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ENT-1: ANATOMY OF EAR — Complete Exam Answers

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QUESTION 1: Describe the Parts of the External Ear and Its Nerve Supply

Parts of the External Ear

1. The Auricle (Pinna)
2. The External Auditory Canal (EAC)
3. The outer epithelial layer of the Tympanic Membrane

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A. The Auricle (Pinna)

• Anterior ligament: External auditory canal to spine of the helix
• Superior ligament: External auditory canal to spine of the helix
• Posterior ligament: Mastoid to the concha

• Anterior auricular muscle
• Superior auricular muscle
• Posterior auricular muscle (These are extrinsic muscles, which may be well developed in some individuals)

• Grows to adult size by 8 to 10 years of age; approximately 60 mm in length
• The superior margin of the pinna lies in line with the eyebrow
• The lower limit of the lobule is in line with the base of the nasal septal columella
• It is inclined approximately 15 to 20 degrees posteriorly and protrudes 15 to 20 mm from the scalp

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B. External Auditory Canal (EAC)

• Length: 3 to 4 cm
• The outer one-third is cartilaginous (fibrocartilage); the medial two-thirds is bony/osseous
• The skin over the cartilaginous canal contains hair follicles, sebaceous glands, and ceruminous glands
• The skin over the bony canal is tightly adherent (no subcutaneous tissue), with only periosteum beneath

Note: The eustachian tube is the reverse — two-thirds cartilaginous and one-third bony.

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C. Nerve Supply of the External Ear

Clinical Note — Herpes Zoster Oticus (Ramsay Hunt Syndrome): Vesicular eruption in the EAC and pinna because the geniculate ganglion of the facial nerve is involved, and Arnold's nerve carries sensory fibres.

Clinical Note — Arnold's Reflex / Ear-Cough Reflex: Stimulation of the posterior EAC (Arnold's nerve / vagal branch) causes reflex coughing. This is why syringing the ear can cause coughing.

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• Greater auricular nerve (C2, C3)
• Auriculotemporal nerve (V3)
• Lesser occipital (C2)
• Vagus - Arnold's nerve (X)
• Facial nerve (VII)
• Glossopharyngeal (IX)

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QUESTION 2: Contents and Boundaries of the Middle Ear

Definition

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Boundaries of the Middle Ear (Six Walls)

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Contents of the Middle Ear

• Malleus: Head, Neck, Manubrium (handle), Anterior process, Lateral (short) process
• Incus: Body, Short process, Long process (ending in Lenticular process)
• Stapes: Anterior crus, Posterior crus, Footplate (average 1.41 mm x 2.99 mm)

• Malleoincudal joint: Diarthrodial joint (synovial)
• Incudostapedial joint: Diarthrodial joint

• Superior malleal ligament (head to roof of epitympanum)
• Anterior malleal ligament (neck to sphenoid through petrotympanic fissure)
• Lateral malleal ligament (neck to tympanic notch)
• Superior incudal ligament (body to tegmen)
• Posterior incudal ligament (short process to floor of incudal fossa)
• Annular ligament of stapes (footplate to margin of oval window)

• Tensor tympani (in canal above the eustachian tube, supplied by V3)
• Stapedius (in pyramidal eminence, supplied by VII)

• Chorda tympani (branch of VII)
• Tympanic plexus (branch of IX — Jacobson's nerve, with contribution from internal carotid plexus)

• Anterior tympanic artery (branch of maxillary artery)
• Stylomastoid artery (branch of posterior auricular artery)
• Petrosal branch of middle meningeal artery

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Divisions/Recesses of the Tympanic Cavity

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• Lateral (membranous)
• Medial (labyrinthine)
• Anterior (carotid)
• Posterior (mastoid)
• Roof (tegmental)
• Floor (jugular)

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QUESTION 3: Middle Ear Cleft — Definition and Anatomy of the Medial Wall

Middle Ear Cleft

1. Eustachian tube (Pharyngotympanic tube)
2. Tympanic cavity (Middle ear proper)
3. Aditus ad antrum (Connecting passage)
4. Mastoid antrum (Central air cell)
5. Mastoid air cells (Pneumatized spaces in the mastoid bone)

Importance: Because all these spaces are lined by continuous mucosa, infection can spread freely throughout the entire middle ear cleft — from a simple otitis media to mastoiditis, and potentially to the meninges or sigmoid sinus.

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Anatomy of the Medial Wall (Labyrinthine Wall) of the Middle Ear

• A rounded bulge on the medial wall formed by the basal coil (first turn) of the cochlea
• Covered by the tympanic plexus of nerves (Jacobson's plexus)
• The tympanic plexus is formed by:
  • Tympanic branch of IX (Jacobson's nerve) — main contributor
  • Caroticotympanic nerves (from internal carotid sympathetic plexus)
• From this plexus, the lesser petrosal nerve is given off (carrying parasympathetic fibres to the parotid gland via the otic ganglion)

• Located posterosuperior to the promontory
• Closed by the footplate of the stapes held by the annular ligament
• Transmits sound vibrations to the perilymph of the vestibule

• Located posteroinferior to the promontory
• Covered by the secondary tympanic membrane (round window membrane)
• Acts as a pressure-relief window for the cochlear fluid
• It faces posterolaterally (important surgically as it can be missed during examination)

• A ridge of bone posterior and superior to the oval window
• Contains the tympanic (horizontal) segment of the facial nerve (VII)
• The facial nerve runs in the fallopian canal and may be dehiscent (lacking bony covering) in 20 to 30% of individuals — this is of great surgical importance

• A broader ridge of bone located just above and posterior to the facial canal prominence
• Produced by the lateral (horizontal) semicircular canal

• A spoon-shaped bony projection at the anterior end of the medial wall
• The tendon of the tensor tympani bends around it at a right angle (like a pulley) to reach the manubrium of the malleus

• A recess in the medial wall, posterior and inferior to the oval window, between the facial nerve canal and the round window
• It is a hidden recess, difficult to visualize — important in cholesteatoma surgery

• Ponticulus: Ridge between the oval window and the sinus tympani
• Subiculum: Ridge between the sinus tympani and the round window

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Diagram Description: Medial Wall of Middle Ear

       ROOF (Tegmen Tympani)
          |
          |--- Prominence of Lateral Semicircular Canal
          |
ANTERIOR  |--- Processus Cochleariformis    POSTERIOR
          |--- Tensor Tympani Canal              |--- Aditus ad Antrum
          |                                      |
          |--- Promontory (Basal coil of cochlea)|--- Pyramid (Stapedius)
          |     [Tympanic Plexus on it]           |
          |                                      |--- Facial Nerve Canal
          |--- OVAL WINDOW (Fenestra Vestibuli)      (above, horizontal)
          |     [Stapes footplate closes it]
          |--- Ponticulus                        
          |--- SINUS TYMPANI (hidden recess)    
          |--- Subiculum                         
          |--- ROUND WINDOW (Fenestra Cochleae)  
          |     [Secondary tympanic membrane]    
          |
       FLOOR (Jugular Wall)

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• Promontory
• Oval window
• Round window
• Facial canal (prominence)
• Fenestra (both windows)
• Lateral semicircular canal prominence
• Sinus tympani

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QUESTION 4: Intratympanic Muscles and Their Functions

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1. Tensor Tympani

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2. Stapedius

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Comparative Table

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Clinical Importance

1. Acoustic (Stapedial) Reflex Testing: Used to assess the integrity of the facial nerve and the auditory pathway. Absent in facial nerve palsy — the Acoustic Reflex Decay test helps diagnose retrocochlear pathology.
2. Hyperacusis in Facial Nerve Palsy: When the facial nerve is damaged proximal to the nerve to stapedius, the stapedius is paralyzed. Without its damping effect, loud sounds are perceived as abnormally loud — this is called hyperacusis.
3. Tensor Tympani Syndrome: Abnormal contractions of the tensor tympani can cause clicking sounds, a sensation of fullness, and discomfort in the ear.

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QUESTION 5: Anatomy of the Middle Ear (With Diagram)

Definition

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The Tympanic Membrane

• Size: approximately 9 to 10 mm in diameter
• Positioned at an angle of about 55 degrees to the floor of the EAC (oblique position)
• It has two parts:
  • Pars tensa (larger, taut part): three layers — outer squamous epithelium, middle fibrous layer (radial and circular fibres), inner mucosal layer
  • Pars flaccida (Shrapnell's membrane) (small, lax part): located superiorly, above the malleolar folds; has no middle fibrous layer — hence lax and flaccid. Located in the Notch of Rivinus
• Umbo = the most depressed point of the tympanic membrane corresponding to the tip of the manubrium of the malleus
• Cone of light (Light reflex): Triangular reflection of light seen anteroinferiorly at 5 o'clock in the right ear and 7 o'clock in the left ear

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Ossicular Chain Function

1. Area ratio: The surface area of the tympanic membrane (~55 sq mm) to stapes footplate (~3.2 sq mm) gives a ratio of approximately 18:1 — this multiplies the force
2. Lever action: The manubrium of the malleus is longer than the long process of the incus — lever ratio of approximately 1.3:1
3. Net gain: Combined mechanical advantage of approximately 22 to 25 dB — sufficient to overcome the 30 dB loss that would occur without the middle ear

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Diagram: Middle Ear Cross Section

              TEGMEN TYMPANI (Roof)
         ________________________________
        |  Epitympanum (Attic)           |
        |  Head of Malleus              |
        |  Body of Incus               |
EAC --> | Mesotympanum                  | --> Aditus ad Antrum
(EAC)   |  TM | Malleus | Incus        |                    --> Mastoid Antrum
        |     |         | Stapes       |
        |     |         |  [Footplate] --> OVAL WINDOW --> Inner Ear
        |     |                        |
        |     |         Promontory     |
        |     |         ROUND WINDOW   |
        |                              |
        |   Hypotympanum               |
        |______________________________|
              FLOOR (Jugular Wall)

        Anterior Wall: ET opening + Tensor Tympani canal
        Posterior Wall: Aditus + Pyramid (Stapedius) + Facial nerve

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QUESTION 6: Medial Wall of Middle Ear — With Diagram and Surgical Importance

Surgical Importance of the Medial Wall

• The tympanic (horizontal) segment of the facial nerve lies in the bony canal on the medial wall, just above the oval window
• In 20 to 30% of individuals, the bony covering of the fallopian canal is absent (dehiscent)
• During middle ear surgery (tympanoplasty, stapedectomy, mastoidectomy), the exposed facial nerve can be inadvertently damaged, causing facial palsy
• The surgeon must identify the facial nerve before proceeding with surgery on the oval window region

• The stapes footplate sits in the oval window on the medial wall
• In otosclerosis, the stapes becomes fixed (ankylosis of footplate) and surgery involves drilling the footplate or making a small opening (stapedotomy)
• The facial nerve canal is just above — risk of injury during drilling
• The saccule of the inner ear lies just medial to the oval window — risk of perforation

• The round window is used as the site of entry for cochlear implant electrode insertion in some approaches
• It acts as a pressure-relief valve; if it ruptures (perilymph fistula), it causes sensorineural hearing loss and vertigo

• The tympanic neurectomy (cutting Jacobson's nerve/tympanic plexus) was used as a treatment for Meniere's disease to reduce endolymphatic secretion
• The lesser petrosal nerve from the tympanic plexus runs towards the otic ganglion and can be used as an anatomical landmark

• This recess posterior to the oval window is a common hiding place for cholesteatoma
• It cannot be visualized directly by looking through the EAC — requires endoscopic or microscopic dissection
• Residual cholesteatoma is the most common cause of recurrence after surgery if this area is not cleared

• The prominence of the lateral semicircular canal on the medial wall is a key landmark in mastoid surgery
• The ampulla of the lateral semicircular canal (the most medial landmark) is the entry point into the inner ear — if breached, it causes vertigo and profound sensorineural hearing loss

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QUESTION 7: Medial and Posterior Wall of Middle Ear — Facial Nerve Relations

Posterior Wall of Middle Ear

• A large opening in the upper part of the posterior wall connecting the epitympanum (attic) to the mastoid antrum
• The body of the incus and the short process of incus project into this opening
• It is the pathway through which middle ear infection spreads to the mastoid antrum

• A hollow, conical bony projection on the posterior wall, just medial to the entry point of the chorda tympani
• Houses the stapedius muscle, whose tendon emerges from the apex to attach to the neck of the stapes
• The facial nerve (VII) passes just behind and lateral to the pyramid in its vertical (mastoid) segment

• A small recess in the posterior wall that holds the short process of the incus
• Located lateral and superior to the aditus

• Enters the middle ear through the posterior wall via the iter chordae posterius (posterior canaliculus)
• Crosses the middle ear by running between the malleus (lateral) and incus (medial)
• Exits anteriorly through the iter chordae anterius (Huguer's canal)

• The facial recess is a triangular space on the posterior wall, bounded:
  • Medially: by the facial nerve
  • Laterally: by the chorda tympani
  • Superiorly: by the fossa incudis
• This is the surgical approach used in cochlear implantation (posterior tympanotomy approach to insert the electrode through the round window)

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Relations of the Facial Nerve on the Medial Wall

• The facial nerve runs horizontally from the geniculate ganglion (anterior) to the second genu (posterior)
• It runs in the fallopian canal on the medial wall, just above the oval window and below the prominence of the lateral semicircular canal
• The nerve lies between the oval window below and the lateral semicircular canal above
• Dehiscence of the bony canal here means the nerve is exposed directly in the middle ear — risk during stapedectomy

• Located just below the lateral semicircular canal and posterior to the oval window
• The nerve bends downward (inferiorly) here to enter the vertical/mastoid segment

• Descends vertically in the posterior wall of the middle ear and mastoid
• Gives off the nerve to stapedius just after the second genu — this enters the pyramidal eminence through a tiny canaliculus
• Gives off the chorda tympani nerve just before it exits the stylomastoid foramen
• Exits the skull at the stylomastoid foramen

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QUESTION 8: Anatomy and Course of the Facial Nerve (VII)

Introduction

1. Motor fibres (to muscles of facial expression)
2. Sensory fibres (taste from anterior 2/3 of tongue via chorda tympani)
3. Parasympathetic secretomotor fibres (to lacrimal, submandibular, and sublingual glands via greater petrosal nerve and chorda tympani)

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Course of the Facial Nerve — Seven Segments

• Arises from the lower border of the pons at the pontomedullary junction (between pons and olive)
• The main motor root and the nervus intermedius (sensory + parasympathetic root) emerge separately at first, then unite
• Traverses the cerebellopontine angle (CPA) accompanied by the 8th cranial nerve (vestibulocochlear)
• Enters the internal acoustic meatus (IAM) at the porus acusticus

• Runs laterally in the internal auditory canal (IAC)
• Position in the IAC: anterosuperior quadrant (above the cochlear nerve; the superior and inferior vestibular nerves are posterior)
• The horizontal crest (crista falciformis) divides the IAC horizontally; the vertical crest (Bill's bar) divides the superior compartment into anterior (VII) and posterior (superior vestibular nerve)

• Passes through the narrowest portion of the fallopian canal
• Lies between the cochlea (anteriorly) and the vestibule (posteriorly)
• Most vulnerable segment — highest risk of injury due to:
  • Narrowest diameter
  • Poorest blood supply
• Ends at the geniculate ganglion (first genu — first bend)

• Greater superficial petrosal nerve (GSPN): Carries preganglionic parasympathetic fibres to the lacrimal gland (via pterygopalatine ganglion) and mucous glands of the nose and palate; also carries taste from the soft palate. Runs in a groove on the floor of the middle cranial fossa.

• Turns posteriorly at the first genu (geniculate ganglion) and runs horizontally
• Lies in the medial wall of the middle ear, above the oval window and below the lateral semicircular canal
• Length: approximately 8 to 11 mm
• Dehiscence of the bony canal is present in 20 to 30% of individuals (most commonly just above the oval window)

• Located posterior to the oval window, just below the lateral semicircular canal
• The nerve bends approximately 90 to 125 degrees and turns inferiorly

• Descends vertically in the posterior wall of the middle ear and mastoid bone
• Length: approximately 13 mm
• Branches in this segment:
  • Nerve to Stapedius — enters the pyramidal eminence (clinical importance: paralysis causes hyperacusis)
  • Chorda Tympani — enters the middle ear posteriorly, crosses the middle ear between the malleus and incus, carries taste from the anterior two-thirds of the tongue and parasympathetic fibres to the submandibular and sublingual glands
• Exits the skull at the stylomastoid foramen (between the styloid and mastoid processes)

• After exiting the stylomastoid foramen, the nerve gives off:
  • Posterior auricular nerve (supplies occipital belly of occipitofrontalis and the auricular muscles)
  • Muscular branch (to posterior belly of digastric and stylohyoid)
• The main trunk enters the parotid gland and divides into:
  • Temporofacial division (upper)
  • Cervicofacial division (lower)
• Further divides into five terminal branches:

• Temporal
• Zygomatic
• Buccal
• Marginal mandibular
• Cervical

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Summary of Branches of Facial Nerve

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QUESTION 9: Anatomy of the External Auditory Canal

Definition

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Dimensions

• Length: 3 to 4 cm (approximately 24 mm in the anterior wall, slightly shorter posteriorly)
• Direction: Not straight — it curves in an S-shape, which is why the pinna must be pulled upward, backward, and outward in adults (or downward and backward in infants) to straighten the canal for otoscopic examination

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Structure

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Boundaries of the EAC

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Isthmus

• The narrowest part of the EAC is the isthmus, located at the junction of the cartilaginous and bony portions
• Foreign bodies, wax, and debris tend to accumulate at the isthmus

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Special Features

• Two or three vertical fissures in the anterior cartilaginous wall of the EAC
• These are gaps in the cartilage filled with fibrous tissue
• Allow spread of infection between the EAC and the parotid gland (parotid abscess can present with EAC swelling and vice versa)

• A bony prominence at the posterosuperior margin of the external auditory meatus (bony part)
• Used as a surgical landmark

• An imperfectly ossified portion on the anterior tympanic plate (floor of EAC)
• Present in some individuals, provides another potential route for spread of infection between the EAC and the parotid/TMJ

• The epithelium of the EAC migrates from the centre of the tympanic membrane outward toward the meatus (centrifugal migration)
• This carries desquamated epithelium and cerumen outward — this is the normal self-cleaning process
• Disruption of this migratory pattern is one proposed mechanism for cholesteatoma formation

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Nerve Supply of EAC

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Blood Supply

• Anterior: Superficial temporal artery (branch of external carotid)
• Posterior: Posterior auricular artery
• Deep auricular artery (from maxillary artery) — supplies medial EAC and outer surface of tympanic membrane

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Lymphatic Drainage

• Anterior: Preauricular (parotid) nodes
• Posterior: Postauricular (mastoid) nodes
• Inferior: Upper deep cervical nodes

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• Otitis externa: Infection of the EAC skin; the Pseudomonas aeruginosa is the most common pathogen. The tight, adherent skin in the bony canal causes severe pain when swollen.
• Malignant otitis externa: Osteomyelitis of the temporal bone in diabetic/immunocompromised patients; Pseudomonas aeruginosa; can involve facial nerve and other cranial nerves.
• Exostoses: Multiple bony outgrowths from the bony canal walls (from cold water exposure — "surfer's ear"); can occlude the canal.
• Osteomas: Single pedunculated bony tumours in the EAC.

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QUESTION 10: Boundaries of MacEwen's Triangle and Its Importance

MacEwen's Triangle (Suprameatal Triangle)

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Boundaries

• Anterosuperiorly: A line drawn as a tangent to the posterior wall of the EAC
• Posteriorly: A vertical line from the supramastoid crest
• Superiorly: The supramastoid crest / temporal line

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Surgical Importance

• The mastoid antrum lies directly deep to (medial to) MacEwen's triangle
• It is approximately 1.25 to 1.5 cm deep to the surface of the mastoid cortex in an adult (much shallower in children)
• In a cortical mastoidectomy, the initial drilling is done over this triangle to open the antrum

• MacEwen's triangle is the standard entry point for cortical (simple) mastoidectomy
• By drilling through this area, the surgeon enters the mastoid antrum and can then drain pus in acute mastoiditis or remove disease in chronic otitis media

• The spine of Henle at the anterior boundary helps identify the posterosuperior wall of the EAC and keeps the surgeon oriented
• The tegmen mastoideum (thin bony plate over the antrum) is the roof of the antrum and the floor of the middle cranial fossa — if the surgeon drills too high, they enter the middle cranial fossa

• The sigmoid sinus is posterior to the antrum (risk of injury if drilling goes too posteriorly)
• The lateral semicircular canal is medial to the antrum (risk of inner ear damage if drilling goes too deep)
• The facial nerve (second genu and vertical segment) lies medial and inferior to the antrum

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• Spine of Henle (anterior landmark)
• Horizontal — supramastoid crest (superior boundary)
• Posterior wall of EAC (anterior boundary)
• Mastoid antrum lies deep to it

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QUESTION 11: Anatomy of the Cochlea (With Diagram)

Introduction

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Gross Anatomy

• The cochlea is a 2.5-turn coiled structure resembling a snail's shell
• It is located in the anterior compartment of the bony labyrinth of the petrous temporal bone
• The base of the cochlea faces the internal auditory canal (IAC); the apex (cupula) points anterolaterally
• Total length when uncoiled: approximately 35 mm
• It coils around a central bony pillar called the modiolus

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Bony Structure

• The central bony pillar/axis around which the cochlear spirals wind
• Contains the spiral ganglion cells (bipolar neurons — the first-order neurons of the auditory pathway)
• The central canal of the modiolus transmits the cochlear nerve (VIII) to the base of the cochlea

• A shelf of bone projecting outward from the modiolus into the cochlear duct
• It partially divides the cochlear canal into two scalae (compartments)
• The basilar membrane stretches from the free edge of the spiral lamina to the spiral ligament, completing the partition

• A small opening at the apex of the cochlea connecting the scala vestibuli and the scala tympani
• Allows low-frequency pressure equalization between the two scalae

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Internal Structure — Three Scalae (Compartments)

Note: Scala vestibuli and scala tympani communicate at the apex via the helicotrema.

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Membranes

• Separates the scala vestibuli from the scala media
• Thin, two-cell-layered membrane
• Important for maintaining endolymph composition

• Separates the scala media from the scala tympani
• The Organ of Corti sits on the basilar membrane
• Tonotopic organization: The basilar membrane is wider and more compliant at the apex (responds to low frequencies) and narrower and stiffer at the base (responds to high frequencies)
  • Base: High frequencies (20,000 Hz)
  • Apex: Low frequencies (20 Hz)

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Organ of Corti

• Inner Hair Cells (IHC): Single row; approximately 3,500 cells; the main sensory transducers — responsible for the actual electrical signal to the auditory nerve (90% of auditory nerve fibres)
• Outer Hair Cells (OHC): Three rows; approximately 12,000 cells; act as amplifiers and fine-tuners; assessed by otoacoustic emissions (OAE); damaged first by noise and ototoxic drugs
• Tectorial Membrane: Gelatinous membrane overlying the hair cells; cilia of the OHCs are embedded in it; deflection of cilia causes depolarization
• Tunnel of Corti: Between the inner and outer pillar cells
• Supporting cells: Pillar cells, Hensen's cells, Deiters' cells, Claudius cells

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Blood Supply

• Cochlear branch of the labyrinthine (internal auditory) artery, a branch of the anterior inferior cerebellar artery (AICA)
• There is no collateral blood supply — hence the cochlea is extremely vulnerable to ischemia

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Cochlear Travelling Wave (Bekesy's Theory)

• Sound vibrations enter the scala vestibuli at the oval window
• A travelling wave propagates along the basilar membrane from base to apex
• The wave reaches maximum amplitude at a specific location determined by frequency (tonotopicity)
• High frequencies: maximum amplitude at the base
• Low frequencies: maximum amplitude at the apex

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Diagram of the Cochlea

                    APEX (Cupula) — Low Frequencies
                    /
     Helicotrema ←/
                 /
Scala     ______/___________________
Vestibuli |  Perilymph              | ← Oval Window (base)
          |_________________________|
          |  Reissner's Membrane    |
Scala     |__________________________|
Media     |  ENDOLYMPH              |
(Cochlear |  [ORGAN OF CORTI sits   |
Duct)     |   on Basilar Membrane]  |
          |__________________________|
          |  Basilar Membrane        |
Scala     |__________________________|
Tympani   |  Perilymph              | → Round Window (base)
          |_________________________|
                    \
                     \__ BASE — High Frequencies
                              (Cochlear nerve enters here
                               via modiolus)

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Clinical Importance of Cochlea

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• Scala Vestibuli (top) → Perilymph
• Scala Media (middle) → Endolymph (+ Organ of Corti)
• Scala Tympani (bottom) → Perilymph

• Reissner = Roof of scala media (separates it from scala vestibuli)
• Basilar = Base/floor of scala media (separates it from scala tympani)

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*Sources: K.J. Lee's Essential Otolaryngology (10th ed.), Scott-Brown's Otorhinolaryngology Head and Neck Surgery, Gray's Anatomy for Students.*All 11 questions have been answered in full. Here is a quick summary guide to help you navigate and revise:

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Quick Revision Summary