The Slip-and-Slide that Keeps You Standing
By Cristian Piraneque, Bioengineering, 2022
Spider-Man’s got nothing on us. If we appreciated our own sense of balance, everyone would be a superhero. It is difficult to fathom the complexity and instantaneous communication that supports balance: visual inputs, sensorimotor (or proprioceptive) receptors laced throughout the body and joints, and, most fascinating of all, the vestibular apparatus. The interactivity of these different systems is often seamless. But anything that disturbs these systems can quickly cause discomfort ranging from simply startling to practically debilitating. Walk out of a spinny ride at Six Flags, and most are aware of how angry the body can get when the brain receives conflicting vestibular, visual and proprioceptive input. Our day-to-day lives are entirely dependent on our ability to continuously remain oriented and aware of where our bodies are in relation to our environment. And while it takes three systems to keep us from falling over, the vestibular apparatus stands out as a uniquely structured system tailored to keep us standing.
Located within the inner ear, this system of tubes and sacs is responsible for the head’s spatial awareness. Composed of three semicircular canals and two gravity-sensitive organs — collectively called the otolith organs, and individually the saccule and utricle — the entire apparatus is about the size of a quarter. The canals are oriented in the X, Y and Z directions and are filled with a fluid called endolymph. When the head moves, so does the endolymph, which tickles hair-like cells within the semicircular tubes to alert the brain that the head has moved, and in a particular direction. The two otolith organs work in a similar way, with the saccule responsible for sensing vertical acceleration and the utricle for horizontal motion. Calcium carbonate crystals attached to the ends of more hair-like cells within the organs respond to changes in gravity. If a crystal falls in response to a change in gravitational force, the hair-like cells alert the brain of movement. This causes the feeling of falling or rising in an elevator, even when the brain isn’t receiving any visual or significant proprioceptive cues.
Our day-to-day lives are entirely dependent on our ability to continuously remain oriented and aware of where our bodies are in relation to our environment.
While the mechanics of this system are often remarkably reliable, they are also extremely sensitive. Say for example, you sit in a chair and begin to spin. Your head begins to move, and with it your semicircular canals. Because of inertia, the endolymph in the tubes resists movement, and so as your head moves to the left, the endolymph pulls the hair-like cells to the right. But after a few moments, the endolymph stabilizes and moves at the same rate as your head. The fluid stops tugging at the hair-like cells, and they straighten out, telling your brain you are no longer moving. But you are! Your visual input says you are whirling around in a chair. This conflict provokes your brain into a tantrum, and you find yourself dizzy and disoriented.
The task of ensuring correct vestibular function is especially important in space. In a microgravity environment, the otolith organs do not function as they do closer to Earth, which causes space motion sickness and hinders the ability of astronauts to perform important tasks. And yet even in the most extreme of environments, the body adapts. In a 2019 report on the human benefits of the International Space Station, NASA describes an ongoing study since 2013 that investigates the effect of weightlessness on vestibular function. So far, this study has shown that there is a dramatic reduction in post-flight symptoms (nausea, dizziness, difficulty tracking objects with eyes) in cosmonauts that have repeatedly been to space over those that have been on prolonged trips for the first time. In collaboration with the Institute of Biomedical Problems at the Russian Academy of Sciences, NASA is using computerized methods — called the OculoStim-CM — to track eye movement with virtual reality glasses that can help explain the complexities of the vestibular function for the cosmonauts in space and, to a greater extent, those affected by vestibular disorders here on earth.
Anything that disturbs these systems can quickly cause discomfort ranging from simply startling to practically debilitating.
Were it not for this complex labyrinth of tubes and sacs, we would be left immobile on the floor. But for those of us fortunate enough to neither be floating around in space or afflicted by a serious vestibular disorder, why not slosh that endolymph around to its fullest potential? Try a handstand, wobble on a slackline, close your eyes and feel those hair-like cells moving. Who knows? There might be a Peter Parker amongst us yet.
Human Physiology (2017). DOI: 10.1134/S0362119717050085
Neuropsychiatric Disease and Treatment (2014). DOI: 10.2147/NDT.S76747
