Understanding the balance system

balance-systemThe 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 from sensory receptors, special nerve endings, to your brain.

Input from the eyes
Nerve endings or sensory receptors in the back of the eye (retina) called rods and cones 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, which are sensory receptors that are sensitive to stretch or pressure in the tissue that surrounds them. As the legs, arms, or other parts of the body move, the receptors respond to the stretch of the muscles surrounding them and send impulses through many sensory nerve fibers to your brain.

Especially 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.

vestibular-systemInput from the vestibular system
The 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 mechanism in your inner ear that regulates the amount and composition of these fluids is important to the proper functioning of your inner ear.

Each inner ear has a hearing component called the cochlea and a balance component called the vestibular apparatus, which consists 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 the head. At the base of each canal is a swelling (ampulla) and within these ampullae are located the sensory receptors for each canal.

Inside each fluid-filled semicircular canal is a sensory receptor (cupula) attached at its base. With head movement in the direction in which this canal is located, the endolymphatic fluid within the canal, because of inertia, lags behind. When this fluid lags behind, the sensory receptor within that canal is bent. The receptor then sends false impulses to the brain about 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 conform to the impulses coming from the left side.

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