Equilibrioception

Balance skill development in children

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 is the result of a number of body systems working together: the eyes (visual system), ears (vestibular system) and the body's sense of where it is in space (proprioception) ideally need to be intact. The vestibular system, the region of the inner ear where three semicircular canals converge, works with the visual system to keep objects in focus when the head is moving. This is called the vestibulo-ocular reflex (VOR). The balance system works with the visual and skeletal systems (the muscles and joints and their sensors) to maintain orientation or balance. Visual signals sent to the brain about the body's position in relation to its surroundings are processed by the brain and compared to information from the vestibular, visual and skeletal systems.

Balance training

Normal balance functioning

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, the utricle and saccule, are called the otolithic organs and are responsible for detecting linear acceleration - 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, organic calcium carbonate crystals crucial to the maintenance of equilibrium. 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.

In humans

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 organs that are responsible for detecting linear acceleration, or movement in a straight line.

In humans, equilibrioception is mainly governed by the visual system. Other systems play roles as well, e.g. the vestibular 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. Bad equilibrium can be a result from Brainstem surgery. Not being very common, but with the advancements in medicine, it is possible to survive brainstem surgery. If a visual nerve is damaged in the process, bad balance may result. Also, a lack of depth perception may occur.

Sense of balance usually deteriorates during the process of aging. It can, however, be improved with the help of special training.

Vestibular system

Main article: Vestibular system

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.

Dysfunction

Main article: Balance disorder

When the sense of balance is interrupted it causes dizziness, disorientation and nausea. Balance can be upset by Ménière'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 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.

In animals

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.

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

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".

In plants

Main article: Gravitropism

Plants could be said to exhibit a form of equilibrioception, in that when rotated from their normal attitude the stems grow in the direction that is upward (away from gravity) while their roots grow downward (in the direction of gravity) this phenomenon is known as Gravitropism and it has been shown that for instance Poplar stems can detect reorientation and inclination.

See also

• Balance disorder
• Broken escalator phenomenon
• Dizziness
• Ernst Mach
• Ideomotor phenomenon
• Illusions of self-motion
• Motion sickness
• Proprioception
• Seasickness
• Spatial disorientation
• Vertigo

Training devices

• Balance ball
• Balance board
• Balance bicycle
• BOSU
• Slackline
• Tight rope