Transcript Secondary Receptor Cell (Hair Cells)

Secondary Receptor Cell
(Hair Cells)
No axon
Output in form of secretion of chemical
transmitter agent
Inner Hair Cells
Outer Hair Cells
Hair Cells
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.
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.
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
Classification of Neurons
1) Afferent or Efferent
2) Direction of travel in cochlea after
passing thru habenula
Radial – travel from center outward
Spiral – travel the length of cochlea
3) Fibers that supply IHC or fibers that
supply OHC
Hair Cells Direction Aff/Eff
Density of
Innervation
Inner
Radial
Afferent
1 : 1 hair cell
Inner
Spiral
Efferent
1 : many IHC
Outer
Radial
Efferent
1 : few OHC
Outer
Spiral
Afferent
1 : many OHC
Efferent Neurons
Nerve supply comes from Olivocohlear
bundle (OCB) from the Superior Olivary
Complex (SOC)
OCB is also called the Ramussen’s
Bundle of the SOC
500 fibers
OCB
OCB
80%
20%
Cochlea
80%
20%
Cochlea
30,000 fibers
CN
100%
Cochlea
CN
100%
Cochlea
HOCB
COCB
homolateral (same side)
crossed (opposite side)
Inner Spiral bundle
enters habenula and
branches off to 3,000
nerve fibers
Outer Radial bundle
enters habenula and
branches off to 8,000 at
Organ of Corti, and
then branches to
40,000 nerve endings
course longitudinally
along inner hair cells
synapse on afferent nerve
rather than on IHC itself
synapse directly on
OHC
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