The inner ear is best known for the cochlea, which converts sound into nerve impulses, for our hearing. But it also contains the vestibular system, which is crucial for balance. The vestibular system is responsible for detecting your head’s orientation in space and its motion. Angular acceleration, or rotational motion, is detected about 3 separate axes by the semicircular canals. Linear acceleration, as well as gravity, are detected in the forward and back direction as well as the side to side direction, by two “otolith organs”, the utricle and saccule. These sensors both directly trigger various reflex movements and provide information to the brain.
The semicircular canals in the figure, together with the Otolith organs and associated nerves, form the vestibular system
I’ll describe how they work below, but first a brief overview of their purpose. If you lean forward or to the side, the otolith organs sense gravity pulling down on your head. This triggers reflex action in muscles to maintain balance. They can also sense acceleration, for example if you are walking briskly then suddenly stop.
The semicircular canals are oriented at right angles to each other in three planes, so can sense your head rotating as it tilts to either side (moving the right ear towards the right shoulder, for example), tilts forward or back, or twists. This is vital in stabilizing your gaze, like “dynamic image stabilization” in cameras. As you twist side to side while looking at a screen, you may notice the image is not bouncing all over but is pretty stable. There is an eye reflex that helps counter the effect of the motion (the “Vestibular Ocular Reflex”), and a neck reflex that works to keep the head stable (the “Vestibulocollic Reflex”). A third reflex of spinal muscles helps with balance (the “The Vestibulospinal Reflex”). In all of these reflexes, the muscles are directly driven by the signal from the inner-ear sensors, without involving the brain. This is much like the “knee-jerk” reflex when the doctor whacks your knee with a little hammer.
In addition to the inner ear’s contribution (the Vestibular system) Your body has two other systems that help with balance. The Somatosensory system, in skin, muscles, and joints, gives information on how the body and its parts are positioned in space (it allows you, for example, to touch your nose with your finger with your eyes closed). Vision gives additional feedback, including where we are in relation to objects around us, and depth perception.
When something goes wrong with the vestibular system, like my episode a couple of weeks ago, we can experience vertigo (perception of motion when nothing is really moving), dizziness, and loss of balance. The Vestibular Ocular Reflex can also be thrown off, leading to involuntary eye movements called nystagmus. Now I’ll quickly go over some common disorders. I’ll go into more detail below after I discuss exactly how the canals and the otolith organs work.
Benign paroxysmal positional vertigo– this is thought to be caused by some crystals in one of the otolith organs becoming dislodged and working their way into one of the semicircular canals. This can happen from an impact, or just occur with aging. The crystals then cause the semicircular canal to give out faulty information, resulting in the symptoms above. A major feature of bppv is that vertigo is brought on by certain head positions or changes in head position (the “positional” part), and it often has sudden onset (“paroxysmal”).
Meniere’s disease– Results from excess fluid in the inner ear. In addition to the vertigo symptoms, it also is characterized by tinnitus (ringing of the ears), and a feeling of pressure in the ears.
Labyrinthitis– both the Cochlea and vestibular systems reside in fluid-filled sacs in the inner ear called the labyrinth. Inflammation in this region, or a nerve emerging from it is called labyrinthitis. It can also cause tinnitus in addition to vertigo.
Vestibular migraine– seems to be caused by overlapping pathways involvling pain and vestibular inputs to the brain. It exhibits the vertigo symptoms discussed, and sufferers will also commonly have a history of motion sickness, such as car or sea sickness.
These are not the only disorders, just the most common. You’ll note there is a lot of overlap in symptoms which is why a proper diagnosis requires sophisticated testing by and ear specialist. They are all treatable but the treatments are different, making a correct diagnosis important. In my case, so far it appears most likely I have some sort of inflammation because my symptoms responded well to a small dose of prednisone. It is also less likely that I have bppv because there is no particular head movement or head position that triggers vertigo or dizziness for me. I do neck stretches and neck roll movements as part of my Yoga practice at night and this has never triggered any symptoms. I’ll be eager to start the process of narrowing it down more on Jan. 25 when I see an ear specialist.
Mechanics of the Vestibular system- details
This goes beyond the overview for those who enjoy learning about the details of biomechanics (or are gluttons for punishment, depending on your point of view). First we’ll take a look at the semicircular canals. They start with an opening at one of the otolith organs (the utricle), then procede in a semicircular shape to their other end, where they widen in a part called the ampula. There the canal is partially blocked by a thickening (the crysta), and blocked the rest of the way by a mass or hair fibers called the cupula. These canals are filled with a fairly viscous fluid (endolymph). When the canal is rotated, fluid flows in the opposite direction. So if the canal in the figure rotates to the right, the angular acceleration causes the fluid to flow around the semicircle to the left, pressing on the hair cells in the ampula. These have protruding fibers which bend, causing the nerve endings of the cell to fire. Since these canals are in three planes at right angles to each other, motion about three separate axes can thus be sensed.
The Otolith organs have a different mechanism. They are also filled with endolymph, and have hair cells protruding out into the fluid. But they also have embedded crystals called otoconia, made of calcium carbonate (the same as limestone or chalk). The crystals are denser than the fluid. So gravity causes them to move through the fluid and bend the hair cells. A similar effect happens with motion: when the organ is accelerated along a line in the appropriate direction, inertia causes the crystals to lag behind and bend the hair cells, causing their nerve endings to fire. There are two otolith organs at right angles to each other, so motion of the head is detected both forward and backwards or side to side.
Note that the otoconia crystals do no belong in the semicircular canals, but these canals share an opening with the utricle. So if one of the crystals becomes misplaced, it can end up in a semicircular canal and cause the bppv symptoms described above. That is also why bppv can be resolved by physical therapy movements like the Epley maneuver.