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the cochlea is the structure of the
inner ear that harbors the cellular
machinery of the auditory apparatus and
its characteristics are very very unique
in order for it to carry out its
marvelous function and so the first
thing that we that I want you to know
about the cochlea is that it has two
labyrinths and one of these is the
osseous labyrinth and so the bony canal
of the cochlea is along and through here
here's the outer bony wall of the
cochlea as it spirals internally within
the inner ear and then here's the other
side of that bony canal that coils
within the inner ear running within this
osseous labyrinth is a membranous
component and this is the memory Ness
labyrinth and we see it in blue and it
too will follow the coiled nature the
bony labyrinth and then it'll end here
finally at the apex of that coiled
cochlea if we take a cross section
through the bony or osseous labyrinth
and the membranous labyrinth this is the
profile that we'll see and in that
profile we will have three Scylla so
here is the scale of a stimuli here's
the outer portion of the osseous
labyrinth on the opposite side we have
the scale a-- timpani and you can
appreciate the bony wall of the osseous
labyrinth here
and then between the scale of a stimuli
and the skaila tympani we have this gala
media and this is also referred to as
the cochlear duct now the cochlear duct
has a specialized fluid called the
endolymph the scale of vestibuli and the
scala tympani have a extracellular fluid
that's termed apparel lymph the
endolymph however is very very unique in
its ionic concentration normally
extracellular fluid is very very low and
potassium
however the scale of media its endolymph
is extremely high in potassium ion
concentration and this assists very
greatly in the depolarization of the
hair cells and reduces the ATP out
requirements of the hair cells as well
the endolymph is secreted by a
specialized epithelium called the
Australia vascular s and that's shown
here on this aspect of the skaila media
the scaly are separated from one another
by membranes this membrane is separating
the scale of a stimuli from the scale of
media and this is aptly termed in blue
here the scale excuse me the vestibular
membrane and then the membrane that
separates the scale of media from the
skaila tympani
company here media here shaded in blue
is the basilar membrane
now when we think about audition the
organ of Corti within the scale of media
is literally the masterpiece of cellular
micro architecture
this is the apparatus that's going to be
responsible for taking the sound waves
and converting them into action
potentials the organ of Corti contains
numerous structures but the ones that
were most interested in are those shaded
in green and these are the hair cells of
the organ of Corti now these are the
outer hair cells and then this would be
a row of inner hair cells and the
stereocilia the stereocilia are embedded
in the tectorial membrane that we see
and through here the hair cells in
association with some supporting cells
are anchored to the basilar membrane
that we see down in through here and
that is labeled here for you the cochlea
along the basilar membrane is frequency
tuned and what you need to understand
about the frequency tuning of the
basilar membrane is that high frequency
sounds well allow the basilar membrane
in the base of the cochlea to start to
vibrate they are more sensitive to high
frequency sound waves and so the basilar
membrane here will start to vibrate and
that will cause movement of the hair
cells because they're embedded in a
tectorial membrane and the nel start to
depolarize in response to high frequency
sound waves
low-frequency sound waves are gonna be
toward the apex of the cochlea and at
this point that area of the basilar
membrane will start to vibrate in
response and then the rest of the
basilar membrane is fine-tuned again
from high to low in between those areas
now I want to guide you through the
innervation of the cochlea and so once
the hair cells have become depolarized
action potentials will be conveyed along
the nerve fibers that make up the
cochlear component of cranial nerve
number 8 and so we see innervation here
of the hair cell with a cochlear nerve
fiber and then that's running through a
bony canal in through here and we're
going to follow that out toward the
central nervous system those nerve
fibers will start to come together and
in this area
we'll have nerve cell bodies of those
cochlear nerve afferent fibers residing
within this spiral ganglion the fibers
will continue in this direction and in
this view will see those fibers
extending away from the ganglion within
the cochlear nerve itself this is within
the inner ear and so it needs to exit
the inner ear to get to the central
nervous system and so it will exit
through the internal acoustic meatus
that we see here and then the cochlear
nerve along with the vestibular nerve
will form cranial nerve number 8 that we
see in through here
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you