Vestibular System

00:02 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.

00:26 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.

01:25 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.

02:53 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.

03:39 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.

03:54 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.

04:33 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.

05:03 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.

06:40 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.

07:17 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.