The Ear

Summary

• There are 3 cavities of the ear that we will consider in turn:
  1. External ear
  2. Middle ear
  3. Internal ear

• For detail on the vestibulocochlear nerve (CNVIII) and the transmission of sound and balance sensation, please see Vestibulocochlear Nerve (CNVIII)

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External ear

• 2 key parts to the external ear:
  • Auricle (aka pinna) which has two further parts:
    • Tragus
    • Conchal bowl
  • External acoustic meatus
  • (Tympanic membrane)

• Auricle
  • Cartilaginous structure
  • Elevations and depressions to capture and localise sound
  • Arterial supply from posterior and anterior auricular arteries as well as occipital artery branches

• External acoustic meatus
  • Extends from the concha to the tympanic membrane
  • Two parts:
    • Cartilagenous part
    • Bony part medially

• Tympanic membrane (the eardrum)
  • Separates EAM from middle ear
    • Attached to handle of malleus at the umbo
  • The TM can help to lateralise which ear is being observed on otoscopy
    • We can tell the below image is the right ear when viewed on otoscopy because:
      • Cone of light is anterior/inferior (i.e. on the right/same side)
      • Lateral process is anterior to malleus (same side)
      • Posterior malleolar fold is posterior to the malleus (opposite – on the left)
  • The membrane has two parts divided by the malleolar folds/suspensory ligaments of the malleus:
    • Pars Tensa – made up of epithelial, fibrous, and mucosal folds
    • Pars Flaccida – lacks fibrous tissueMost likely to be teared

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Middle Ear

• The main function of the middle ear is to communicate and amplify vibrations of the tympanic membrane
  • Otherwise the sound is insufficient to stimulate the cochlea
  • Accomplished through the ossicles

    

• 3 ossicles (hammer hits anvil to make the stirrup):
  1. Malleus – connects to tympanic membrane
  2. Incus – connects malleus to stapes
  3. Stapes – connects to internal ear via oval window which leads into vestibule

     

• Each transfer of vibration increases sound

• Ossicles are stabilised by 2 muscles:
  • Tensor tympani – Tenses tympanic membrane to reduce oscillation
  • Stapedius – Reduces oscillation of stapes 

    

• The middle ear communicates with nasopharynx via pharyngotympanic tube (aka Eustachian Tube)
  • Equalises pressure in middle ear
  • Normally closed – only opens with ↑Extrauricular pressure
  • Made up of bony and cartilaginous parts

    

• The middle ear requires air to effectively function
  • Therefore fluid – e.g. in infection – greatly impairs this

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Inner ear

• 2 parts
  1. Outer bony labyrinth (aka the otic capsule)
     • Contains semicircular canals and otolithic organs (saccule/utricle)
  2. Within this is the membranous labyrinth
     • Contains the Cochlea

• Supplied by labrynthine artery
  • Enters via Internal acoustic meatus (along with CNVII and CNVIII)
  • Blockage can lead to loss of balance

• 3 parts to the bony labyrinth:
  1. Vestibule
  2. Semicircular canals
  3. Cochlea

     

• Within the bony labyrinth is the membranous labyrinth
  • The two are separated by perilymph

• Hearing process:
  • Stapes vibrations enter vestibule of inner ear via oval window
  • This causes perilymph movement within cochlea
  • Movement of inner hair cells within the cochlea stimulates receptors of Corti

    

• Semicircular canals are also present in bony labryinth
  • There are 3 of these:
    • Horizontal – detects rotation around the vertical axis
    • Superior – detects rotation around the horizontal/transverse axis
    • Posterior – detects rotation around the AP axis
  • These detect rotation about a given axis
  • Project posterosuperiorly from vestibule
  • Contain endolymph which is responsible for balance  (vs perilymph for sound)

• Detection of movement:
  • Semicircular canals detect rotational movement
  • Utricle detects left, right, forward, and back
  • Saccule detects up and down
  • Each of these structures sends signals via the vestibulocochlear nerve