The crista ampullaris, remember, are located,
there's three of them, they're located
in the ampulla or the ballooned or increased
sort of bulbous projection of the semicircular
canal as it comes in contact with the vestibule.
They detect angular acceleration of the head.
And what you see in the diagram on the top
is the section through the membranous labyrinth,
within the bony or osseous labyrinth. You
can see the bone down the bottom, the
osseous component. The cristae are projections of
the membranous labyrinth into the endolymph
of the membranous labyrinth. Remember there's
endolymph in the membranous labyrinth and
there's perilymph in the osseous labyrinth.
And those projections have hair cells on them,
the sensory epithelium. They're called the
cristae. The cristae are these projections
into the endolymph. And these hair cells have
on them a gel-like structure called the cupula.
Hard to see in this diagram, and also hard
to see in the section below that I'll show
you in a moment because getting good sections,
good fixed preserved sections of this part
of the ear is very difficult because this
is embedded in the temporal bone, in the head
as you would know. On the lower image, you
can see hair cells. These are sitting on the
surface of the crista or the projection into
the endolymph. And the cupula is just the
very pale yellowish glycoprotein gel-like
substance sitting on the surface of the hair
cells. And the hair cells, of course as we saw
earlier in the diagram, are supported by supporting
cells or sustentacular cells.
Very hard to understand really the
histological details here, as I said before
because the details aren't preserved
that well. Let's go to a diagram to see the details
of these structures, the crista. In the diagram,
have a look at the projection of the crista
into the endolymph, lined by supporting cells
and the hair cells. Type I hair cells
are on the ridge predominantly of the crista.
Type II cells tend to be towards the centre.
You can see nerve fibres coming from the hair
cells. Then you can see the cupula, the gelatinous
structure that is supported or suspended on
top of the hair cells, supported by a glycoprotein
called otogelin. That sort of binds the gelatinous
structure to the hair cells. And between the
two are these very fine pores or fluid filled
spaces to allow the hair cells to move.
And what happens when there's movement of
the head? When there is the displacement of
the endolymph with base this cupula.
Remember, they're projecting into the endolymph.
And that displacement causes the bending of
the hair cells in relation to the kinocilium.
They might bend away from the kinocilium.
Or they might bend towards the kinocilium,
and that sends a different message to the
brain as I explained earlier in the lecture.
So that stimulation, that movement of endolymph
then is what stimulates action potential from
these hair cells and gives information about
the movement of that fluid, and therefore,
the movement of the head. The macula are
very similar. They have the hair cells, again,
shown on this diagram, type I and type II.
Again, they have otogelin which is a glycoprotein
that cements the structure above it to those
hair cells. And the structure above it is
just like the cupula except that's called
the otolithic membrane. These macula, remember,
there's two of them, one on the saccule
and one on the utricle. And they detect the linear
movement of the head or position of the head.
And they do so because the cupula is different
here, or at least I should say the otolithic
membrane is similar to the cupula except that,
it has within it, tiny calcium carbonate structures
called the otoliths. So again, when
there's movement of the head,
these move as well. The endolymph moves pass
these structures projecting into the utricles,
and then that moves hair cells, stimulates
the hair cells. The stereocilia again move
in relation to the kinocilium. And that again
sparks an impulse back to the central nervous
system. So, the actual process is very similar.
The only difference is that the macula have
these otolithic membranes containing the otoliths
or calcium carbonate deposits, whereas, the
semicircular canals contain the crista, and
the crista are the parts within the ampulla
region of the semicircular canal. They have
the cupula on them, and the position, or at
least the way in which it works
is exactly the same.
Here's a high magnification picture taken
through the macula. You can see in the image
there're the hair cells and then they're supporting
the otolithic membrane, and embedded in that
membrane are these very small calcium carbonate
structures, the otoliths. And they move, the
membrane moves, and therefore, that stimulate
the hair cells. So, we've looked at the
crista ampullaris. We've looked at the macula
that sits in the utricle and the saccule.
They're involved with movements of the head,
turning of the head, acceleration of the head,
etc, position of the head in space because
of the movement of that endolymph and the
fact that that endolymph then moves either
the cupula or the otolithic membrane, and
that's detected by the hair cells.
Let's now look at the cochlea. And again, it's
labelled down the bottom there, number 4, you
can see the yellow component. That is the
cochlea duct containing the organ of Corti.