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The Balance System
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Dizziness is a symptom of the vestibular (balance) system not functioning properly.

The ability to maintain balance depends on information that the brain receives from three different sources—the eyes, the muscles and joints, and the vestibular organs in the inner ears. All three of these sources send information in the form of nerve impulses to your brain. 

Sensory inputs from the eyes give visual cues that aid in balance. For example, if a person is walking down the street, buildings appear to be aligned straight up and down, and the horizon appears horizontal.

Sensory information from the muscles and joints is called proprioception. As the legs, arms, or other parts of the body move, nerve endings respond to the stretch of the muscles surrounding them and send impulses to your brain. Especially important are the impulses that come from your neck, which indicate the direction the head is turned

Each inner ear, or labrynth,  contains the balance (vestibular) organs. These consist of the otolith, which responds to acceleration and gravity, and three semicircular canals. These are located at right angles to each other and match up with those in the opposite ear. They function rather like 'spirit-levels', giving the brain information about movement in 3 dimensions.

When the vestibular apparatus on both sides of the head are functioning properly, the impulses coming from the right side agree with the impulses coming from the left side. Disagreement between signals from the left and right sides results in dizziness.

Integration of Sensory Input

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 the brain stem, where it is sorted out and integrated with contributions from other parts of the brain. 

The brain stem also receives input from two other main areas of the brain—the cerebellum, which functions as the coordination centre, may contribute information about automatic movements that have been learned through constant practice, e.g. adjustments in balance needed to serve a tennis ball. Secondly, the cerebral cortex, which functions in thinking and memory. The cerebral cortex contributes previously learned information, e.g., that icy sidewalks are slippery and require a person to walk on them with a different pattern of movement pattern in order to maintain balance.

Motor Output

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. These muscles 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.

The motor impulses that go to the eyes coordinate their movement to produce clear vision during active head movements (e.g., while running or watching a tennis match) or passive head movements (e.g., while sitting in a moving car).

The motor impulses that are sent from the brain to the other muscles of the body control their movement so that balance is maintained whether a person is sitting, standing, or turning cartwheels.

For example, when a person is turning cartwheels, some of the impulses that leave the brain stem go back to the cerebral cortex, carrying information to the thinking centers that acknowledge that it's okay to see trees whirling in circles. With practice at this and similar new activities, the brain learns to "read" 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 then out to the muscles form a pathway. With repetition, it becomes easier for the impulses to travel over the same pathway, until maintaining balance during any activity becomes automatic. 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 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.

In effect, balance becomes the "sixth sense."