Vestibular function is hard for me

[scilearner]

Hello everyone,

 

 

When you turn your head in the plane of the canal, the inertia of the endolymph causes it to slosh against the cupula, deflecting the hair cells. Now, if you were to keep turning in circles, eventually the fluid would catch up with the canal, and there would be no more pressure on the cupula. If you stopped spinning, the moving fluid would slosh up against a suddenly still cupula, and you would feel as though you were turning in the other direction. This is the explanation for the phenomenon you discovered when you were 5.

 

First of all I don't understand the pic. Why is the endolymph moving in opposite direction to the movement of head, shouldn't it move the same way. How is inertia involved in this and what is the paragraph saying.

 

Thanks

Edited January 23, 2011 by scilearner

[SMF]

Scilearner. This textbook description is poorly done. To get the principle, imagine a glass of water on the counter. Rotate the glass clockwise and the water stays still. If you had fastened a piece of some bendable plastic sheeting to the inside wall of the glass (artificial cupula), rotating the glass clockwise, with the water staying mostly still, would bend the plastic in the counterclockwise direction. If you continue to rotate the glass, friction will eventually cause the water to also turn clockwise along with the glass so that when you stopped rotating the glass the water would continue to rotate clockwise and bend the plastic in the clockwise direction. One big difference between the glass and a semicircular canal is the amount of friction between the fluids and the container. You would have to turn the glass for a minute or so before the water caught up, while in the semicircular canal it is 10 or 15 seconds. I am not exactly sure regarding this time, but sit in a desk chair that you can spin around in and do the experiment. SM

[scilearner]

Scilearner. This textbook description is poorly done. To get the principle, imagine a glass of water on the counter. Rotate the glass clockwise and the water stays still. If you had fastened a piece of some bendable plastic sheeting to the inside wall of the glass (artificial cupula), rotating the glass clockwise, with the water staying mostly still, would bend the plastic in the counterclockwise direction. If you continue to rotate the glass, friction will eventually cause the water to also turn clockwise along with the glass so that when you stopped rotating the glass the water would continue to rotate clockwise and bend the plastic in the clockwise direction. One big difference between the glass and a semicircular canal is the amount of friction between the fluids and the container. You would have to turn the glass for a minute or so before the water caught up, while in the semicircular canal it is 10 or 15 seconds. I am not exactly sure regarding this time, but sit in a desk chair that you can spin around in and do the experiment. SM

 

Thanks SMF, that was a great answer , much better than the explantion I noted before. I understood everything, how relatively water moves in other direction, but I have simple question since my physics is not good. When you rotate the water bucket, why does water stay still. Is it because you are only rotating the bucket and not the water, so water remains still. If you move it hard though water would also move right. I understood all the rest of the explantion though.

[SMF]

Scilearner, you are correct that rotating the water container doesn't rotate the water, at least directly. Newton's first law of motion says that stuff (pardon the informal language) stays still, unless it is moved by something, and once moving will continue until something stops it. The water glass, and your semicircular canals, connect to the fluids they contains by friction with the walls of the containers. A smooth water glass will not have very much friction with the water and it takes a while for the moving glass to cause the water to turn. If you made a glass with a rough internal surface the water would catch up with the rotating glass much faster. A semicircular canal has quite a bit of friction between its internal membrane and the fluid (endolymph), because there is much more surface area, relative to the endolymph volume, than a glass of water. This friction relationship has been evolutionarily tuned to give the best sensory function for typical head movements, and this gets fooled when you get dizzy.

 

Here is some added information. You can move your head all around and up and down while looking at some place on a wall, and your visual world will appear to stay still and constant. We all take this for granted, but if you took a video camera and rotated all around while holding the camera to your body the video you made would be an impossible to watch blur. To make a good video you would have to try to keep the video camera pointed at exactly the same place on the wall while you rotated, and this is hard to do. How about if you were sitting in a desk chair and somebody else turned you back and forth irregularly, keeping the camera straight would be impossible. This is the problem that your visual system has to deal with when you turn your head or walk around. There are several mechanisms that help you keep your visual world steady while you are moving and the most important one is the input from the semicircular canals that tell your eyes to counter-rotate when your head rotates. It is amazing that this system is so accurate. When you spin until you get dizzy, and the world seems to be spinning around you, the endolymph keeps moving and your visual-motor control system thinks you are turning when you are not. What this tells you is that you are a well tuned device and someone who has had an infection in, or damage of, their vestibular system can tell you about the horrors of having this system broken.SM

Edited January 24, 2011 by SMF