The vestibular system, or balance system, is the sensory system that provides the dominant input about movement and equilibrioception. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of the inner ear, situated in the vestibulum in the inner ear (Figure 1). As our movements consist of rotations and translations, the vestibular system comprises two components: the semicircular canal system, which indicate rotational movements; and the otoliths, which indicate linear translations. The vestibular system sends signals primarily to the neural structures that control our eye movements, and to the muscles that keep us upright. The projections to the former provide the anatomical basis of the vestibulo-ocular reflex, which is required for clear vision; and the projections to the muscles that control our posture are necessary to keep us upright. Equilibrioception or sense of balance is one of the physiological senses. ... The cochlea is the auditory portion of the inner ear. ... The auditory system is the sensory system for the sense of hearing. ... This is a page about the part of the ear. ... Inner ear The inner ear is the bony labyrinth, a system of passages comprising two main functional parts: the organ of hearing, or cochlea and the vestibular apparatus, the organ of balance that consists of three semicircular canals and the vestibule. ... An otolith, (oto-, ear + lithos, a stone), also called statoconium[1] or otoconium is a structure in the saccule or utricle of the inner ear, specifically in the vestibular labyrinth. ... Figure 3 Three-neuron arc, during a head movement to the right. ...

Figure 1 The labyrinth of the inner ear, from the left ear. It contains i) the cochlea (yellow), which is the peripheral organ of our auditory system; ii) the semicircular canals (brown), which transduce rotational movements; and iii) the otolithic organs (in the blue/purple pouches), which transduce linear accelerations. The light blue pouch is the endolymphatic sac, and contains only fluid.

Vestibular System File links The following pages link to this file: Vestibular system ...

Semicircular canal system

The semicircular canal system detects rotational movements. More precisely, it detects change in rotational movements. The semicircular canals are its main tools to achieve this detection.

Structure

Main article: Semicircular canal

As the basis of our perception of a three-dimensional world, our vestibular system contains three semicircular canals in each labyrinth. They are approximately orthogonal to each other, and are called the horizontal (or lateral), the anterior semicircular canal (or superior) and the posterior (or inferior) semicircular canal. Anterior and posterior canals may be collectively called vertical semicircular canals. inner ear illustration showing semicircular canal, hair cells, ampulla, cupula, vestibular nerve, & fluid The semicircular canals are three half-circular, interconnected tubes located inside each ear that are the equivalent of three gyroscopes located in three planes perpendicular (at right angles) to each other. ... See also Labyrinth, an article treating the mythical maze that imprisoned the Minotaur. ... For more uses of the word labyrinth, see Labyrinth (disambiguation) The labyrinth is a system of fluid passages in the inner ear, including both the cochlea which is part of the auditory system, and the vestibular system which provides the sense of balance. ... In mathematics, orthogonal is synonymous with perpendicular when used as a simple adjective that is not part of any longer phrase with a standard definition. ... The lateral or horizontal canal (external semicircular canal) is the shortest of the three canals. ... The posterior semicircular canal, vertical like the superior, is directed backward, nearly parallel to the posterior surface of the petrous bone; it is the longest of the three canals, measuring from 18 to 22 mm. ...

• Movement of fluid within the horizontal semicircular canal corresponds to rotation of the head around a vertical axis (i.e. the neck), as when doing a pirouette.
• The anterior and posterior semicircular canal detect rotations of the head in the sagittal plane (as when nodding), and in the frontal plane, as when cartwheeling. Both anterior and posterior canals are oriented at approximately 45° between frontal and sagittal planes.

The movement of fluid pushes on a structure called cupula, which contains hair cells that transducts the mechanical movement to electrical signals ^[1] A significant part of ballet terminology is in French language. ... The anatomical planes The anatomical position is a schematic convention for describing the relative morphology of the human body. ... A front, in addition to its common dictionary meanings, may specifically refer to: a weather front, a boundary of two airmasses a military front, an area where armies are engaged in conflict a Front (Soviet Army), a major military subdivision of the Soviet Army a front organization or front company... The cupula forms the apex of the cochlea. ...

Push-pull systems

Figure 2: Push-pull system of the semicircular canals, for a horizontal head movement to the right.

The canals are arranged in such a way that each canal on the left side has an almost parallel counterpart on the right side. Each of these three pairs works in a push-pull fashion: when one canal is stimulated, its corresponding partner on the other side is inhibited, and vice versa. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

This push-pull system allows us to sense all directions of rotation: while the right horizontal canal gets stimulated during head rotations to the right (Fig 2), the left horizontal canal gets stimulated (and thus predominantly signals) by head rotations to the left.

Vertical canals are coupled in a crossed fashion, i.e. stimulations that are excitatory for an anterior canal are also inhibitory for the contralateral posterior, and vice versa.

Vestibulo-ocular reflex (VOR)

The vestibulo-ocular reflex. A rotation of the head is detected, which triggers an inhibitory signal to the extraocular muscles on one side and an excitatory signal to the muscles on the other side. The result is a compensatory movement of the eyes.

Main article: Vestibulo-ocular reflex

The vestibulo-ocular reflex (VOR) is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa. Since slight head movements are present all the time, the VOR is very important for stabilizing vision: patients whose VOR is impaired find it difficult to read, because they cannot stabilize the eyes during small head tremors. The VOR reflex does not depend on visual input and works even in total darkness or when the eyes are closed. The extraocular muscles are the six muscles that control the movements of the eye. ... Figure 3 Three-neuron arc, during a head movement to the right. ... For other uses, see Reflexive (disambiguation). ... An eye is an organ that detects light. ... Human eye cross-sectional view. ...

This reflex, combined with the push-pull principle described above, forms the physiological basis of the Rapid head impulse test or Halmagyi-Curthoys-test, in which the head is rapidly and forcefully moved to the side, while controlling if the eyes keep looking in the same direction.

Mechanics

The mechanics of the semicircular canals can be described by a damped oscillator. If we designate the deflection of the cupula with θ, and the head velocity with , the cupula deflection is approximately

α is a proportionality factor, and s corresponds to the frequency. For humans, the time constants T₁ and T₂ are approximately 3 ms and 5 s, respectively. As a result, for typical head movements, which cover the frequency range of 0.1 Hz and 10 Hz, the deflection of the cupula is approximately proportional to the head-velocity (!). This is very useful, since the velocity of the eyes must be opposite to the velocity of the head in order to have clear vision.

Central Processing

Signals from the vestibular system also project to the Cerebellum (where they are used to keep the VOR effective, a task usually referred to as Learning or Adaptation) and to different areas in the cortex. The projections to the cortex are spread out over different areas, and their implications are currently not clearly understood.

Otolithic organs

While the semicircular canals respond to rotations, the otolithic organs sense linear accelerations. We have two on each side, one called utricle, the other Saccule. Figure 4C shows a cross section through an otolithic organ: the otoconia crystals in the Otoconia Layer (Fig. 4, top layer) rest on a viscous gel layer, and are heavier than their surroundings. Therefore they get displaced during linear acceleration, which in turn deflects the ciliary bundles of the Hair cells (Fig. 4, bottom layer) and thus produces a sensory signal. Most of the utricular signals elicit eye movements, while the majority of the saccular signals projects to muscles that control our posture. While the interpretation of the rotation signals from the semicircular canals is straightforward, the interpretation of otolith signals is more difficult: since gravity is equivalent to a constant linear acceleration, we somehow have to distinguish otolith signals that are caused by linear movements from such that are caused by gravity. We can do that quite well, but the neural mechanisms underlying this separation are not yet fully understood. The utricle, larger than the saccule, is of an oblong form, compressed transversely, and occupies the upper and back part of the vestibule, lying in contact with the recessus ellipticus and the part below it. ... Categories: Stub ...

Experience from the vestibular system

Experience from the vestibular system is called equilibrioception. It is mainly used for the sense of balance and for spatial orientation. When the vestibular system is stimulated without any other inputs, one experiences a sense of self motion. For example, a person in complete darkness and sitting in a chair will feel that he or she has turned to the left if the chair is turned to the left. A person in an elevator, with essentially constant visual input, will feel she is descending as the elevator starts to descend. Of more importance are illusions of the vestibular system. For example, a person in a descending elevator does not feel it is descending once its initial acceleration has ceased. Illusions include: Equilibrioception or sense of balance is one of the physiological senses. ... For other uses, see Elevator (disambiguation). ... For other uses, see illusion (disambiguation). ...

Vestibular/Somatogyral Illusions

See Sensory illusions in aviation. Because human senses are adapted for use in the ground, navigating by sensory input alone during flight can be dangerous. ...

Vestibular/Somatogravic Illusions

See Sensory illusions in aviation. Because human senses are adapted for use in the ground, navigating by sensory input alone during flight can be dangerous. ...

Pathologies

Diseases of the vestibular system can take different forms, and usually induce vertigo and instability, often accompanied by nausea. The most common ones are Vestibular neuritis, a related condition called Labyrinthitis, and BPPV. In addition, the function of the vestibular system can be affected by tumors on the cochleo-vestibular nerve, an infarct in the brain stem or in cortical regions related to the processing of vestibular signals, and cerebellar atrophy. Vestibular Neuronitis, also called Vestibular neuritis, is a paroxysmal, single attack of vertigo. ... Labyrinthitis is a balance disorder that usually follows an upper respiratory tract infection (URI). ... Benign paroxysmal positional vertigo (BPPV) (or Benign paroxysmal vertigo) is a condition caused by problems in the inner ear. ...

Alcohol can also cause alterations in the vestibular system for short periods of time and will result in vertigo and possibly nystagmus. This is due to the variable viscosity of the blood and the endolymph during the consumption of alcohol. The common term for this type of sensation is the "Bed Spins".

• PAN I - The alcohol concentration is higher in the blood than in the vestibular system, hence the endolymph is relatively dense.
• PAN II - The alcohol concentration is lower in the blood than in the vestibular system, hence the endolymph is relatively dilute.

It is interesting to note that PAN I will result in subjective vertigo in one direction and typically occurs shortly after ingestion of alcohol when blood alcohol levels are highest. PAN II will eventually cause subjective vertigo in the opposite direction. This occurs several hours after ingestion and after a relative reduction in blood alcohol levels.

BPPV

Main article: Benign paroxysmal positional vertigo

BPPV, which is short for Benign Paroxysmal Positional Vertigo, is probably caused by pieces that have broken off from the Otoliths, and have slipped into one of the semicircular canals. In most cases it is the posterior canal that is affected. In certain head positions, these particles shift and create a fluid wave which displaces the cupula of the canal affected, which leads to dizziness, vertigo and nystagmus. Benign paroxysmal positional vertigo (BPPV) is a condition caused by problems in the inner ear. ... Nystagmus is involuntary eye movement that can be part of the vestibulo-ocular reflex (VOR), with the eyes moving first in the direction of the lesioned side (slow phase) followed by a quick correction (fast phase) to the opposite side or away from the lesioned side. ...

External links

• SensesWeb, which has been created by Tutis Vilis, contains animations - of all sensory systems, as well as the corresponding PDF-Files, and additional further links.
• Vestibular Primer (by J. David Dickman, Ph.D., professor of neurobiology, Washington University in Saint Louis) An up-to-date introduction to the vestibular system.
• Dizzytimes.com Online Community for Sufferers of Vertigo and Dizziness

Neurobiology is the study of cells of the nervous system and the organization of these cells into functional circuits that process information and mediate behavior. ... Washington University in St. ...

Footnotes

1. ^ Medical Physiology, Boron & Boulpaep, ISBN 1-4160-2328-3, Elsevier Saunders 2005. Updated edition. 1300 pages.

References

• S. M. Highstein, R. R. Fay, A. N. Popper, editors (2004). The vestibular system. Berlin: Springer. ISBN 0-387-98314-7.  (Comment: A book for experts, summarizing the state of the art in our understanding of the balance system)

• Thomas Brandt (2003). Vertigo : Its Multisensory Syndromes. Berlin: Springer. ISBN 0-387-40500-3.  (Comment: For clinicians, and other professionals working with dizzy patients.)

• Driver Drowsiness: Is something missing? J. Christopher Brill, Peter A. Hancock, Richard D. Gilson. University of Central Florida (2003) link (Comment: Research on driver or motion-induced sleepiness aka 'sopite syndrome' links it to the vestibular labrynths.)