SENSORY INPUT
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.
- Input from the eyes
Nerve endings or sensory receptors in the back of the eye (retina) are sensitive to light. When light rays strike them, their nerve fibers send impulses to the brain with 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. - Input from the muscles and joints
The input received by the brain from the muscles and joints comes from proprioception—sensory receptors that are sensitive to stretch or pressure in the tissue that surrounds them. These sensory impulses are then sent to the brain for cues on what the body is doing in that moment of time. Most important are the impulses that come from your neck, which indicate the direction the head is turned, and the impulses that come from the ankles, which indicate the body's movement or sway relative to the ground while standing. - Input from the vestibular system
The vestibular system (inner ear) is a complex series of passageways and chambers within the bony skull. These chambers are filled with and surrounded by fluid. There are identical systems located in the right and left ear. Each inner ear has a hearing component (the cochlea) and a balance component (the vestibular apparatus consisting of three semicircular canals and otolith organs). Inside each fluid filled canal is a sensory receptor that responds to head movement. When the head moves, the fluid in the ear lags behind causing the sensory receptor to bend, which creates an impulse to the brain about your movement. When the vestibular apparatus on both sides of the head are functioning properly, they send symmetrical impulses to the brain. That is, the impulses coming from the right side agree with the impulses coming from the left side. When impulses are not symmetrical on the right and left sides, one can become off balance and report dizziness.
CENTRAL NERVOUS SYSTEM PROCESSING
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.
Conflicting sensory input
There are times that the sensory input that we receive from one of the sources conflicts with the input from the other sources. For example, when standing next to a bus that is pulling away from the curb, your visual input from the large rolling bus may indicate that you are moving. You may lean forward a little to compensate for that sensation, or 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.
MOTOR OUTPUT
After the brain processes all of the information coming in regarding your movement and the environment around you, it must respond quickly and tell the body what to do in order for you to maintain balance and keep upright. It sends impulses back to muscles in your head, neck, eyes, legs and the rest of your body to allow a state of balance to occur.
- Motor output to the eyes.
The impulses that go to the eyes coordinate their movement to produce clear vision during head movements. This is controlled automatically by the vestibular system. If this did not occur, anytime you moved, your vision would be blurry. Hence, the inner ear controls gaze stabilization. - Motor output to the muscles and joints
The 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, walking or playing a sport. Through practice and repetition, the impulses from the sensory receptors to the brain and then out to the muscles form a pathway. With repetition, it becomes easier for the impulses to travel over the same network or pathway, until maintaining balance during any activity becomes automatic. This is the basis for physical therapy in treating people with a damaged vestibular or balance system—the exercises mimic the movements that make them feel dizzy and lose their balance. After 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.