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Sensing the senseless: Dizziness in the brain



Getting dizzy used to be fun. Tire swings and merry-go-rounds offered endless entertainment for my young friends and me. We would spin and spin and spin until we could no longer walk and we just collapsed into a pile of giggling goons. Not so these days. It is rarer and rarer that I find myself getting dizzy on purpose and more and more common for me to get dizzy without doing any spinning at all. Dizziness is one of those weird phenomena that everyone experiences, whether you’re a hyper kid or plagued with bouts of vertigo. But how do we get dizzy anyways?



Imagine yourself in your car with a full mug of piping hot coffee in the cup holder next to you. You know that you have to drive really carefully to keep all the coffee in the cup, because the motion of the coffee is directly affected by changes in your car’s acceleration. If you brake too hard or take a curve too fast the coffee sloshes out of the cup.



Similar movements are happening in your vestibular organs all the time to help keep you oriented. Deep inside your inner ears there are three loop-like structures called the semicircular canals that are filled with fluid. Just as the coffee detects the motion of your car, the fluid in these canals detects the motion of your head. Each of these canals also has a set of cilia—cells with hair-like projections that sway and bend as the fluid sloshes over them. When the force of the fluid on the hairs exceeds a threshold a signal is sent to your brain that says “Oh, your head just swiveled 62 degrees counterclockwise” or “you just did a cartwheel”. The motion you perceive depends on the amount that the hairs were bent and in which direction the bending occurred. This process is called mechanoelectrical transduction because a mechanical signal (motion of fluid) is being transduced into an electrical signal (neuron firing). Researchers are still trying to figure out the precise mechanism behind mechanoelectrical transduction in the vestibular system but they have two main ideas.



The first is that channels in the cilia cell membranes get stretched open as the hair bends. When the channel is open, charged atoms can enter the cell and cause the neuron to fire. The second idea is that the channels on each cilia are plugged with molecular stoppers which are connected by spring-like proteins. When the cilia are bent or deformed, the springs stretch and the stoppers get pulled out of the channels allowing charged atoms to enter the cell and cause the neuron to fire.



But what about dizziness? If you’ve been spinning like a top for 30 seconds or so the fluid in your canals is going to be sloshing around quite a bit. And even when your body has stopped spinning, it takes a little while for the fluid in your ears to stop sloshing. Your brain continues to get some signals from your semicircular canals that says “whoa we’re spinning in circles” while your eyes are telling your brain that you are standing still. This discrepancy between what your visual and vestibular senses tell your brain is the source of the sensation of dizziness.



What I want to know now is why getting dizzy was so fun when I was younger but is so nausea-inducing now. Do all kinds of dizziness come from similar molecular mechanisms? How does spinning dizziness differ from vertigo dizziness or heat stroke dizziness? If you know the answers to any of these questions or have any hypotheses of your own comment below!



Sources and Further Reading
St George, R. J. and Fitzpatrick, R. C. (2011) The sense of self-motion, orientation and balance explored by vestibular stimulation. The Journal of Physiology, 589: 807-813. doi: 10.1113/jphysiol.2010.197665


Pierre-Paul Vidal and Matthieu P. Robert (2012) Ocular vestibular evoked myogenic potentials: the missing link. J Physiol, 590.13, p2953


Martin Chalfie (2009) Figure 4: Dual Tether Model from Neurosensory mechanotransduction. Nature Reviews Molecular Cell Biology, 10, 44-52.
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