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The Vestibular System
From The Human Balance System, by Mary Ann Watson, M.A.

A healthy person usually takes his or her sense of balance for granted.

When you are healthy you don't even think about walking on the beach, or from the sidewalk on to the grass, or across a gravel driveway. Or, if you get out of bed in the middle of the night, it isn't too difficult to get around your house in the dark without stumbling or losing your balance.

Your ability to maintain your balance depends on information that your brain receives from three different sources -- your eyes, the muscles and joints of your body, and your inner ears. All three of these sources send information in the form of nerve impulses from sensory receptors, special nerve endings, to your brain. This is the sensory input that has to do with balance...

Your inner ear or labyrinth is a complex series of passageways and chambers within the bony skull. Within these passageways are tubes and sacs filled with a fluid called endolymph. Around the outside of the tubes and sacs is a different fluid -- the perilymph. Both of these fluids are of precise chemical compositions, and they are different. The fact that there is a mechanism in your inner ear that regulates the amount and composition of these fluids is very important to the proper functioning of your inner ear.

Part of each labyrinth, or inner ear, is a snail-shaped organ called the cochlea. It functions in hearing. Located right next to the cochlea is the part of the inner ear that has to do with balance. The is called the vestibular apparatus. On each side of the head it is composed of three semicircular canals and a utricle and saccule.

Each of the semicircular canals is located in a different plane in space. They are located at right angles to each other and to those on the opposite side of your head. At the base of each canal is a swelling (ampulla) and within these ampullae are located the sensory receptors for each canal.

Let's look inside a semicircular canal. The sensory receptor (cupula) is attached at its base, but the top of it remains free. When you move your head in the direction in which this canal is located, the endolymphatic fluid within the canal, because of inertia, lags behind. The same thing happens when you spin a glass of water between your hands.  When the fluid lags behind, the sensory receptor within that canal is bent. The receptor then sends impulses to the brain.  The receptor is only sensitive while it is actually moving -- just like the hairs on your arm. Try to move just one hair -- you can feel it as you bend it. When you stop, you don't feel anything anymore. (Clothes are continually bending hairs -- you are not aware of that.) The same thing happens in the hair cells of the cupula.

All of the sensory input concerning balance, from the eyes, from the muscles and joints, and from the two sides of the vestibular system, is sent to a central area in the brain, called the brain stem, where it will be sorted out and integrated.  When you are healthy and both sides of your vestibular system are functioning properly, the two sides of the vestibular system send impulses to the brain that are symmetrical. That is, the impulses coming from the right side conform to the impulses coming from the left side.

The brain stem also receives input from two other areas of the brain -- the cerebellum, which is your coordination center, and the cerebral cortex, which functions in thinking and memory. As the brain stem is integrating all the input it receives concerning balance, the cerebellum may contribute information about automatic movements that have been learned through constant practice, e.g. adjustments in balance needed to serve tennis ball.

The cerebral cortex contributes previously learned information. For example, you have learned that icy sidewalks are slippery and that you have to step on them in a different way in order to keep your balance.

Sometimes the integrating activities that take place in the brain stem are more complicated than at other times. For instance, there are times that the sensory input that we receive from one of the sources conflicts with the input from the other sources. This may occur when you are standing next to a bus that is pulling away from the curb. Your visual input from the bus may be indicating to your brain that you are moving. You may find yourself leaning forward a little to compensate for that sensation. You may even feel dizzy. But your muscles and joints send input that you are not moving, and other visual input finally indicates that other objects are stationary, and a correction is made.

As integration of all the sensory input takes place, the brain stem sends out impulses along motor-nerve fibers that begin in the brain stem and end in the muscles that make your head and neck, your eyes, your legs, and the rest of your body move and allow you to maintain your balance and have clear vision while you are moving.

Some of the impulses that leave your brain stem go back to the cerebral cortex, carrying information to your thinking centers that tell you it's okay to see trees whirling in circles as you turn cartwheels. As you practice these and similar new activities, your brain learns to "read" all different kinds of sensory input as normal.

This is exactly what happens as a baby learns to balance through practice and repetition.  The impulses from the sensory receptors to the brain stem and out to the muscles form a pathway. With repetition, it becomes easier for the impulses to travel over the same network or pathway, until many activities of keeping your balance become automatic.  Physiologists say that these nerve pathways become "facilitated." This is the reason why dancers and athletes practice their activities over and over again. Even very complex movements become almost automatic over a period of time. Anyone who has learned to ride a bicycle, swim or ski can relate to this idea.

This is also the basis for physical therapy in treating people with a damaged vestibular system -- the exercises mimic the movements that make them feel dizzy and lose their balance. After a period of time, the brain "learns" that the input from this activity is "normal" for the damaged system, and the side effects of dizziness and balance decrease.

(This document is not intended as a substitute for professional health care.)
Rev. 1-87

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Last Edited: Friday, November 01, 2002