Audiology revision

• Ear Structure and Function: An Introduction to the Anatomy and Physiology of the Ear
• Paul Turner, Senior Lecturer in Audiology, City, University of London
• Sound


  The stimulus of hearing, a vibratory form of mechanical energy
• Compressions and rarefactions


  Sound waves travel as alternate regions of high and low pressure
• Individual particles of the medium do not travel with the wave, but only vibrate back and forth around their equilibrium position
• Frequency


  Measured in hertz (Hz) with one Hz being equal to one complete cycle (one complete compression and rarefaction) in a second
• The human ear can detect a range of frequencies from 20 to 20,000Hz
• Sound lower than 20Hz is called infrasound, sound above 20,000Hz is called ultrasound
• Sound amplitude


  Relates to the size of the pressure changes above and below atmospheric pressure, often quoted in decibels (dB)
• The softest sound that can be heard is 0dB
• Each increase of 10dB represents an approximate doubling of the perceived loudness of the sound
• Outer, middle and inner ear

  • The ear can be divided into the outer, middle and inner ears
• Outer (external) ear


  Consists of the pinna and external auditory meatus or ear canal
• Pinna


  • Collects sound and funnels it down the ear canal to the tympanic membrane
• External auditory meatus (EAM)

  • Lined with skin and leads inward from the bottom of the concha of the pinna to the tympanic membrane, approximately 2.5cm long in adults
  • Skin in the canal is supplied with specialized cerumen (wax) glands
  • Outer part is supported by the cartilage of the pinna, while the medial 1.5cm is supported by the temporal bone
• Outer ear function


  • Non-acoustic: Protects tympanic membrane from direct assault & injury, self-cleansing action
  • Acoustic: Collects sound and generates resonances that increase sound pressure at the tympanic membrane, aids sound localisation
• Pressure gain of the outer ear

  The outer ear is an acoustic cavity which resonates, helping to increase the sound pressure at the tympanic membrane, with a peak at 2-5kHz of 15-20dB
• This is why human hearing is most sensitive at these frequencies
• The external ear produces spectral modulations of incoming sounds that assist in sound localisation from the front & back</b>
• Tympanic membrane


  • About 8-10mm across, made up of a thin fibrous layer covered by skin on the outside and mucosa on the internal surface
  • Vibrates back and forth with incoming compressions and rarefactions of the sound wave
• Middle ear


  An air-filled chamber situated between the ear canal and the inner ear, connected to the nasopharynx via the Eustachian tube, containing the auditory ossicles (malleus, incus and stapes)
• Function of middle ear

  • To transform acoustic energy in air at the eardrum to acoustic energy in the fluids of the cochlea
  • To increase the efficiency of sound energy transfer to the cochlea by increasing the sound pressure at the oval window
• Surface area difference


  The eardrum has a surface area about 17 times larger than the stapes footplate, so sound pressure at the footplate is 17 times greater than pressure at the eardrum
• Lever action of ossicles

  The arm of the malleus is 1.3 times longer than that of the incus, resulting in a lever action that increases the force and produces a 1.3 factor increase in pressure at the oval window
• Eustachian tube


  Connects the middle ear with the nasopharynx, the main function is to aerate the middle ear so that the pressure remains the same as atmospheric pressure
• Eustachian tube anatomy


  • 36mm long, comprised of a 12mm osseous portion and a 24mm cartilaginous portion
  • The ET remains closed except whilst swallowing or yawning, when it opens briefly to allow air to enter the middle ear
• The ET in children is almost horizontal and much shorter than in adults, making middle ear infections and effusions more common
• Inner ear


  Called the labyrinth because of the complexity of its shape, contains the cochlea and structures for equilibrium
• Anatomy of inner ear


  • Comprises the bony labyrinth and the membranous labyrinth, situated within the temporal bone
  • The bony labyrinth contains perilymph, the membranous labyrinth contains endolymph
• Cochlea


  The shape of the cochlea resembles a snail shell with 2¾ turns, divided into the scala vestibuli, scala media and scala tympani
• Within the modiolus is the spiral ganglion, whose central processes form the cochlear nerve
• Sitting on the basilar membrane is the auditory receptor organ, the organ of Corti
• Sound transmission to the cochlea

  1. The stapes pushes against the oval window of the cochlea

  2. Causes fluid in cochlea to vibrate at the same frequency as the sound & sets up 'travelling wave' deflections in the basilar membrane

  3. Pressure relieved by bulging round window
• Spiral ganglion


  The central processes of spiral ganglion neurons form the cochlear nerve
• Organ of Corti


  The auditory receptor organ sitting on the basilar membrane
• Structures in a cross section of one turn of the cochlea

  • Tectorial membrane
  • Inner hair cells
  • Outer hair cells
  • Auditory nerve fibres
• Scala in the cochlea

  • Scala media
  • Scala vestibuli
  • Scala tympani
• Spiral ganglion


  The spiral ganglion (4)
• Auditory nerve fibres


  The auditory nerve fibres (5)
• Sound transmission to the cochlea

  1. Stapes pushes against oval window

  2. Fluid in cochlea vibrates

  3. Travelling wave deflections in basilar membrane

  4. Pressure relieved by round window