A balanced argument

Why is your sense of balance centered in your ears? How come get dizzy always comes with nausea? Will Ashley-Fenn gives us a level argument...

The vestibular system is a series of fluid-filled tubes within the 'inner ear' on either side of the skull; it assists in your sense of balance by detecting movements of the head and body. The vestibular system is made up of two distinct parts; the semicircular canals and the otolith organs.

As the name suggests, semicircular canals are tubes shaped a bit like bicycle tyres. There are three of these canals on each side of the head. One is horizontal and the other two are vertical and are at roughly 90° to each other.

The canals are responsible for detecting head rotation; because of their orientation, each canal is particularly sensitive to movement in a particular plane.

When you nod, shake or tilt your head, the fluid in the canals doesn’t start to move as fast as your head does. A bit like how when a car accelerates you’re pushed backwards into the seat. This causes the bending of a membrane at the base of the canals, and this bending is detected by bundles of sensitive hair cells.

The activation of the three canals on each side can be compared in order to work out what head movement has occurred.

The other component of the vestibular system is the otolith organs; these are responsible for detecting linear accelerations; forward/backwards movements (like in an accelerating or braking car) and up/down (like when you go up or down in a lift). The otolith organs are embedded with crystals of calcium carbonate called otoconia. When move forwards or backwards, or up or down, these small crystals move about, and displace bundles of hair cells. The body can then work out accelerations by piecing together all the forces it feels from the otolith organs.
 
But sometimes the vestibular system can also be responsible for making you feel ill, like during car sickness. 

A lot of the research around motion sickness has focused on the idea of ‘sensory mismatch’. Your eyes are seeing the things in car that aren’t moving relative to you, but your ears can feel every bump, speed change and turn.

Another cause of mismatch is the so – called 'Coriolis/Cross-Coupling effect'. This phenomenon is well documented in pilots but has also been studied in people during car journeys.

The effect occurs because a passenger has their head tilted to the side at a particular angle – for example, their head may be resting on their left shoulder – while the car is going round a right hand bend in the road.

The combination of the head tilt and the turn round a bend, tricks the brain into thinking that the head has rotated when it hasn't. This then conflicts with a signal from the otolith organs.

In fact, one trick to reduce car sickness may be to tilt your head in the same direction as the bend in the road!

It seems bizarre that the brain thinks that the way to deal with this sensory mismatch is by making you feel ill and then emptying your stomach. There is a theory that the brain interprets the mismatch as the presence of a toxin, which it would try to get rid of by vomiting.

But you are massively reliant on your vestibular system, just to be able to walk down the street. Hopefully that’s some comfort next time you feel car sick!