PHY238Y Lecture 12

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PHY238YLecture 12

• The human inner ear
• Physics of hearing (IV)
• References
• Hallett et al. Physics for the life sciences,
  4th ed., Ch.2 (2.6)
• Some of the pictures were taken from Hyper
  Physics
• Hyper Physics http//hyperphysics.phy-astr.gsu.ed
  u/hbase/sound/ear.htmlc1
• Thanks to dr. Rod Nave for the permission to use
  the above resource

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PHY238YLecture 12

• The inner ear contains specialized sense organs
  for transducing sound into neural impulses
• - the semicircular canals which serve as the
  body's balance organ
• - the cochlea which serves as the body's
  microphone, converting sound pressure impulses
  from the outer ear into electrical impulses which
  are passed on to the brain via the auditory
  nerve.

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PHY238YLecture 12

• The semicircular canals are the body's balance
  organs, detecting acceleration in the three
  perpendicular planes. These accelerometers make
  use of hair cells which detect movements of the
  fluid in the canals caused by angular
  acceleration about an axis perpendicular to the
  plane of the canal.
• The canals are connected to the auditory nerve.

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PHY238YLecture 12

• Cochlea has three fluid filled sections.
• The perilymph fluid in the canals (almost
  identical to spinal fluid)
• the endolymph fluid in the cochlear duct.
• The fluids differ in terms of their
• electrolytes and if the membranes
• are ruptured so that there is mixing of
• the fluids, the hearing is impaired.
• The organ of Corti is the
• sensor of pressure variations.

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PHY238YLecture 12

• Cochlea is a coiled tube (10 mm diameter 35 mm
  long) bisected by cochlear partition and
  supporting the basilar membrane
• Peak frequencies along the basilar membrane high
  frequencies close to the base low frequencies at
  the apex end.

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PHY238YLecture 12
Traveling waves of different frequencies along
the basilar membrane (Fig. 6.5 from K. Bogdanov
Physics in Biology)
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PHY238YLecture 12

• The basilar membrane of the inner ear plays a
  critical role in the perception of pitch
• The theory that won the Nobel prize for
  Physiology or Medicine (1961) Traveling Waves
  propagate along the basilar membrane (Georg von
  Bekesy)
• Basilar membrane does not respond to vibrations
  in a simple waybut to sinusoidal vibrations and
  traveling waves
• - High frequencies peak near base
• - Low frequencies peak near apex

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PHY238YLecture 12

• Organ of Corti is situated on the basilar
  membrane in one of the three compartments of the
  cochlea.
• It contains four rows of hair cells which
  protrude from its surface.
• Above them is the tectorial (tectoral) membrane
  which can move in response to pressure variations
  in the fluid - filled tympanic and vestibular
  canals.
• Movement of tectorial membrane relative to
  basilar membrane causes hair cells to tilt. A
  lilt of 0.3 nm can be converted into a nervous
  signal

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PHY238YLecture 12

• Taking electrical impulses from the cochlea and
  the semicircular canals, the auditory nerve makes
  connections with both auditory areas of the
  brain.

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PHY238YLecture 12

• Cochlear implants electronic devices designed to
  provide sound information in people with intact
  auditory nerve, but injured cochlea.
• 22 electrodes are surgically inserted inside the
  inner ear
• They receive harmonics from a microphone speech
  processor system
• They send signals to the corresponding nerve
  terminals from the inner ear