The Structure and the Function of the Cochlea

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The Structure and the Functionof the Cochlea
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Learning Objectives

• By the end of todays session, you should be able
  to
• Describe the organisation of the organ of Corti.
• Recall how displacement of fluids in the cochlea
  results in the generation of neural signals in
  the sensory hair cells of the organ of Corti.
• Explain how the organ of Corti plays a role in
  frequency analysis.

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• Structure of the Cochlea
• Fluid filled tube, coiled to save space
• When straightened out, tube 34mm long
• Closed at one end (apical cochlea)
• Basal end of cochlea contains 2 flexible
  membranes, the oval window, on which the stapes
  sits, and the round window that acts like a
  pressure release surface
• Main structural feature of cochlea is basilar
  membrane (BM)
• BM composed of collagen fibres which provide
  support for sensory cells of inner ear
• BM divides cochlea tube into upper (scala
  vestibuli) and lower compartment (scala tympani)
• Third compartment also present, scala media,
  which is a sub-compartment of scala vestibuli

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A diagram of the ear with the cochlea unwound.
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• The 3 compartments in the cochlea, the scala
  media (SM), the scala vestibuli (SV) and the
  scala tympani (ST) are fluid filled.
• The scala media is separated from the scala
  vestibuli by Reissners membrane and from the
  scala tympani by the basilar membrane.
• The scala media provides a special environment
  for the organ of Corti which rests on the basilar
  membrane
• This structure is responsible for transducing
  sound waves into neural signals

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Reissners membrane
Scala Vestibuli
Scala Media
Scala Tympani
Basilar membrane
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• Cochlea fluids
• Fluids maintain correct physiological state of
  cells of cochlea
• Have physical properties of water
• Fluid in SV and ST termed perilymph
• Principle ion is Na
• Perilymph has same composition as CSF as it
  arises from capillary circulation around cochlea
• Fluid in SM different in composition.
• Termed endolymph
• Principle ion K

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• The Structure of the Organ of Corti
• Organ of Corti runs the length of the cochlea
  tube
• It consists of sensory hair cells and non sensory
  cells, supporting cells (Deiters cells)
• It sits on the basilar membrane and moves with
  the motion of the basilar membrane

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• There are two types of sensory hair cells one
  row of inner hair cells and three rows of outer
  hair cells.
• The apex (top part) of these hair cells contain
  stereocilia in which are located ion channels
• The tectoral membrane overlies the sensory hair
  cells.
• When the stereocilia of the hair cells move
  against the tectoral membrane , their ion
  channels are opened.

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Inner Hair Cells
1. Nucleus2. Stereocilia3. Cuticular plate4.
Radial afferent ending (dendrite of type I
neuron)5. Lateral efferent ending
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Outer Hair Cells
1. Nucleus2. Stereocilia3. Cuticular plate6.
Medial efferent ending7. Spiral afferent ending
(dendrite of type II neuron)
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• In the human cochlea, there are
• - 3,500 IHCs and
• - 12,000 OHCs.
• Distributed in rows along length of cochlea
• This number is ridiculously low, when compared to
  the millions of photo-receptors in the retina or
  chemo-receptors in the nose!
• In addition, hair cells share with neurons an
  inability to proliferate
• This means that the final number of hair cells is
  reached very early in development (around 10
  weeks of foetal gestation) from this stage on
  our cochlea can only lose hair cells.

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• Mechano-electrical Transduction
• This describes the process by which the sound
  waves (mechanical stimuli) are transformed into
  neural signals (transduction).
• Displacement of fluids in the scala tympani cause
  displacement of the basilar membrane.
• As the basilar membrane is displaced, the
  stereocilia of the sensory hair cells are pushed
  against the tectoral membrane. This opens the
  ion channels in the stereocilia.
• Sodium ions diffuse into the hair cells to result
  in the generation of a neural signal that is
  referred to as a receptor potential.

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• Basilar membrane acts like a mechanical spectrum
  analyser
• It responds to sounds by vibrating in a pattern
  dependent upon intensity and frequency of
  incoming sound
• Inner hair cells relay information about this
  pattern to auditory nerve
• Different frequencies excite populations of hair
  cells along cochlear duct
• High frequencies excite cells at basal end of
  cochlea near stapes
• Low frequencies excite cells at apical end of
  cochlea
• Larger pressure differences across basilar
  membrane displace hair cell steriocilia even more

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• Tonotopic mapping
• Conversion of sound frequency to coding as
  position of excitation
• Experiments by Von Bekesy in the 1960s
  established that sounds of different frequencies
  cause varied displacement of the basilar
  membrane.
• Displacement of the basilar membrane begins at
  the base and progresses to the apex of the
  cochlea.
• High frequency sound waves peak at the base of
  the cochlea. Hence sensory hair cells at the
  base of the cochlea transduce high frequency
  sounds.
• Low frequency sounds peak at the apex of the
  cochlea.
• Hence, sensory hair cells in the apex of the
  cochlea transduce low frequency sounds.

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The Auditory Nerve 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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• Auditory Area of Brain
• The schematic view of some of the auditory areas
  of the brain shows that information from both
  ears goes to both sides of the brain
• When the auditory nerve from one ear takes
  information to the brain, that information is
  directly sent to both the processing areas on
  both sides of the brain.

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