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


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
The auricle. There are numerous distinct parts of the auricle.
  • External acoustic meatus
    • Extends from the concha to the tympanic membrane
    • Two parts:
      • Cartilagenous part
      • Bony part medially
The external acoustic meatus. Note the two parts.
  • 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

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 muscles acting on the ossicles
  • 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

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

      Simplified view of the inner ear canal
  • 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

      A more detailed vie w of the inner ear (and middle ear)
  • 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