General features of the ear

structure 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)

image

  • 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
Very quiet (whisper) 30 dB
Quiet (radio) 40 dB
Moderate (vacuum cleaner) 70 dB
Very noisy (alarm clock) 80 dB
Possible hearing loss 90 dB or more

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

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