Lecture 16 – Anatomy of Hearing.docx

Lecture 16 – Anatomy of Hearing

19.03.24

• 1. The external ear
  • Everyday use of the word “ear” means technically the external ear
  • External ear consists of
    • auricle (pinna)
    • Meatus (ear canal)
    • Tympanic membrane (ear drum)
  • Task of the auricle: localize the sound source (important: evolution)
  • Tympanic membrane works like a loudspeaker or microphone membrane
• External auditory meatus
  • External ear canal
  • 7 mm in diameter and 2.5 cm long
  • This generates resonane frequencies at 3400 Hz
  • From an acoustic point of view: ear canal is a filter than amplifies frequencies between 2 kHz and 5 kHz
  • Terminates at the tympanic membrane
    • Two-third of ear canal housed in bone (osseus portion)
    • One-third of ear canal composed of cartilageous parts
  • Resonating cavity that contributes to hearing
    • Determine resonant frequency
  • Outer third-line with hair cells and cerum (ear wax) – protects by trapping dirt and insects
• Tympanic membrane
  • Also known as eardrum
  • Separates the middle ear from the outer ear
  • Oval shaped, 10 mm in diameter
  • Thin three-layered sheets of tissue
  • Landmakrs
    • Umbo – point of attachment for maleus, middle ear bone – location is cone of light (reflects light from otoscope)
  • Responsible for initiating mechanical impedance-matching process of middle ear
    • First layer: outer (cuticular) layer
    • Second layer: intermittent (fibrous) layer
    • Third layer: inner (mucous) layer
• 2. The middle ear
  • The middle ear consists of the tympanic cavity
    • This cavity contains the smallest moving bones of the human body – the ossicles
      • Malleus (hammer): touches the tympanic membrane and transmits to
      • The incus (anvil) which transmits to the
      • Stapes (stirrup) which transmits to the internal ear (oval window)
    • Malleus and stapes are attached to muscles (may attenuate to transmission of sound by these bones
  • The stapes connects directly to the internal ear through the oval window 🡪 transmission of stapes movement to the lymphatic fluid inside the internal ear
• Ossicles
  • Malleus
    • Largest of the ossicles (9 mm long and weighs only 25 mg)
    • Provides point of attachment with tympanic membrane
    • Bulk of bone is the head or caput
  • Incus
    • Shaped like an anvil
    • Weight 30 mg and is around 7 mm long
    • Provides intermediate link of ossicular chain
    • Incus and malleus articulate by means of a saddle joint
  • Stapes (stirrup)
    • Third bone of ossicular chain
    • Weights 4 mg with an area of 3.5 mm^2
    • Helps to transmit sound vibrations from eardrum to oval window
    • Articulation of the incus and stapes of ball and socket type
  • Ossicular chain is held in place by ligaments
• Tympanic muscles
  • Muscles of middle ear attached to ossicles
  • Smallest muscles of human body
    • Stapedius muscle
      • Imbedded in posterior wall of middle ear
      • Pulls stapes posteriorly
    • Tensor tympani
      • Pulls malleus anterior and medial
• Sound attenuation in the middle ear
  • The middle ear performs a kind of “volume control” 🡪 muscles of malleus can be tensed, resulting in a low frequency damping
  • It must be activated though by neural impulses 🡪 in order to be activated as noise control, the noise has to be processed by the internal ear 🡪 damage could have been occurred already
  • Additionally, these muscles are activated just before a person starts to speak 🡪 damping mechanism to protect against own voice
• Pressure increase in the middle ear
  • Sound waves are mechanically transmitted by ossicles of the middle ear to the internal ear, which is filled with watery liquid
  • Ossicles perform conversion of pressure changes from an elastic medium (air) to pressure changes of an incompressible liquid (water)
  • Ossicles function like a cone: from large surface (tympanic membrane) to a smaller surface (stapes)
    • This leads to a pressure increase 🡪 pressure variations at the interal ear are about 20 times stronger than original air pressure variation
    • This pressure increase is necessary to generate the necessary activation of the liquid (otherwise reflection would occur)
• Pressure equalization in the tympanic cavity
  • The middle ear is not completely airtight, a connection with the eustachian tube allows for pressure equalization (e.g. meteorological pressure changes)
    • The Eustachian tube leads from the middle ear to the nasopharynx
    • Without pressure equalization, the meteorological changes would “push” the ear membrane inwards 🡪 feeling of “pressure on the ear” (felt e.g. when going downhill, or in an airplane, or fast elevator)
• 3. The internal ear
  • Cochlea: part of the inner ear relevant for hearing 🡪 sound waves are transformed into neural impulses
    • Is shaped like a snail shell
    • Contains two passages, separated by basilar membrane
      • Upper: scala vestibuli
      • Lower: scala tympani
      • These two passages meet at the apex (the tip) in the helicotrema
    • Scala vestibuli connects to middle ear (stapes) through the oval window
• The “uncoiled” cochlea
  • 
• Physiology of hearing
  • The pressure waves from the middle ear (stapes) reach the cochlea through the oval window 🡪 longitudinal pressure waves are generated in internal ear fluid through scala vestibuli to the apex
  • These pressure waves return via scala tympani to the round window
  • Round window serves as pressure release, since the fluids are incompressible