The Middle Ear: Structure and Function

Middle Ear

• Located behind the eardrum
• Comprised of three bones: hammer, anvil, and stirrup
• Transmits vibrations from the tympanic membrane to the oval window of the inner ear
• Functions as a lever system, amplifying pressure from the eardrum to the oval window by a factor of 22
• Filters out noise generated in the body
• Protects the ear from high amplitude vibrations by switching to a less-efficient mode of vibration at high sound levels

Inner Ear

• Transforms sound wave energy into nerve impulses for the brain
• Components:
  • Cochlea: a snail-shaped structure generating fluid waves from the stapes' movement
  • Reissner Membrane: separates the scala vestibuli from the scala media
  • Basilar Membrane: separates the scala media from the scala tympani and contains sensory cells
• Basilar Membrane functions:
  • Contains outer and inner hair cells responding to different frequencies
  • Hair cells are covered by the tectorial membrane
  • Vibrations cause hair cells to shear against the tectorial membrane, triggering chemical and electrical changes
• Sound Transmission:
  • Sound energy causes basilar membrane vibrations
  • Vibration triggers hair cell shearing and electrical charges
  • Electrical charges produce neural impulses traveling along the auditory nerve to the brain
  • The brain interprets these impulses as sounds
• Frequency Detection:
  • High frequency sounds produce greatest motion near the oval window
  • Low frequency sounds produce greatest motion near the apex
  • Different areas on the basilar membrane stimulate different frequencies, resulting in varying nerve cell firings
  • The brain interprets these impulses as sounds of different pitches