nerve	Vestibular Nerve
Ganglion	Spiral ganglion (in modiolus)	Scarpa ganglion (in IAC)
Neurons	Bipolar neurons	Bipolar neurons
Function	Carries auditory impulses	Carries balance impulses
Receptor	Organ of Corti hair cells	Utricle, saccule, SCC cristae

Vestibular nerve divisions: Superior division (utricle, superior and lateral SCCs); Inferior division (saccule and posterior SCC).

Auditory Pathway

Hair cells (Organ of Corti)

↓

Spiral ganglion (1st order neuron)

↓

Cochlear nerve (CN VIII)

↓

Cochlear nuclei (dorsal + ventral) — at pontomedullary junction

↓

Superior olivary complex — first site of BINAURAL interaction; sound localization

↓

Lateral lemniscus

↓

Inferior colliculus — auditory reflex centre

↓

Medial geniculate body (thalamus)

↓

Auditory cortex — Brodmann areas 41 & 42; superior temporal gyrus

SECTION 9 | Physiology of Hearing

9.1 Sound Transmission

Complete Sound Conduction Pathway

Sound waves → Pinna

↓

External auditory canal

↓

Tympanic membrane vibrates

↓

Malleus → Incus → Stapes

↓

Stapes footplate → Oval window

↓

Perilymph wave (scala vestibuli)

↓

Basilar membrane displacement

↓

Hair cell stereocilia deflection

↓

Ion channel opening → depolarization → neurotransmitter release

↓

Cochlear nerve impulse → Auditory cortex

9.2 Air Conduction vs Bone Conduction

Feature	Air Conduction	Bone Conduction
Pathway	External ear → Middle ear → Inner ear	Skull bone vibration → Inner ear directly
Efficiency	Better (normal physiological pathway)	Lesser
Normal result	AC > BC (Rinne positive)	BC < AC
Mechanism types	—	Compressional, Inertial (ossicular lag), Osseotympanic

9.3 Theories of Hearing

Theory	Principle	Status
Travelling Wave Theory (Bekesy)	Sound creates travelling wave on basilar membrane; different frequencies peak at different locations	Most accepted
Place Theory	Pitch determined by site of maximal basilar membrane vibration	Basis of tonotopy
Frequency Theory	Frequency coded by rate of nerve impulse firing	Cannot explain high-frequency hearing
Volley Theory	Groups of neurons fire in coordinated bursts for high frequencies	Extends frequency theory

9.4 Hair Cell Transduction

Cochlear Signal Transduction

Basilar membrane movement

↓

Stereocilia deflect toward tallest → tip links open K⁺/Ca²⁺ channels

↓

Potassium influx from endolymph (high K⁺) → depolarization

↓

Voltage-gated Ca²⁺ channels open at basolateral membrane

↓

Glutamate neurotransmitter release at ribbon synapse

↓

Cochlear nerve (spiral ganglion) fires

9.5 Acoustic Reflex

Reflex contraction of the stapedius muscle in response to loud sound (>70–80 dB). Protects the cochlea and improves speech discrimination.

Acoustic Reflex Pathway

Loud sound → Cochlea

↓

Cochlear nerve

↓

Cochlear nucleus

↓

Superior olivary complex

↓

Facial nerve nucleus

↓

Stapedius muscle contraction (bilateral)

SECTION 10 | Physiology of Balance

10.1 Overview

Balance is maintained by integration of the vestibular system, vision, proprioception, and cerebellum.

Type of Equilibrium	Receptor	Stimulus
Static equilibrium	Macula of utricle and saccule	Gravity and linear acceleration
Dynamic equilibrium	Crista ampullaris of SCCs	Angular / rotational acceleration

10.2 Vestibulo-Ocular Reflex (VOR)

The VOR maintains stable vision during head movement by moving the eyes in the opposite direction to head movement.

VOR Pathway

Head movement → Semicircular canal stimulation

↓

Vestibular nerve (Scarpa ganglion)

↓

Vestibular nuclei (brainstem)

↓

Ocular motor nuclei (III, IV, VI)

↓

Compensatory eye movement (opposite to head movement)

10.3 Nystagmus Physiology

Nystagmus is rhythmic involuntary oscillatory eye movement with a slow phase (vestibular origin) and a fast corrective phase (cerebral). Direction is named according to the fast component.

Caloric Test

Stimulus	Endolymph Movement	Nystagmus Direction (fast phase)
Warm water (44°C)	Rises (ampullopetal)	SAME side as irrigated ear
Cold water (30°C)	Falls (ampullofugal)	OPPOSITE side to irrigated ear
Mnemonic — COWS C — Cold water → Opposite side nystagmus O — (Opposite) W — Warm water → Same side nystagmus S — (Same)

SECTION 11 | Master Comparison Tables

Cartilaginous vs Bony EAC

Feature	Cartilaginous Part	Bony Part
Extent	Outer 1/3	Inner 2/3
Wall material	Elastic cartilage	Temporal bone
Skin	Thick	Thin; tightly adherent to periosteum
Hair follicles	Present	Absent
Ceruminous glands	Present	Absent
Fissures of Santorini	Present	Absent

Pars Tensa vs Pars Flaccida

Feature	Pars Tensa	Pars Flaccida
Size	Larger	Smaller
Fibrous layer	Present	Absent
Strength	Strong	Weak / lax
Site	Lower TM	Upper TM (above malleus lateral process)
Cholesteatoma	Less common	COMMON — Prussak space

Endolymph vs Perilymph

Feature	Endolymph	Perilymph
Potassium	High	Low
Sodium	Low	High
Location	Membranous labyrinth	Between bony & membranous labyrinth
Produced by	Stria vascularis	Blood / CSF filtration
Resembles	Intracellular fluid	CSF
Potential	Endocochlear +80 mV	~0 mV

Utricle vs Saccule

Feature	Utricle	Saccule
Size	Larger	Smaller
Position	Superior posterior	Anteroinferior
Detects	Horizontal linear acceleration	Vertical linear acceleration
Macula orientation	Horizontal	Vertical

Tensor Tympani vs Stapedius

Feature	Tensor Tympani	Stapedius
Nerve supply	Mandibular nerve (V3)	Facial nerve (VII)
Attachment	Upper handle of malleus	Neck of stapes
Function	Tenses TM; reduces excessive vibration	Protects from loud sound; improves speech discrimination
Paralysis causes	Hypermobile TM	Hyperacusis

Cochlear vs Vestibular Nerve

Feature	Cochlear Nerve	Vestibular Nerve
Function	Hearing	Balance
Ganglion	Spiral ganglion	Scarpa ganglion
Receptor	Organ of Corti	Maculae & cristae

SECTION 12 | High-Yield Exam Pearls

ANATOMY MUST-KNOW FACTS

Stapes = smallest bone in the body

Stapedius = smallest skeletal muscle

Tensor tympani supplied by V3 (mandibular nerve)

Stapedius supplied by CN VII (facial nerve)

Handle of malleus attached to TM; stapes footplate attached to oval window

Ossicles fully ossified at BIRTH

Otosclerosis most commonly affects the stapes footplate (at fissula ante fenestram)

Chorda tympani passes MEDIAL to handle of malleus

Labyrinthine segment of facial nerve = narrowest + poorest blood supply → site of Bell palsy

MacEwen triangle = surface landmark for mastoid antrum

Prussak space = most common origin of cholesteatoma

Mastoid antrum present at birth; air cells develop postnatally

ET length = 36 mm; cartilaginous part = medial 2/3; bony part = lateral 1/3

Tensor veli palatini = chief muscle opening the Eustachian tube

Children have horizontal ET → higher risk of otitis media

Anteroinferior quadrant of TM = safe site for myringotomy

Posterosuperior quadrant = dangerous area; cholesteatoma common

Cone of light seen in anteroinferior quadrant of normal TM

Notch of Rivinus = superior deficiency of tympanic annulus

Pars flaccida lacks fibrous layer → weak → cholesteatoma

PHYSIOLOGY MUST-KNOW FACTS

Labyrinthine artery usually from AICA; it is an END ARTERY — no collateral circulation

Endolymph: HIGH potassium (like intracellular fluid); produced by stria vascularis

Perilymph: HIGH sodium (like CSF)

Endocochlear potential = +80 mV

Base of cochlea → HIGH frequency; Apex → LOW frequency

Inner hair cells = primary hearing receptors (1 row)

Outer hair cells = cochlear amplification + OAE source (3 rows)

Utricle detects HORIZONTAL linear acceleration; Saccule detects VERTICAL

Crista ampullaris in SCC ampulla → detects ANGULAR (rotational) acceleration

Otoconia = calcium carbonate crystals; dislodged → BPPV

VOR: head moves → eyes move opposite → stabilizes gaze

Caloric test: COWS — Cold Opposite, Warm Same (fast phase of nystagmus)

Superior olivary complex = first site of BINAURAL interaction

Auditory cortex = superior temporal gyrus; Brodmann areas 41 and 42

Travelling wave theory (Bekesy) = most accepted theory of hearing

Acoustic reflex: loud sound → stapedius contraction (via facial nerve)

OAE: generated by outer hair cells; used for neonatal screening

BPPV treated with Epley manoeuvre (canalith repositioning)

EMBRYOLOGY MUST-KNOW FACTS

EAC from first branchial cleft (ectoderm); canalized by 7th month

Middle ear from first pharyngeal pouch (endoderm) — tubotympanic recess

Malleus + Incus from first branchial arch; Stapes suprastructure from second arch

Inner ear from otic placode (ectoderm) → otocyst → membranous labyrinth

Pinna from hillocks of His (6 hillocks around first branchial groove)

Hillock 1 = tragus (first arch); hillocks 2–6 = from second arch

Scheibe dysplasia = most common congenital inner ear malformation

Michel aplasia = complete absence of inner ear

Mondini dysplasia = cochlea with only 1.5 turns (normal = 2.5); risk of CSF gusher

ANATOMY OF EAR

Topic Overview

SECTION 1 | Introduction & Divisions of the Ear

1.1 Definition & Functions

1.2 Divisions of the Ear

1.3 Clinical Importance of Ear Anatomy

1.4 Flowchart — Divisions of the Ear

SECTION 2 | Embryology of the Ear

2.1 Development of the External Ear

Pinna — Hillocks of His

External Auditory Canal

2.2 Development of the Tympanic Membrane

2.3 Development of the Middle Ear

Ossicular Origins

Mastoid Development

2.4 Development of the Inner Ear

2.5 Germ Layer Derivation — Summary

2.6 Congenital Anomalies

SECTION 3 | External Ear

3.1 Auricle (Pinna) — Surface Anatomy

Cartilaginous Framework

3.2 Muscles, Blood Supply, Lymphatics & Nerve Supply of Pinna

Extrinsic Muscles

Blood Supply

Lymphatic Drainage

Nerve Supply

3.3 Applied Anatomy of Pinna

3.4 External Auditory Canal (EAC)

General Anatomy

Parts of EAC

Relations of EAC

Blood & Nerve Supply of EAC

3.5 Applied Anatomy of EAC

SECTION 4 | Tympanic Membrane

4.1 General Anatomy

4.2 Parts of the Tympanic Membrane

4.3 Layers of the Tympanic Membrane

4.4 Nerve Supply of Tympanic Membrane

4.5 Quadrants of Tympanic Membrane

4.6 Applied Anatomy of Tympanic Membrane

SECTION 5 | Middle Ear

5.1 Tympanic Cavity — General Anatomy

5.2 Six Walls of the Middle Ear

5.3 Important Spaces and Recesses

5.4 Contents of Middle Ear — Summary

5.5 Ossicles

Ossicular Muscles

Ossicular Joints

Transformer Mechanism of the Middle Ear (Impedance Matching)

5.6 Eustachian Tube

5.7 Mastoid Antrum and Air Cell System

Relations of Mastoid Antrum

Pneumatization Types

Surgical Landmarks

SECTION 6 | Facial Nerve in the Temporal Bone

6.1 Introduction

6.2 Intratemporal Course — Segment by Segment

6.3 Branches Inside the Temporal Bone

6.4 Blood Supply of Intratemporal Facial Nerve

6.5 Surgical Landmarks for Facial Nerve

6.6 Applied Anatomy

SECTION 7 | Inner Ear

7.1 Overview — Bony Labyrinth

7.2 Inner Ear Fluids

7.3 Cochlea — Detailed Anatomy

Scalae of the Cochlea

Membranes of the Cochlea

Organ of Corti

Tonotopic Organization of Cochlea

7.4 Vestibular Apparatus

Components and Functions

Crista Ampullaris and Cupula

Otoconia

Vestibular Nuclei and Connections

SECTION 8 | Blood Supply & Nerve Supply of the Inner Ear

8.1 Blood Supply

8.2 Nerve Supply — CN VIII Vestibulocochlear Nerve

SECTION 9 | Physiology of Hearing

9.1 Sound Transmission

9.2 Air Conduction vs Bone Conduction