Middle Ear

General features of the ear

3 parts of the ear:

• External ear
  • involved in hearing
  • Anatomy:
    • auricle: funnel shaped, elastic cartillage
      • 3 parts:
        • helix
        • concha
        • lobule
    • external auditory meatus
      • 2.5cm curved tube
      • Outer 1/3: cartilage
      • Inner 2/3: Bony (temporal bone)
    • Modified sweat glands –> secrete cerumen
      • prevents drying
      • protects from insects
    • Blood supply
      • posterior artery
      • auricular a.
      • superficial a.
      • temporal a.
    • Sensory nerve
      • Auriculo-temporal
        • branch of V3
      • Great auricular
        • C2, C3
      • Lesser occipital
        • C2
      • Auricular branch of 10th CN
        • Vagus
• Middle ear
  • involved in hearing
• Internal ear
  • involved in hearing & equilibrium

Hearing & Balance

Tympanic membrane (eardrum)

• Thin connective tissue membrane
  • vibrates in response to sound
  • Transfers sound energy to middle ear ossicles
• Boundary between outer & middle ear
• Anatomy:
  • pearly gray membrane
  • oblique
  • facing – towards floor
    • downward
    • forward
    • lateral
  • Layers
    • Outer: Skin
    • Middle: Fibrous tissue
      • circular & radial fibres maintain elastic tension
        • helps to multiply magnitude of sound wave
    • Inner: Mucous membrane
  • Lateral part
    • concave
    • depression: umbo
    • cone of light
      • part of tympanic membrane that can be seen
      • anterior inferior part
    • 2 parts
      • pars flaccida
      • pars tensa
• Nerve supply:
  • External surface
    • auriculo-temporal nerve
    • vagus nerve (vomiting)
  • Internal surface
    • glossopharyngeal nerve
• Very sensitive
  • if got perforation, be careful of damaging chorda tympani nerve
    • facial nerve
    • running along tympanic membrane & supply the tongue (taste)

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Middle ear (tympanic cavity)

• A small, air-filled mucosa-lined cavity
  • flanked laterally by the eardrum
  • flanked medially by the oval & round window
• Epitympanic recess
  • superior portion of the middle ear
• Pharyngptympanic (Eustachian) tube
  • connects the middle ear to the nasopharynx
  • equalizes pressure in the middle ear cavity with the external air pressure
  • in the floor (tympanic plate)
    • has internal jugular vein
    • if fracture, can cause severe bleeding
• Boundaries
  • Superior face
    • paper thin plate of bone (Tegmen tympani/tegmental wall)
      • separates the cavity from cranium and the meningeal coverings of the brain
      • has implications regarding pathways for infection
  • Inferior face
    • Tympanic plate of temporal bone
      • spearates the cavity from the jugular fossa
        • large groove that accomodates the internal jugular vein
  • Lateral wall
    • tympanic membrane
      • above it: epitympanic recess
  • Medial wall
    • cochlea/inner ear
    • important landmarks
      • oval window
        • a kidney shaped opening into the vestibule of the inner ear
        • occupied by the footplate of the stapes
        • are of tympanic membrane is 17x the oval window.
          • as the area increases, pressure increases
          • amplitude of sound wave increased at oval window
      • round window
        • a circular opening into the basal turn of the cochlea which is covered by a thin membrane
      • promontary
        • a rounded prominence caused by the 1st turn of the bony labyrinth of the inner ear
  • Anterior wall
    • carotid wall
      • internal carotid artery runs behind this wall
    • major auditory landmark
      • eustachian tube
        • above it: bony canal for tensor tympani
      • malleous, incus &
        stapes
        • transmit vibratory motion of the eardrum to the oval window
          • move as a unit
          • inertia is small, so once sound vibrations have ceased, the vibrations of the ossicles also terminate abruptly (bcos of the ligaments)
          • the 2 media crossed by sound: air & cochlear fluid
            • the more different the characteristics of the 2 media, the more sound energy will be reflected at the boundary
            • acoustic resistance of cochlear fluid MORE than air
        • dampened by the tensor tympani & stapedius muscle
  • Posterior wall
    • mastoid wall
    • landmarks
      • aditus to mastoid antrum
        • entrance to mastoid antrum –> mastoid air cells
      • pyramidal eminence contains stapedius muscle
      • canal for facial nerve
        • chorda tympani nerve

Sound transmission -factors

1) Difference between air and cochlear fluid

• transmission

2) Difference between area of tympanic membrane & oval window

• increased amplitude of sound at oval window

Acoustic reflex

• In response to loud sounds (80-90 dB above)
  • contraction of tympanic muscles as a reflex (smallest striated muscles in the body)
    • tensor tympani
    • stapedius
  • ossicles become more stiff, and less force is delivered to the cochlea

Eustachian Tube

• Orientation
  • downward
  • forward
  • medially
• Parts
  • Osseous portion
    • begins in the anterior wall of the middle ear cavity
  • Cartilaginous portion
    • most of the tube is cartilaginous
  • Isthmus
    • where the 2 portions meet
• Tensor palatini muscle
  • opens blocked tube
  • by yawning or swallowing

Cochlea

• spinal, conical, body chamber
  • extends from the anterior vestibule
• divided into 3 chambers
  • Scala vestibuli
    • filled with perilymph
  • Scala media
    • filled with endolymph
  • Scala tympani
    • terminates at the round window
    • filled with perilymph
• Similarities between Scala vestibuli & scala tympani
  • filled with perilymph
  • basilar membrane supports the organ of Corti
  • contains hearing receptor
  • cochlear branch of 8th nerve runs from the organ of Corti to the brain

Q: What is the sensory apparatus of the ear?
A: Organ of Corti (type of neuron: bipolar neuron)

Route of sound transmission

• Outer ear
  • pinna
  • auditory canal
  • eardrum
• Middle ear
  • malleous
  • incus
  • stapes
  • -> to oval window
• Inner ear
  • scala vestibuli & scala tympani
  • -> to cochlear duct
    • sound waves penetrate thru cochlear duct
  • vibrate the basilar membrane
    • stimulation of organ of corti
    • -> excite specific hair cells according to frequency of sound
  • Bending cilia (generate AP)
    • opens mechanically-gated ion channels
    • causes a graded potential
      • release of a neurotransmitter (probably glutamate)
  • neurotransmitter causes cochlear nerve to transmit impulses to the brain
    • where sound is perceived

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Structural abnormalities (conductive deafness)

• earwax (impacted)
• eardrum perforation/inflammation
• autosomal disorder (sclerosis of ossicles)

Sensory deafness

• cochlear nerve damage

Physical examination

• Weber test
  • test for lateralization
  • press the tuning fork on the top of patient’s head in the midline and ask if patient hear the sound
  • normally,
    • sound is head in the center of head/equally in both ears
  • Conductive hearing loss
    • vibration louder louder on the side of the conductive hearing loss
• Rinne test
  • compares air conduction to bone conduction
  • place the tuning fork (512 hz) on the mastoid eminence firmly & tell patient to say ‘now’ when they can no longer hear the vibration
  • remove the tuning fork and place it near the ear (without touching), compare the sound
  • Normally,
    • greater air conduction than bone conduction, therefore can hear the vibration longer with the fork in the air.
    • sound is equal on both sides
  • If air conduction more than bone conduction (usually normal)
    • sensorineuronal deafness/hearing loss
      • right sided deafness, left side louder
  • If bone conduction more than air conduction (cannot hear the vibration via air conduction)
    • conductive deafness/hearing loss

Referred pa
in to ear

• from the 5th, 7th, 9th, 10th cranial nerves
  • TMJ
  • Tonsils –> vagus nerve –> external ear
  • Throat
  • Eustachian tube
  • Teeth –> inf & sup alveolus (V2, V3)
  • Tongue

Otitis media

• From obstruction of Nasopharynx
  • adenoid hypertrophy
  • tumours
  • cleft palate
• Obstruction of auditory tube
  • due to infection/allergy
  • metaplasia of epithelium – chronic infection
• Complications
  • acute mastoiditis
  • facial nerve paralysis
  • acute labyrinthitis
  • sigmoid sinus thrombophlebitis
  • CNS infection

Otosclerosis

• A primary bone dyscrasia
  • abnormal bone growth near the middle ear
• Involvement of temporal bones
  • autosomal dominant
• Involvement of oval window
  • footplate fixation & persistent conductive hearing loss
• Involvement of cochlear endosteum
  • produce sensorineuronal hearing loss
    • thickened bone causes vibration of perilymph to become less