SCIENCE 101: The Vestibulo part of Cranial Nerve VIII, the Vestibulocochlear Nerve

Jun 20 2011 Published by under Basic Science Posts, Neuroanatomy, Uncategorized

Sci loves Portal. I think it's a magnificently clever game, and Portal 2 is the in the same vein. I love the songs, I love the humor, and I love the concept (puzzle solving with a giant gun! That makes portals!), and I DESPERATELY want to play.

I can't.

Sci, not unlike many people on this planet, is a sufferer of motion sickness. I've never been able to read in the car, and sitting in the back seat is often too much. Heck, sitting in the FRONT seat is sometimes bad! And Portal. Whooooo, Portal. Just WATCHING Portal makes me green in about 2 minutes, and attempting to play myself...well I only lasted about 30 seconds.

Motion sickness is caused by a mismatch between the perception of movement (say, moving in a car), and what your sense of balance perceives (that you're sitting still). Finally the difference in perception causes dizziness and nausea. It's stronger in some than in others, and for those of us who are really weak of stomach, well, we can thank an overactive vestibular system.

Last week we covered the basics of the auditory system, controlled by cranial nerve VIII, the vestibulocochlear nerve. But the "vestibulo" part of the vestibulocochlear nerve is, though in the same general location, something else entirely. Because cranial nerve VIII not only controls your hearing (the -cochlear part), it also controls your BALANCE and sense of how you're moving through space. While this may seem to most people like it's merely a matter of avoiding sharp objects and not embarrassing yourself on the dance floor, knowing your orientation in space is a universal issue for all living creatures. It's something we don't really think about, but are you facing up or down? Where is your head relative to your feet? When you're moving forward, how do you know how FAST you're going? These are all questions for your vestibular system.

And the vestibular system is OLD. All jawed vertebrates, you and the fish you just ate, and the chicken you had for dinner, pretty much have the same vestibular system setup.


You can the the cochlea sitting there, like a cute little snail, right where we covered it last time. Now, look just above it and slightly to the left. You see those two arches? Those are part of your semicircular canals. And those loops detect position and acceleration...and determine whether you're feeling motion sick.

If you'll excuse me, I'm feelin' a bit queasy talking about all that acceleration...

You have three semicircular canals, one lying flat (the horizontal canal, appropriately enough), and two that sit relatively upright, the anterior and posterior canals. They are actually at about 45 degree angles. Between the three of them, they are capable of detecting where your head is pretty much at all times (except when fooled by modern inventions like cars and Portal). And the way they do this many ways similar to the auditory system, but in a lot of ways different as well. And it's also completely mind boggling. When it comes down to it, the simple vestibular system, passed down all the way from the first jawed fishes, is so complex that it can blow your mind to think how it evolved.

All three of your semicircular canals are filled with fluid called endolymph. The canals, filled with endolymph, each end in an area called the utricle. At the end of the utricle is a bulged out bit called the ampulla. The ampulla has a little ridge of tissue running along the bottom of it called the crista, and this is where the vestibular magic happens. The crista contains a little pile of sensory hair cells (kind of like in the auditory system), and the hair cells have little cilia on them, outward, in the same direction, into the open space. The cilia are embedded in a gelatin called the cupula.


So you've got these little cilia sticking up surrounding hair cells. The whole thing is surrounded with FLUID, the endolymph that fills the semicircular canals. When you tilt, turn, or lift your head (or whip it around by spinning around in circles. Go on, you know you want to), the fluid in your semicircular canals moves too, slightly delayed. THIS moves your gelatinous cupula, which bends the hair cells. Bending one direction will increase action potentials, and bending the other direction will decrease action potentials.

Now, the cupula bends as the endolymph shifts. But what happens if you KEEP turning your head? Then the endolymph will stop moving, and your cupula will stop bending. Once you STOP turning, your endolymph will stop just a little behind, and the cupula will briefly bulge in the opposite direction. This means that the semicicular canals respond by far the best to CHANGES in the speed of rotation, rather than just continuous movement.

At the bottom of your semicircular canals lies your utricule and saccule. These don't have cristae in the same way, but they do have the hair cells. The hair cells in the utricule lie horizontal when you're upright, while those in the saccule lie vertical. Their hair cells are in gelatin just like in the semicircular canals, only this gelatin also has little nuggets of CALCIUM in it. These bits of calcium make the gelatin heavy, such that when you flop around, the gelatin flops with you, and stays flopped instead of springing back. Not only that, in the utricle and saccule, the hair cells don't all point the same direction. Instead, they bristle out all everywhere. This means that any movement of the calcium will activate some parts of the hair cells more than others. This then stimulates the hair cells, which send signals about the position of your head. This area is technically called the otolithic membrane, and the little calcium bits are call "otoconia" which actually means "ear dust". So if you have the constant feeling you've got little bits of something in your're not actually wrong.

And what do the action potentials of the hair cells activate? Your eighth cranial nerve, of course. The neurons of the vestibulocochlear nerve synapse onto the hair receptor cells, and the signals from the hair cells get passed on via neurotransmission.

These cells then go in two main branches, joining up briefly with the nerve from the auditory system to head into the brain, and then branching out on their own because they don't need those stupid hearing guys. One branch heads to the cerebellum, the tiny brain at the base of your big one, which plays a big role in motor control. The other heads off to a group of aptly named vestibular nuclei. And this is the part where you realize just HOW important the vestibular system is. I mean, most other cranial nerves have like one or two nuclei. The auditory system just stops in at one primary one, etc,etc. Vestibular system? It's got FOUR. Cause it is BIG STUFF. The four are the inferior, medial, lateral, and superior vestibular nuclei, and they are in the space between the medulla and the pons.

Each of the three semicircular canals and your otolithic membranes will send neurons to these vestibular nuclei in a particular pattern. And the vestibular nuclei ALSO receive inputs from the cerebellum, the spinal cord, and...the other vestibular nuclei (remember what you've got on one side you've got on the other, so technically you have eight vestibular nuclei).

And of course the vestibular nuclei themselves then project to the spinal cord, the cerebellum, and cranial nerve III (oculomotor), IV (trochlear), and VI (abducens). Why all these inputs and projections, you might ask? Well, knowing where your head is only works if you can adjust your posture and where you're looking to accomodate it. So all of these interconnections serve to keep you apprised of where your head is in space, and allow you to unconsciously adjust your posture to reflect it.

Now you'll note in that picture up there that the vestibular nerves head ALL the way down the spinal cord. That's actually very key, because the vestibular nerve feeds information to the neck muscles (keeping your head in position), and your antigravity muscles in your torso (keeping you upright).

All of these interconnections are both really interesting, and they are also really cool in that they allow you to sometimes FOOL your vestibular system. This is something that Sci learned back in the days when she was a dancer. As you've probably seen, dancers spin around a lot.

Now those who aren't dancers see that and think we're super human. Try spinning around a bunch. You get really dizzy really quickly, as you move faster than your poor vestibular system can handle. But dancers have figured out how to FOOL their vestibular systems, using the fact that your vestibular system is more sensitive to changes in acceleration than it is to actual movement. When we turn (and you can see it in the video if you watch REALLY carefully) we actually turn our heads FASTER than the rest of our bodies, and continually fix our eyes on a single point. This is called "spotting", and it fools the vestibular system into thinking we're in one continuous motion, rather than a series of fast turns. By focusing our eyes and moving our heads quickly, we do not allow the semicircular canals to stop moving, so the fluid keeps going. Remember that if the fluid keeps going, the cupula won't move, so the body loses the feeling that we're continually changing position, and thinks that we're just a straight line. And then the confused signals aren't sent, and dancers don't get dizzy! (Ok, ok, we get a little dizzy, it takes a lot of practice, and you can never stop moving your head, but we're a lot LESS dizzy than we would be. Trust me, you can't do a series of foutte turns without spotting a LOT.)

And so, the next time you get motion sick, try to spin around in a circle, or even just turn your head, remember your vestibular system. It's one of the oldest, and yet most complicated, systems we have. And you'd probably be covered in bruises without it. ๐Ÿ™‚

And finally, in honor of the awesome game that I will probably never be able to play, due to my charming vestibular system:

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