Cochlea cross section


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.

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