In this video I want to discuss a cross section through the area of what is known as the cochlea.
If you remember from lecture, the cochlea is where we are going to take sound vibrations
from the air, pass them into vibrations within liquid, and then turn that into a nerve impulse
going to the brain.
All of this occurs within the petrous portion of the temporal bone.
And in that portion of the bone we can see a structure rolled up about 2 1/2 times around,
looks somewhat like a snail shell, and for diagrammatic purposes, when we cross section
through we always refer to the three chambers from top to bottom.
Although as we coil around of course top and bottom change his position.
The top chamber is known as the scala vestibuli.
The scala vestibuli is full of a fluid known as perilymph.
The bottom chamber is known as the scala tympani.
And the scala tympani also is filled with perilymph.
The smaller, intermediate chamber, is known as the cochlear duct.
And the cochlear duct is filled with fluid known as endolymph.
Now when we have sound vibrations that pass through the ossicles of the ear, the stapes
will move on the oval window, and the oval window is the beginning of the scala vestibuli.
So when the stapes moves the oval window that begins a vibration of the fluid, the perilymph
within the scala vestibuli.
And that will pass down along the length of the scala vestibuli.
But what it also does, is it vibrates this membrane, right here, known as the vestibular
membrane.
The vestibular membrane actually forms the roof of our cochlear duct.
So when we vibrate the perilymph in the scala vestibuli that vibrates the vestibular membrane,
which then causes vibration of the endolymph within the cochlear duct.
When the endolymph in the cochlear duct begins to vibrate, this structure right here, known
as the tectorial membrane, starts to vibrate as well.
So I want to focus in on just this area right here.
This area is known as the organ of Corti.
This is we are we actually convert the vibrations in the fluid into nerve impulses.
If we look carefully at the organ of Corti we can see that there are some cells which
form a little hill like structure.
These are known as supporting cells.
And at the top of this column of supporting cells we will have some hair cells.
They are referred to as hair cells.
They don't have hair like hair on your scalp, but they have a little extensions which will
be embedded into the tectorial membrane.
So on the top of this little hill we have the hair cells, so we have supporting cells,
hair cells and the extensions of the hair cells will be embedded in the tectorial membrane.
Here we have a small open area.
And then we have another little role of support cells with another hair cell on it.
These hair cells, it is important, have their hairs in bedded in the tectorial membrane.
So when the endolymph of the cochlear duct causes movement of this tectorial membrane,
that in turn moves the extensions of the hair cells, which causes these cells to send an
impulse along their nerve fibers into this structure here, known as the spiral ganglion.
Then from here the nerve fibers will join into the cochlear portion of the vestibulocochlear
nerve.
In the vestibulocochlear nerve then carries the impulse to the brain.
So that is the beginning of the cochlear portion of cranial nerve eight, the vestibulocochlear
nerve.
So recapping real quickly, scala vestibuli filled with perilymph.
Scala tympani, also perilymph.
Cochlear duct, filled with endolymph.
The top of the cochlear duct is the vestibular membrane.
Vestibular membrane vibrates, that starts a vibration in the endolymph.
Endolymph vibrates, that vibrates the tectorial membrane which sends an impulse through the
hair cells of this organ of Corti.
The organ of Corti is often referred to as the functional unit of hearing because that
is where vibration is actually turned into a nerve impulse, at these hair cells.