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Equilibrioception or sense of balance is one of the physiological senses. It helps prevent humans and animals from falling over when walking or standing still.

Balance skill implemented

[edit] Normal balance functioning

Illustration of the flow of fluid in the ear, which in turn causes displacement of the top portion of the hair cells that are embedded in the jelly-like cupula. Also shows the utricle and saccule-otolithic organs that are responsible for detecting linear acceleration, or movement in a straight line.

This figure shows nerve activity associated with rotational-induced physiologic nystagmus and spontaneous nystagmus resulting from a lesion of one labyrinth. Thin straight arrows - direction of slow components; thick straight arrows - direction of fast components; curved arrows - direction of endolymph flow in the horizontal semicircular canals: AC - anterior canal, PC - posterior canal, HC - horizontal canal.

Balance is the result of a number of body systems working together. Specifically, in order to achieve balance the eyes (visual system), ears (vestibular system) and the body's sense of where it is in space (proprioception) ideally need to be intact. Also the brain, which compiles this information, needs to be functioning normally.

The semicircular canals, found within the vestibular apparatus, let us know when we are in a rotary (circular) motion. The semicircular canals are fluid-filled. Motion of the fluid tells us if we are moving. The vestibule is the region of the inner ear where the semicircular canals converge, close to the cochlea (the hearing organ). The vestibular system works with the visual system to keep objects in focus when the head is moving. This is called the vestibulo-ocular reflex (VOR).

Movement of fluid in the semicircular canals signals the brain about the direction and speed of rotation of the head - for example, whether we are nodding our head up and down or looking from right to left. Each semicircular canal has a bulbed end, or enlarged portion, that contains hair cells. Rotation of the head causes a flow of fluid, which in turn causes displacement of the top portion of the hair cells that are embedded in the jelly-like cupula. Two other organs that are part of the vestibular system are the utricle and saccule. These are called the otolithic organs and are responsible for detecting linear acceleration, or movement in a straight line. The hair cells of the otolithic organs are blanketed with a jelly-like layer studded with tiny calcium stones called otoconia. When the head is tilted or the body position is changed with respect to gravity, the displacement of the stones causes the hair cells to bend.

The balance system works with the visual and skeletal systems (the muscles and joints and their sensors) to maintain orientation or balance. For example, visual signals are sent to the brain about the body's position in relation to its surroundings. These signals are processed by the brain, and compared to information from the vestibular, visual and the skeletal systems.

[edit] In humans

In humans, equilibrioception is mainly sensed by the detection of acceleration, which occurs in the vestibular system. Other senses play roles as well, e.g. the visual system and proprioception. The importance of visual input for balance is illustrated by its being harder to stand on one foot with eyes closed than with eyes open.

The sense of balance, usually, deteriorates in the process of aging of a person. However, it can be improved considerably with the help of special training.

[edit] Vestibular system

Balance skill development in children

In the vestibular system, equilibrioception is determined by the level of fluid properly called endolymph in the labyrinth - a complex set of tubing in the inner ear.

[edit] Dysfunction

When the sense of balance is interrupted it causes dizziness, disorientation and nausea. Balance can be upset by Meniere's disease, superior canal dehiscence syndrome, an inner ear infection, by a bad common cold affecting the head or a number of other medical conditions. It can also be temporarily disturbed by quick or prolonged acceleration, for example riding on a merry-go-round. See also vertigo. Blows to the head can also affect equilibrioreception, especially those to the side of the head or directly to the ear.

Most astronauts find that their sense of balance is impaired when in orbit because they are in a constant state of weightlessness. This causes a form of motion sickness called space adaptation syndrome.

[edit] In animals

Some animals have better equilibrioception than humans, for example a cat uses its inner ear and tail to walk on a thin fence. [1]

Equilibrioception in many marine animals is done with an entirely different organ, the statocyst, which detects the position of tiny calcareous stones to determine which way is "up".

[edit] Training devices

• balance ball
• balance board
• balance bicycle
• BOSU
• slackline
• tight rope

[edit] External links

• McCredie, Scott (2007). Balance: In search of the lost sense. New York: Little, Brown. 296 pp.