Physiology of Hearing

Function of Hearing
1. Sounds are created when objects vibrate
  1. Vibrations of object cause molecules in object’s surrounding medium to vibrate as well, which causes pressure changes in medium.
2. Sound waves and air pressure
  1. Sound travels through air at 340 m/s
3. Basic sound wave qualities
  1. Amplitude
  2. Intensity
  3. Frequency
4. Perceptual components of sound
  1. Pitch
  2. Loudness
  3. Duration
  4. Timbre
5. Hearing
  1. 20 - 20000 Hz
  2. Ratio between the faintest and loudest sound is more than 1:1,000,000.
Mammalian Auditory System
1. Outer ear
  1. Sounds collected by pinnae
    1. Funnelled from pinnae to ear canal
      1. Length and shape of ear canal enhance sound frequencies
  2. Main function is to insulate the tympanic membrane
2. Middle ear
  1. Ossicles
    1. Stapes
      1. Transmits vibrations to oval window
    2. Incus
    3. Malleus
  2. Muscles
    1. tensor tympani
    2. Stapedius
    3. Muffle pressure changes
      1. But there's a delay of 1/5 of a second so cannot protect against abrupt noises
3. Inner ear
  1. Changes in sound pressure transform into neural signals
  2. Cochlea
    1. Contains organ of Corti
      1. specialised neurons called hair cells, dendrites of auditory nerve fibres that terminate at base of hair cells, and scaffold of supporting cells
      2. Inner Hair Cells carry most (90%) of the information out of the cochlea
      3. Outer Hair Cells receive majority of information into the cochlea
        sharpen the displacement pattern of the basilar membrane
        improve sensitivity to sound; they make response thresholds lower.
        Works by motility of the hair cell body and/or the stereocilia
        Damage leads to broad tuning
    2. Canals
      1. Tympanic
      2. Vestibular
      3. Middle
    3. Membranes
      1. Reissner's
      2. Basilar
        cannot account for the sharpness of frequency in the auditory system
4. The auditory nerve
  1. Responses of individual AN fibres to different frequencies are related to their place along the cochlear partition
  2. Frequency selectivity: clearest when sounds are very faint.
  3. Threshold tuning curve
  4. rate saturation
    1. isointensity curves
  5. Rate intensity
5. Auditory cortex
  1. Tonotopic organisation
    1. Maintained in primary auditory cortex (A1)
    2. Neurons from A1 project to belt area, then to parabelt area
Hearing Impairment
1. Obstruction
  1. Ear wax
2. Conductive hearing loss
  1. Caused by problems with the bones of the middle ear, (e.g., during ear infections, otitis media)
3. Otosclerosis
  1. More serious type of conductive loss.
  2. Caused by abnormal growth of middle ear bones; can be remedied by surgery
4. Sensorineural hearing loss
  1. Injured hair cells
5. Hearing loss
  1. Consequence of aging

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Physiology of Hearing

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