Neuro Primer: Vestibular System

Spanish Translation also available here: Neuro Cartilla: Sistema Vestibular

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Have you ever watched shaky camera footage taken by someone running frantically through the forest? The camera shot shakes and bounces, making it difficult to get a clear view of the environment, much less the details of the trees that whiz past. Why is it then, that when we are running through a forest or across a soccer field in real life, our view of the path ahead of us is fairly stable and easy to focus on? The answer lies in a crucial part of our sensory system called the vestibular system.

The vestibular system is mainly centered around a few key receptors in our inner ear, but it also includes the pathways that stretch between them and out from them to the brain. In general, this system helps our brain make sense of how our body is moving relative to its environment and also signals our body to make certain adjustments when needed (like stabilizing our eyesight when running). It all starts in the labyrinth of the inner ear. When we move, either linearly (like walking forward) or rotationally (spinning around in a pirouette), there are corresponding parts of the inner ear that respond to these motions. These parts are called the semicircular canals, of which there are three for each axis of motion (x, y, and z axes), and the otolith organs (made up of the saccule and the utricle). The semicircular canals are responsible for detecting rotational movement, whereas the otoliths detect linear acceleration. How exactly these receptors “detect” movement and acceleration is through the body’s clever mechanical interpretation of gravity.

When the body makes a rotational motion, like spinning around a vertical axis or even nodding our head “up” and “down”, gravity pushes down on a fluid called endolymph that is found inside the semicircular canals. Depending on the amount of fluid that shifts and how quickly it shifts, the movement of the fluid then acts on a gelatinous structure called the cupula. Certain kinds of hair cells are embedded in the cupula, and when the entire gelatinous mass shifts, it bends those hair cells, which causes them to fire and transmit signals to the brain. Our brain interprets these signals, integrates them with other kinds of sensory information (like what our eyes are seeing), and then sends signals to the rest of the body to react accordingly.

Our brain’s interpretation of linear acceleration works much the same way. When we make translational movements – such as moving forward or backward or side-to-side – the force of gravity and inertia act on our otoliths. Both otolithic “organs” – the saccule and the utricle – have a patch of hair cells that are embedded in a gelatinous matrix. On top of this gelatinous mass is a layer of calcium carbonate crystals. When the body moves linearly, gravity pushes down on the crystals and shifts them, which then moves the gelatinous layer below it, thus bending the hair cells embedded in it and transmitting another set of signals to the brain.

Together, the semicircular canals and the otoliths help give the brain a better understanding of where the body is located relative to its environment and how it is moving. By combining these signals with information from our other senses, such as vision, the brain can help the rest of the body accommodate for different kinds of motion. One major example of this is called the vestibular ocular reflex (VOR). When our head moves, the VOR helps our eyes compensate for the movement and allows us to focus on a specific target. For example, if the vestibular system detects rotation in one direction, it will send those signals to the brain, which then triggers certain eye muscles to move in the other direction to compensate. All of this happens so quickly that the end result may seem trivial to most human beings – stable vision during swift motion. Yet without it, our vision would instead look like a blurry photograph taken with shaky hands.

This vital sensory system, however, is not completely fool-proof. Spin around in an office chair for too long and once you stop, you will still feel like your body is spinning even if your eyes are telling you a different story. This is because the endolymph in your inner ear is still moving with the inertia of your spin, so it keeps on triggering a signal to your brain that you are about to fly off the chair – even though you have already stopped. Numerous issues with dizziness and orientation actually result from various disorders of the inner ear, such as benign paroxysmal positional vertigo (BPPV), which is when those inner ear crystals become detached from the gelatinous matrix and end up floating in the endolymph.

Most of the time, however, the vestibular system works just fine, all thanks to our planet’s secret ingredient: gravity. As you might expect, astronauts in outer space do not have the force of gravity pushing down on these parts of the inner ear, especially on the calcium carbonate crystals in the otoliths. This means that those crystals are not shifting the gelatinous membrane underneath it, and the sensitive hair cells at the bottom of the membrane are not getting bent or triggered. What ends up happening is a disorienting experience – imagine tilting your head forward to look down at your shoes and feeling instead like you’re falling forwards. Tilting your head slightly to the left also makes your entire body feel like its leaning heavily to the left. These changes to the vestibular system are one reason why astronauts can become so disoriented when they first get into orbit. Combine the disorientation with the fact that astronauts are basically floating about in space, and the concept of “up” and “down” pretty much loses its meaning – maybe along with what they had for breakfast that morning.

What questions do you have about our vestibular system or our sense of balance and orientation, in general? Leave them in the comments down below!

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— Written by Alexa Erdogan. Illustrated by McCall Sarrett.

References:

• Buckey, J. C., & Homick, J. L. (Eds.). (2003). The Neurolab Spacelab mission: neuroscience research in space: results from the STS-90, Neurolab Spacelab mission (No. 535). Government Printing Office.
• Churchill, S. E. (Ed.). (1997). Fundamentals of space life sciences (Vol. 1). Krieger Publishing Company.
• Clément, G. (2003). Perception of the spatial vertical during centrifugation and static tilt. The neurolab spacelab mission: Neuroscience research in space, 5-10.

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