Secondary Receptor Cell Hair Cells

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Secondary Receptor Cell(Hair Cells)

• No axon
• Output in form of secretion of chemical
  transmitter agent

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Hair Cells
Inner Hair Cells
Outer Hair Cells
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Inner Hair Cells (IHC)
There is one row of approx. 3,500 inner hair
cells. These cells receive about 95 of the
innervation from the afferent nerve fibers from
the acoustic portion of the VIII nerve.
FYI--The cuticle capped apex of an IHC is
slightly concave and bears about 48 stereocilia,
oriented parallel to the longitudinal axis of the
cochlea. The cell body height of the IHC is
fairly constant over the extent of the entire
cochlea. Stereocilia vary in length and diameter
within an individual hair cell and also among the
individual turns of the cochlea. The number of
cells also varies along the length of the basilar
membrane. Bredberg (1968) found about 80 inner
hair cells per mm at the basal end and 155 cells
per millimeter at the apical end.
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Outer Hair Cells (OHC)
There are three rows of approx. 13,500 outer hair
cells. Although they are much greater in number
than the inner hair cells, they receive only
about 5 of the innervation of the afferent nerve
fibers from the acoustic portion of the VIII
nerve. These cells contain muscle-like filaments
that contract upon stimulation and fine tune the
response of the basilar membrane to the movement
of the traveling wave. The OHC cilia are in a W
shape.
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Innervation of the Organ of Corti

• AFFERENT
• ascending between cochlea to cochlear nucleus
  (CN) going to brain
• 30,000 fibers, located in the bony canal
  (Internal Audiotory Meatus) at junction of
  Modiolus Spiral Lamina.
• MYELINATED central to habenula, and nonmyelinated
  peripheral to habenula

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Classification of Neurons

• Afferent or Efferent
• Direction of travel in cochlea after passing thru
  habenula
• Radial travel from center outward
• Spiral travel the length of cochlea
• Fibers that supply IHC or fibers that supply OHC

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Hair Cells Direction Aff/Eff Density of
Innervation
Inner
1 1 hair cell
Radial
Afferent
Inner
Spiral
Efferent
1 many IHC
1 few OHC
Outer
Radial
Efferent
1 many OHC
Outer
Spiral
Afferent
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Efferent Neurons

• Nerve supply comes from Olivocohlear bundle (OCB)
  from the Superior Olivary Complex (SOC)
• OCB is also called the Ramussens Bundle of the
  SOC

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500 fibers
OCB
OCB
80
80
AFFERENT
20
20
Cochlea
Cochlea
30,000 fibers
CN
CN
EFFERENT
100
100
Cochlea
Cochlea
10
HOCB COCB

• homolateral (same side)
• Inner Spiral bundle enters habenula and branches
  off to 3,000 nerve fibers
• course longitudinally along inner hair cells
• synapse on afferent nerve rather than on IHC
  itself

• crossed (opposite side)
• Outer Radial bundle enters habenula and branches
  off to 8,000 at Organ of Corti, and then branches
  to 40,000 nerve endings
• synapse directly on OHC

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The VIII Nerve

• The VIII nerve is twisted like a rope with low
  frequencies (apical fibers) on the inside high
  frequencies (basal fibers) around the outside.

HIGHER
HI
LO
LOWER