The cochlea is the structure of the inner ear that harbors the cellular machinery of the auditory apparatus.
Its characteristics are very, very unique in order for it to carry out its marvelous function. The first thing
that I want you to know about the cochlea is that it has two labyrinths. One of these is the osseous labyrinth.
The bony canal of the cochlea is along in through here. Here is the outer bony wall of the cochlea as it
spirals internally within the inner ear. 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. This is the membranous labyrinth.
We see it in blue. It too will follow the coiled nature of the bony labyrinth. Then, it will 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. In that profile, we will have three scalae.
Here is the scala vestibuli. Here’s the outer portion of the osseous labyrinth. On the opposite side,
we have the scala tympani. You can appreciate the bony wall of the osseous labyrinth here. Then between the
scala vestibuli and the scala tympani, we have the scala media. This is also referred to as the cochlear duct.
Now, the cochlear duct has a specialized fluid called the endolymph. The scala vestibuli and the scala
tympani have extracellular fluid that’s termed perilymph. The endolymph, however, is very, very unique
in its ionic concentration. Normally, extracellular fluid is very, very low in potassium. However, the scala media,
its endolymph is extremely high in potassium ion concentration. This assists very greatly in the depolarization
of the hair cells and reduces the ATP requirements of the hair cells as well. The endolymph is secreted by a
specialized epithelium called the stria vascularis. That’s shown here on this aspect of the scala media.
The scalae are separated from one another by membranes. This membrane is separating the scala vestibuli
from the scala media. This is aptly termed in blue here, the vestibular membrane. Then the membrane that
separates the scala media from the scala tympani, tympani here, media here shaded in blue, is the basilar membrane.
When we think about audition, the organ of Corti within the scala media is literally the masterpiece of
cellular microarchitecture. 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
we’re most interested in are those shaded in green. These are the hair cells of the organ of Corti.
These are the outer hair cells. Then, this would be a row of inner hair cells. The stereocilia are
embedded in the tectorial membrane that we see in through here. The hair cells, in association with
some supporting cells are anchored to the basilar membrane that we see down in through here.
That is labeled here for you. The cochlea along the basilar membrane is frequency tuned.
What you need to understand about the frequency tuning of the basilar membrane is that high
frequency sounds will allow the basilar membrane in the base of the cochlea to start to vibrate.
They are more sensitive to high frequency sound waves. So the basilar membrane here will start to vibrate.
That will cause movement of the hair cells because they are embedded in the tectorial membrane.
Then they’ll start to depolarize in response to high frequency sound waves. Low frequency sound waves
are going to be toward the apex of the cochlea. At this point, that area of the basilar membrane will
start to vibrate in response. 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.
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 at number eight. So we see innervation here
of the hair cell with the cochlear nerve fiber. Then that’s running through a bony canal in through here.
We’re going to again follow that out toward the central nervous system. Those nerve fibers will start to
come together. In this area, we’ll have nerve cell bodies, those cochlear nerve afferent fibers
residing within the spiral ganglion. The fibers will continue in this direction. In this view, we’ll see those
fibers extending away from the ganglion within the cochlear nerve itself. This is within the inner ear.
So it needs to exit the inner ear to get to the central nervous system. So it will exit through the
internal acoustic meatus that we see here. Then the cochlear nerve along with the vestibular nerve
will form cranial nerve number eight that we see in through here.