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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 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.
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, 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.
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.
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".
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.
- Gaerlan, M., Alpert, P., Cross, C., Louis, M., & Kowalski, S. (2012). Postural balance in young adults: The role of visual, vestibular and somatosensory systems. Journal Of The American Academy Of Nurse Practitioners, 24(6), 375-381.
- "Equilibrioception". ScienceDaily. Retrieved 15 January 2011.
- Azri, W.; Chambon, C.; Herbette, S. P.; Brunel, N.; Coutand, C.; Leplé, J. C.; Ben Rejeb, I.; Ammar, .; Julien, J. L.; Roeckel-Drevet, P. (2009). "Proteome analysis of apical and basal regions of poplar stems under gravitropic stimulation". Physiologia Plantarum 136 (2): 193–208. doi:10.1111/j.1399-3054.2009.01230.x. PMID 19453506.
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