The system that keeps us upright and headed in the right direction!
Our vestibular system consists of two organs, one in each ear, that communicate to various parts of our brain. The actual vestibular organ is no larger than a penny (yes, penny, remember those?). The vestibular system as a whole (the two organs and brain connections) tells our brain where we are in space and in which direction we are moving. In that respect, it acts as a sort of biological gyroscope. But how it actually works, is all together quite different...not to mention, elegant and impressive.
The "A" word - Anatomy
As we learned, two organs, one in each inner ear. Each organ has 2 otoliths and 3 semicircular canals, so 5 components total in each ear, 10 components total. We're good on the math.
Otoliths
Otoliths are membranous sacs that sense linear movement and gravity. The utricle is more or less oriented in the horizontal plane and senses translational movement -- left, right, forward, backward (or any combination). The saccule is oriented in the vertical plane and senses the body's orientation to gravity -- which way is up and which way is down. How do they do this, you ask? Read on.
Within each otolith (the utricle and saccule) there is a gelatinous slab. I think of it as that polarizing potluck dish of green jello with fruit inside. Except inside this jello dish are a bunch of hair cells. If hair in your jello is less appealing, think of these as little carrot shavings standing on end. On top of this hair-infested jello dish, are little "crystals" of calcium carbonate, or otoconia. In keeping with the food theme, think of these as a bunch of pomegranate seeds.
When we tilt our head back, gravity pulls on the otoconia (pomegranate seeds) on top of the jello, causing the hair cells (carrot shavings) to move within the jello. When these hair cells move, they stimulate the vestibular nerve, initiating a series of electrical impulses. The vestibular nerve carries these electrical signals to other nerves leading to various parts of the brain, telling our brain that our head is tilted backwards.
Semicircular Canals
The semi-circular canals have a shared, but unique function to the otoliths. They also have hair cells, that when displaced send electrical signals to the brain via the vestibular nerve. However, rather than linear motions, the semicircular canals sense rotational movements, or acceleration. So if you think of the otoliths as being active when we are riding in a car (forward/backward) or in an elevator (up/down) -- translational/linear movement, think of the semicircular canals as being active when were are spinning in a circle, or simply turning our head. Because their function is a little different, so is their anatomy, that "A" word again...
The semicircular canals are basically bony tubes with fluid inside (endolymph) and fluid surrounding them (perilymph). At the base of each of these bony tubes is another jello mold! known as the cupula. Inside that cupula is...you guessed it! Hair cells. So when we turn our head, the endolymph fluid inside the semicircular canal pushes the cupula a certain direction and moves the hair cells. These hair cells act just as the ones in the otoliths do in that they directly communicate to the vestibular nerve, sending a signal to our brain that our head is turning.
Remember, there are 3 semicircular canals in each ear -- the anterior canal, posterior canal, and horizontal canal. These canals are oriented in a particular way to sense all the various types of rotational movement of the head -- yaw, pitch, roll, and any combination of the three.
So for example, if you turn your head to the LEFT, you stimulate the LEFT horizontal canal and inhibit (or dampen) the RIGHT horizontal canal. Both these canals send messages to the brain telling your brain not only that you are turning your head left, but how far and how fast you are turning it.
Vestibular Reflexes
So our brain knows our head is tilted back or that it's turning left. What does it do with that information? As a caveat, we've spent a lot of time describing how the vestibular organ operates, but the entire vestibular system incorporates multiple neuronal circuits in various parts of the brain. This is another blog post for another time. I just don't want to downplay that part of the system because well, it's pretty freaking important. But to respect your reading time, once the brain receives signals from the vestibular organ via the vestibular nerve, a lot of processing happens throughout...yada, yada, yada... and the brain then sends signals to various parts of our body to control muscles that keep us upright and balanced. It mostly sends these signals to three areas: 1) through our spine to the muscles of our legs, back, and stomach, 2) to the muscles surrounding our eyes, and 3) to the muscles around our neck.
Vestibulo-Spinal Reflex - stabilizes the body
The Vestibulo-Spinal Reflex (VSR) keeps our body upright against gravity. This reflex literally prevents us from just toppling over. As the vestibular organ sends signals to the brain about what direction is up and what direction is down, the brain sends signals to muscles in our legs, back, and core to keep us held upright. It does this as we stand still and as we move. If someone accidentally bumps into us, the vestibular system sense this sudden movement and our brain responds by taking a step to prevent us from falling. With certain neurodegenerative diseases, such as Parkinson's, or injury to the brain as in stroke, this reflex pathway can be interrupted or impaired making it difficult for us to maintain balance or react in a timely manner to sudden losses of balance.
Vestibulo-Ocular Reflex - stabilizes the eyes
The Vestibulo-Ocular (eyes) Reflex (VOR) keeps our eyes stable on the world around us while our head is in motion. When we walk or run, our body and head move up and down with each step. If our eyes moved with our head, the visual world would like it was bouncing up and down, similar to watching bad videography of someone running with a camera on their shoulder or their iPhone in their hand. In order to see the world around us without it bouncing or shifting, our eyes have to move in the exact opposite direction as our head and at the same velocity. This is accomplished with the VOR. As your head moves, your brain sends signals to specific muscles around your eyes to contact and move your eyes in the opposite direction. The reflex is particularly fine tuned in certain athletes who my be running around a field or gym and have to focus on a ball, player, or apparatus while doing so. If the VOR is not functioning properly due to inflammation as in vestibular neuritis or concussion, we get sensations of blurred or "bouncy" vision, or even a spinning sensation (vertigo) when moving.
Vestibulo-Colic Reflex - stabilizes the head
The Vestibulo-Colic (neck) Reflex (VCR) keeps our head stable while we are moving. While the functional role of the VCR in humans is not clearly understood, or necessarily prevalent, we do know there is a reflexive connection between stimulation of the vestibular organ and muscles of the neck. For example, if a soccer ball hits you unexpectedly in the back of the head, your head pitches forward. This stimulates the anterior semicircular canals in both the left and right vestibular system. When these canals are stimulated, the muscles in the back of your neck automatically engage. This may be to bring your head back to neutral or to prevent your head from pitching too far forward and sustaining an injury. Although the VCR reflex is not well understood, we do know that there are significant connections between the neck and the vestibular system. Patients who have experienced whiplash may report sensations of dizziness described as "fogginess" or "floating". When we work on releasing tight muscles and strengthening weak muscles in their neck, their symptoms resolve. Still, we have lots to learn!
So, it is easy to see how if your vestibular system is not working properly, you can get sensations of dizziness, spinning (vertigo), being off-balance, and fogginess. The good news is vestibular rehabilitation can greatly improve or completely resolve many of these symptoms. I do this by re-educating the vestibular system to process information accurately, adapt to specific triggers which cause your symptoms, and integrate fluidly with other sensory systems (vision, hearing, sensation) using hands-on therapy, targeted exercises, and lifestyle modifications.