Before we move on and look at those sensory
structures in more detail, it's important
firstly to make sure we understand the histology
and the ultrastructure of the hair cells.
There are two hair cells shown here, type
I and type II. Hair cells are very similar,
whether they're in the crista ampullaris
or in the maculae. They differ in the organ
of Corti in one way that I'll mention at
the end of this slide. But essentially, they're
very similar. Type I and type II hair cells,
I'm not going to really talk about any functional
difference between these cells, but you can
look at the slide and know that they're
The type I hair cells have a more flask or
bulbous type of appearance or shape. The type
II hair cells have a more elongated shape.
At their apex, they both have projections
called stereocilia. And notice that these
stereocilia are different in length.
They have a graded increase in length from one
side of the cell to the other, and that's
repeated along all the hair cells. And on
the lateral, on the left lateral side of each
of these hair cells, you'll see one cilium.
It's called the kinocilium. That's a very
important structure because that defines
the polarity of the cell.
What we're going to learn later on is that
those stereocilia are actually affected by
movement of the endolymph when we move our
head in certain directions. And that movement
of the endolymph actually moves those stereocilia
towards the cilium or away from the cilium.
And that sparks off opposite impulses, and
it provides information about the movement
of that fluid, and therefore, the position
of our head. Those hair cells are supported
by supporting or sustentacular cells. You can see they surround
the hair cells that colored very light grey in this picture.
And then the hair cells that innervate it, the bipolar neurons
from the vestibular ganglion, send out a dendritic
branch, the efferent nerve fibre you see here,
that wrap around or form a bulbous type of
connection to the type I hair cells, or simply
just have isolated buttons or elongated buttons
on the type II hair cells. When those hair
cells are stimulated, when
those stereocilia move, then those efferent
fibres will send information back to the central
nervous system about that movement of endolymph,
and then the central nervous system interprets
that into defining our position of our head
in space and our movement of our head.
There is also an efferent nerve supply. These efferent
fibres probably alter the sensitivity of the
hair cells. They may also amend in some way
the information travelling back to the central
nervous system by the efferent or via the
efferent nerve fibres.
The difference between these hair cells in
this diagram and the hair cells that I will
show you in the organ of Corti, the organ that
detects sound, is that there's no kinocilium.
And that's because the stereocilia don't
bend in any direction towards that kinocilium
in the organ of Corti. Their impulse generation
is due to hearing force and contact with the
tectorial membrane that touches them on their
apical surface, on the apical surface of the
stereocilia. So it's a slightly different
method of stimulating these hair cells.
But it does not require the kinocilium in
the hair cells of the organ of Corti.