Human Vestibular System in Space
How does the human body maintain a sense of body position and balance on Earth, while flying in an airplane, or traveling through space?
Astronauts from STS-45 tumbling in space
The presence of sensory and response systems is a universal attribute of life as we know it. All living organisms on Earth have the ability to sense and respond appropriately to changes in their internal and external environment. Organisms, including humans, must sense accurately before they can react, thus ensuring survival. If our senses are not providing us with reliable information, we may take an action which is inappropriate for the circumstances and this could lead to injury or death.
How Many Senses?
We are all familiar with the question, "How many senses do humans have?" The answer we hear most often is five: sight, taste, smell, hearing, and touch. (Touch itself includes heat, cold, pressure, and pain.) Actually, there are many other senses - hunger, thirst, kinesthetic, etc. One of the most powerful of the other senses is the vestibular sense, provided by the vestibular system. It is our ability to sense body movement combined with our ability to maintain balance (equilibrium). The human body has a remarkable ability to sense and determine the direction and speed in which it is moving and maintain balance (postural equilibrium).
Human beings have the ability to walk a tight rope, do repeated pirouettes in a ballet performance, combine twists and turns when diving, or perform triple toe loops while ice skating... all (usually) without losing balance and while keeping track of the relative position of arms and legs with respect to the rest of the body. Incredible!
How does the human body sense and control the movement so precisely? How do we maintain balance while putting ourselves through a wide variety of spinning and tumbling activities that are inherently "unbalancing"? When we are in motion, how do we know in what direction and at what speed we are moving? How do these important body senses change or adapt when we fly in an aircraft or enter the microgravity environment of low Earth orbit? Can these sensory and response systems, which work so well here on Earth, provide us with inaccurate and potentially harmful information when we fly as pilots or astronauts? Let's find out!
Maintaining postural equilibrium, sensing movement, and maintaining an awareness of the relative location of our body parts requires the precise integration of several of the body's sensory and response systems including visual, vestibular, somatosensory (touch, pressure, and stretch receptors in our skin, muscles, and joints), and auditory. Acting together, these body systems constantly gather and interpret sensory information from all over the body and usually allow us to act on that information in an appropriate and helpful way.
Body movements undertaken in our every day "Earth-normal" environment usually do not upset our sense of balance or body orientation. However, we have all experienced dizziness and difficulty walking after spinning around in a circle. How does the unique gravitational condition encountered in space flight affect an astronaut's sense of body orientation, movement, and balance?
Astronauts experience similar sensations of dizziness and disorientation during their first few days in the microgravity environment of space. Upon returning to Earth after prolonged exposure to microgravity, astronauts frequently have difficulty standing and walking upright, stabilizing their gaze, and walking or turning corners in a coordinated manner. An astronaut's sense of balance and body orientation takes time to re-adapt to Earth-normal conditions.
Something about the vestibular system obviously adapts to changing conditions, but what? Why? How? Might a better understanding of this microgravity-induced vestibular function help people back on Earth prevent the dizziness, disorientation, and susceptibility to falling that some older people experience? Answers to these important and interesting questions require us to know more about the anatomy (structure) and physiology (function) of the human vestibular system on Earth as well as in space.
For many years, NASA has been investigating the human vestibular system's adaptation to the space environment. Important experiments were performed on STS-40 (Spacelab-1), STS-58 (Spacelab-2), and STS-90 (Neurolab). Future flight experiments will help us to better understand the physiology of our vestibular system by building on what we have learned from previous missions and ground-based research.
Things to Know: Vocabulary Building
Diagram of the ear
The ear is made up of several smaller structures that can be organized into three distinct anatomical regions: an outer ear which extends from outside the body through the ear canal to the tympanic membrane (ear drum); a middle ear, an air-filled cavity containing three tiny bones (ossicles) that transmit and amplify sound between the ear drum and the cochlea (where the sense of hearing is located); and the inner ear, composed of the cochlea and the vestibular system.
The Vestibular System
The vestibular system, which is key to our senses of balance, motion, and body position, is comprised of three semicircular canals connected to two membranous sacs called the saccule and utricle. The saccule and utricle are often referred to as the otolith organs. The otolith organs allow us to sense the direction and speed of linear acceleration and the position (tilt) of the head. The semicircular canals allow us to sense the direction and speed of angular acceleration.
The semicircular canals are oriented along three planes of movement with each plane at right angles to the other two. Pilots and astronauts call these three planes of rotation pitch (up and down; nod your head "yes"), roll (tumbling left or right; move your head from your left to your right shoulder or vice versa), and yaw (lateral movement left and right; shake your head "no").
What's the difference between angular and linear acceleration? Linear acceleration is a change in velocity (speed increasing or decreasing over time) without a change of direction (straight line). Angular acceleration is a change in both velocity and direction at the same time. For example, imagine you are in a stopped car. The driver of the car steps on the accelerator and you accelerate straight ahead. The driver steps on the brake pedal and you decelerate to a stop. Then the driver puts the car in reverse and you accelerate straight backwards, and then the driver slams on the brakes once again. You have just experienced linear acceleration and deceleration in both forward and backward directions. Your movement was along a straight line and your otolith organs helped you sense these linear accelerations and decelerations.
Imagine yourself on a roller coaster. You start out accelerating straight ahead, just like in the car. Suddenly, the track dips almost straight down and you "pitch" forward. Then the nose of your car (and you) comes almost straight up. You have just experienced downward and upward pitch. The roller coaster, while staying perfectly flat on the track, now takes a severe left turn followed by a right turn. You have just "yawed" to the left and right. Now comes the really fun part. Your roller coaster and the track do a complete 360-degree roll, first to the left and then to the right. Makes you dizzy just thinking about it, right? You have just experienced the three planes of angular acceleration; pitch, yaw, and roll. An aircraft, a spaceship, or any vehicle operating in three-dimensional space can accelerate in these three planes of rotation and often along more than one plane at the same time. Your semicircular canals enable you to sense these angular accelerations.
Although they are both located within the vestibular apparatus of your inner ear, are interconnected, and operate using similar physical principles, the sensory mechanisms which allow you to detect linear acceleration (otolith organs) are structurally and functionally different than those which allow you to detect angular acceleration (semicircular canals).
The vestibular system also helps you maintain a fixed gaze on a stationary or moving external object while you are undergoing complex head and body movements. Look at the clock on the wall. Now move your head sideways or up and down, or even in a circle. Your eyes stay fixed on the clock. With slow movement, the eyes are kept stationary by visual mechanisms only. As the speed of movement increases, the vestibular system takes over the image stabilization process.
Vestibular Physiology: How Structure Supports Function
Now that you understand the location and overall design of your vestibular system and its role in providing you with reliable sensory input, let's investigate the structure and functions of its two different components.
The Semicircular Canals
Diagram of the crista
The semicircular canals are a set of three membranous tubes embedded within a bony structure of the same shape. The central cavity of each canal is filled with a fluid called endolymph. Each endolymph-filled canal has an enlarged area near its base called an ampulla.
Parts of the vestibular nerve penetrate the base of each ampulla and terminate in a tuft of specialized sensory hair cells. The hair cells are arranged in a mound-like structure called the ampullary crest. Rising above the ampullary crest is the cupula, consisting of the hair-like extensions of the hair cells surrounded by a gelatinous material arranged into a wedge-shaped structure. This structure consisting of the ampullary crest and the cupula is called a crista.
When the endolymph moves (or the cupula moves and the fluid remains stationary), the gelatinous tip of the cupula and the hair cell extensions embedded within it are displaced to one side or the other. When the embedded hair cells bend, they send a signal via the vestibular nerve to the brain where the information is evaluated and appropriate action is initiated.
The mechanics of how the semicircular canals actually function to "sense" angular acceleration may be more easily understood by reviewing the physics of inertia. The Law of Inertia states that "a body at rest remains at rest unless acted upon by an unbalanced force." This is important because angular acceleration and deceleration primarily affect the semicircular canals and entirely depend on the relative movement of endolymph with respect to the cupula.
The effects of angular acceleration on the semicircular canals
This means that if you were to begin accelerating along one of the three planes of rotation (pitch, roll, or yaw), structural components of the corresponding semicircular canal would begin moving immediately since they are attached to the rest of your head. However, the endolymph within that particular semicircular canal would tend to "remain at rest" due to inertia. It would lag behind the structural components, deflecting the cupula and generating a nerve impulse to the brain.
Initially, the membranous tubular and cellular structures move but the fluid does not. Thus, there is relative movement between the fluid and the rest of the semicircular canal. Eventually, due to friction and the drag it induces, the fluid begins to move at the same speed as the components within which it is contained. When this occurs, the cupula is not deflected and, even though your body is continuing to angularly accelerate, the acceleration is not "sensed". You incorrectly perceive that you are stationary.
Now, let's stop your angular acceleration suddenly. What happens? The moving fluid now has momentum and so it continues to move until friction and drag bring it to a stop. In other words, fixed structures of your semicircular canal stop immediately (since they are still attached to your head which is still attached to your body) but the endolymph fluid continues to move in the direction of the previous movement. The Law of Inertia also states that a body in motion will continue in motion in a straight line unless acted upon by an unbalanced force. Now, the cupula and the embedded hair cells are bent in the opposite direction. This causes you to incorrectly sense that you are accelerating in the direction opposite to your previous acceleration, even though you are completely stopped!
Saccule and Utricle
The saccule and utricle are referred to collectively as "the otolith organs". They sense linear acceleration and are affected by gravity. They also provide you with information concerning changes in head position (tilt). Because of the way they are situated within the vestibular apparatus, the saccule is more sensitive to vertical acceleration (like riding in an elevator) and the utricle is more sensitive to horizontal acceleration (riding in a car).
Both the saccule and the utricle contain a thickened patch of specialized cells called a macula that consists of sensory hair cells interspersed with "supporting" cells. The free hair-like tufts extending from the hair cells are embedded in a gelatinous otolithic membrane which supports small piles of calcium carbonate crystals on its surface. Collectively, these calcium carbonate crystals are called otoliths. The otoliths increase the mass of the otolithic membrane and give it more inertia. On Earth, when the head is tilted to the left or right, forward or back, the otoliths tend to move along the gravity gradient (downwards). Even a slight movement of the otolithic membrane is enough to bend hair cells and send sensory information to the brain. A similar inertia and gravity-dependent process occurs when you accelerate linearly -- up or down, forward or backward.
Diagram of the otolith organ
The underlying physiology and functioning of the otolith organs are remarkably similar to those of the semicircular canals. Both systems depend upon inertia and the mechanical deflection of hair cells to initiate nerve impulses that are sent to the brain and interpreted as body movement. The brain then reflexively initiates appropriate "corrective" actions within the nervous, visual, and muscular systems to ensure that situational awareness and balance are maintained.
Let's re-examine our previous example of rapidly accelerating straight ahead in a car. During forward acceleration, inertia causes the utricle's otolithic membrane and its associated otoliths to lag behind the portion of the utricle that is firmly attached to the head. This in turn causes the hair cells, whose hair-like extensions are embedded within the otolith membrane, to be deflected backwards. This backward deflection stimulates sensory nerves to fire and this provides the brain with information on the direction and speed of acceleration. A similar process occurs within the saccule when you are in an elevator and it either begins to rise or descend rapidly.
Humans sense position and motion in three-dimensional space through the interaction of a variety of body proprioceptors, including muscles, tendons, joints, vision, touch, pressure, hearing, and the vestibular system. Feedback from these systems is interpreted by the brain as position and motion data. Our vestibular system enables us to determine body orientation, senses the direction and speed at which we are moving, and helps us maintain balance.
When there is no visual input as is common in many flight situations, we rely more heavily on our vestibular sense for this information. However, in flight and in space, our vestibular system, which is designed to work on the ground in a 1g environment, often provides us with erroneous or disorienting information.
Some of these spatial disorientation effects result in illusions that can be induced for the purpose of scientific research, or even just for fun. Filmmakers and designers of high-tech amusement park rides often use these techniques to pull us into the action and give us a more thrilling adventure.
Understanding the workings of the various organs that comprise this system will lead to improved adaptation strategies for astronauts entering a microgravity environment and returning to an Earth-normal environment. It will also help military and civilian pilots and people on Earth who are prone to dizziness and disorientation. We all benefit from NASA's scientific research on the vestibular system.
- (pl = ampullae) expanded area within each semicircular canal which contains a crista; used to detect angular acceleration.
- a simultaneous change in velocity and direction (as in spinning); sensed by the semicircular canals.
- (pl = cristae) within ampullary region of semicircular canal; name given to structure composed of ampullary crest (hair cells) combined with the cupula.
- One component of a crista; sits atop ampullary crest and is composed of hair-like extensions of sensory hair cells embedded within a gelatinous mass.
- fluid within semicircular canals which, when it moves, deflects the cupula and initiates the sensation of angular acceleration.
- common name given to sensory cells located within the ampullary crest of semicircular canals and the macular region of saccule and utricle (otolith organs).
- the fundamental property of inert material tending to resist changes in its state of motion. Macula (pl = maculae) thickened area within saccule and utricle consisting of hair cells and supporting cells. In both the saccule and utricle, the macula is covered by the gelatinous otolithic membrane containing otoliths.
- a change in velocity without a change in direction (up and down or side to side); sensed by the otolith organs.
- thickened area within saccule and utricle consisting of hair cells and supporting cells. In both the saccule and utricle, the macula is covered by the gelatinous otolithic membrane containing otoliths.
- tendency of a body in motion to resist a change in that motion.
- repeated rapid eye movements.
- calcium carbonate crystals adhering to and embedded within otolithic membrane of saccule and utricle (otolith organs).
- saccule and utricle, primarily responsible for sensing linear acceleration as well as head position (tilt).
- rotational motion carried out along a front to back vertical plane.
- rotational motion carried out along a lateral vertical plane.
- one of the two types of otolith organs of the vestibular system; used to sense linear acceleration and the position (tilt) of the head.
- three fluid filled circular tubular structures within each inner ear which are arranged at right angles to each other and are responsible for sensing angular acceleration.
Sensory hair cells
- common name given to sensory cells located within the ampullary crest of semicircular canals and the macular region of saccule and utricle (otolith organs).
- integrated sensory system which combines individual inputs from skin, muscles, tendons and stretch receptors throughout the body.
- the semicircular canals and the otolith organs (Responsible for sensing angular and linear acceleration, respectively.)
- rotational motion carried out along a horizontal plane.
For more information on the Human Vestibular System in Space
+ View site
Office of Biological and Physical Research
+ View site