Vestibular system

The system that subserves the bodily functions of balance and equilibrium. It accomplishes this by assessing head and body movement and position in space, generating a neural code representing this information, and distributing this code to appropriate sites located throughout the central nervous system. Vestibular function is largely reflex and unconscious in nature.

The centrifugal flow of information begins at sensory hair cells located within the peripheral vestibular labyrinth. These hair cells synapse chemically with primary vestibular afferent nerve fibers, causing them to fire with a frequency code of action potentials that include the parameters of head motion and position. These vestibular afferents, in turn, enter the brain and terminate within the vestibular nuclei and cerebellum. Information carried by the firing patterns of these afferents is combined within these central structures with incoming sensory information from the visual, somatosensory, cognitive, and visceral systems to compute a central representation of head and body position in space. This representation is called the gravito-inortial vector and is an important quantity that the central nervous system employs to achieve balance and equilibrium. See also Brain; Nervous system (vertebrate); Postural equilibrium; Reflex.

The vestibular labyrinth is housed within the petrous portion of the temporal bone of the skull along with the cochlea, the organ of hearing (Fig. 1). The receptor element or primary motion sensor within the labyrinth is the hair cell (Fig. 2). Hair cells respond to bending of their apical sensory hairs by changing the electrical potential across their cell membranes. These changes are called receptor potentials, and the apical surface of the hair cell thus functions as a mechanical-to-electrical transducer. The frequency of the resulting action potentials in the VIIIth cranial (vestibulocochlear) nerve encodes the parameters of angular and linear motion. See also Biopotentials and ionic currents; Ear (vertebrate); Synaptic transmission.

The vestibular labyrinth is located within the inner ear. It communicates with the brain via the VIIIth nerve. Each of the three semicircular canals has an ampulla and a long and slender duct. The utricle primarily senses motion in an earth-parallel plane, while the saccule primarily senses motion and gravity in an earth-perpendicular plane.

Otolithic macula at rest. The arrows within the primary afferents or VIIIth nerve fibers indicate spontaneous activity in these fibers in the absence of motion of the otolithic mass relative to the hair cell stereocilia. (The otolithic membrane is not illustrated for clarity.)

Hair cells are the common sensory element in both the angular and linear labyrinthine sensors as well as within the cochlea. The particular frequency of energy that hair cells sense within these diverse end organs arises because of the accessory structures surrounding the hair cells. Thus, angular motion is sensed by the semicircular canals, linear motion by the otolith organs, and sound energy by the cochlea.

The primary afferents innervated by hair cells are the peripheral processes of bipolar neurons having cell bodies located in Scarpa's ganglion within the internal auditory meatus. The central processes of these cells contact neurons in the brainstem of the central nervous system. The vestibular nuclei complex is defined as the brainstem region where primary afferents from the labyrinth terminate. It is composed of four main nuclei: the superior, medial, lateral, and descending nuclei. The axonal projections of vestibular nuclear neurons travel to all parts of the neuraxis, including the brainstem, cerebellum, spinal cord, and cerebrum. See also Motor systems.

In all vertebrates, there is an efferent system that originates from cell bodies within the central nervous system and terminates upon labyrinthine hair cells and primary afferents. The efferent vestibular system is presently a subject of intense study but undoubtedly is in place to enhance vestibular function. It is interesting that evolution felt it necessary to modify incoming vestibular information before it could enter the central nervous system.

Human beings, in common with other vertebrates, possess a set of sense organs that provide information to the brain concerning orientation and motion of the body. They reside in the inner ear and are collectively referred to as the vestibular system. They form a very small part of the human anatomy, the main components being no larger than 10 mm across. Most people are unaware of the vital role they play in everyday life — except when something goes wrong with one of the elements of this system. The vestibular system is deeply embedded in the temporal bone alongside the cochlea (which is responsible for hearing) and it contains two distinct types of sensory organs; the semicircular canals and the otolith organs.

The semicircular canals respond to rotational movements of the head, whether induced passively during activities such as running or riding a horse or a motorcycle, or actively, as occurs when voluntary head movements are made during visual search. Each canal forms a cavity in the temporal bone and each contains a membranous duct filled with a viscous fluid (endolymph). There are three on each side of the head and the plane of each canal is perpendicular to the others, so that between all six of them they can provide information related to rotational acceleration of the head during movement around any axis. During such acceleration in the plane of a particular canal, the endolymph tends to remain stationary because of its inertia, so that there is relative motion of the endolymph within the duct. Motion of the endolymph is resisted by viscous friction at the fluid-duct boundary and by the elasticity of a gelatinous structure inside each canal, the cupula. The cupula contains sensory hair cells that consequently become deflected, causing stimulation of associated nerve fibres, leading to the transmission of signals to the brain.

The otolith organs respond to linear motion. They lie at the point at which all three semicircular ducts converge. There are two of them on each side of the head, and each contains sensory receptors in a structure known as a macula. With the head erect, the macula in each utricle is oriented horizontally, and in the saccule vertically. The base of each macula carries hair cells that project into a gelatinous substrate in which are embedded minute crystals of calcium carbonate (the otoconia) forming plaque with an area of only 1.5-2 mm²; the otolith membrane separates this complex from the more fluid endolymph. When linear acceleration occurs in the plane of the macula, the inertia of this dense complex causes it — and therefore the hair cells within it — to be deflected in the direction opposite to that of the movement. These deflections set up trains of nerve impulses, with frequencies proportional to the extent of deflection. In the otolith organs hair cell clusters are tuned to different directions of motion, all directions of motion in the plane of the otolith being represented. The utricle can thus send signals to the brain representing a combination of fore-aft and lateral motion of the head, whereas the saccule principally conveys information about vertical motion.

Maintenance of visual and postural stability

In general, the function of the vestibular apparatus is (via connections in the brain) to generate activity in various muscle systems, which will compensate for the head and body motion, and result in the maintenance of visual and postural stability. The area in the brain stem (the vestibular nucleus) that receives the output of the canals and otoliths has direct connections with muscles controlling eye movements and with muscles of the neck and limbs. In the case of the eyes, the vestibulo-ocular reflex generates eye movements that compensate for head motion with a very short delay (around 10 milliseconds).

As we walk or run, the head generally bobs up and down. Stabilization of the eye prevents movement of visual images on the retina, which would otherwise cause images to be blurred. Individuals who have been unfortunate enough to lose the function of the vestibular system (through damage to the inner ear) often experience apparent motion of the visual world (oscillopsia) under these circumstances.

Fortuitously, the vestibulo-ocular reflex, ‘designed’ to deal with maximum running speed, also allows modern man to view stationary objects in the outside world when travelling in high-speed vehicles, where there is often considerable linear and angular vibration. However, sometimes the reflex is inappropriate. Reading a newspaper in a train is often difficult because, when looking at objects within the moving vehicle, the stabilizing reflex is no longer appropriate. To suppress the eye movements we rely largely on the ocular pursuit system, a mechanism that we normally use to track moving objects with the eyes when we are stationary. But ocular pursuit has a very limited range of operation and does not function at frequencies of vibration above about 2 cycles per second. Unfortunately, in moving vehicles frequencies of vibration are frequently much higher — between 2 and 20 cycles per second.

Stabilization mechanisms similar to those for the eye operate for control of the head, limbs, and other postural systems, but they are necessarily more complex than those controlling the eye.

Perception of motion and orientation

As well as controlling actions within the body, vestibular stimulation also engenders powerful sensations of motion and orientation in space. Stimulation of the canals gives a sensation of turning, so that someone who is rotated on a swivelling chair will experience a sensation of rotation even in the absence of any other cues such as vision (i.e. with their eyes closed). However, because the canals are really responsive to angular acceleration, during a constant rate of angular rotation (constant angular velocity) the sensation gradually decays over 10-20 seconds. And when rotation stops, the individual experiences rotation in the opposite direction, even though actual motion has ceased, because the fluid in the canal continues to move when the head has stopped. In everyday life, prolonged rotation is not often encountered, but it does occur frequently when flying. Pilots must therefore be aware that they cannot always rely on the sensation of motion, particularly in circumstances where there are no other reference cues such as sight of land (e.g. when flying in cloud).

Stimulation of the otolith organs also gives rise to sensations, but in this case they may be of either linear motion or of orientation with respect to the vertical. When linear acceleration is sustained it causes a continuous deflection of the otoconia. The most common situation in which this occurs is when the head is tilted, when gravitational acceleration causes the otoconia to be deflected in proportion to the degree of tilt. Consequently, application of sustained linear acceleration is usually interpreted as tilt, so that, when accelerating forward in a high-speed vehicle, a sensation of being tipped backwards is experienced. Again, this is particularly important when flying because, on take-off, an aircraft is normally accelerating and climbing at the same time. The combination of vehicle and gravitational accelerations gives rise to a sense of tilt that is greater than it should be, and the pilot must learn not to misinterpret this sensory information.

When linear motion changes frequently, for example during vibration, a true sense of linear motion is normally experienced. This is most sensitive at frequencies close to those of natural head movements (around 2 cycles per second).

In normal circumstances, linear and angular motion stimuli are combined, as when we bend down to tie a shoelace in a moving train. In such circumstances, the sensations can be complex and unexpected as a result of the coriolis components of acceleration that accompany motion in three dimensions. The individual may experience a disturbing sensation of tumbling in these circumstances, which may be sufficient to bring on motion sickness.

Allied to the experience of real linear or angular body motion are similar sensations that may arise from motion of the visual world when the body itself is stationary. These sensations of self-motion are referred to as linear or angular vection respectively.

Disorders of the vestibular system

One of the major consequences of a failure of the vestibular system is the occurrence of vertigo, or dizziness, which is experienced by large numbers of individuals. Acute vertigo can occur when the vestibular system on one side of the head suddenly stops working effectively, which can be due to factors such as vestibular neuritis or haemorrhage in the cerebellum or brain stem. In such cases there is sudden onset of a strong sense of rotation, often accompanied by a flicking back and forth of the eyes (nystagmus). It generally disappears within hours or days. More persistent vertigo can occur, for example, as a result of the migration of calcite crystals from the otolith organs on to the cupula of the semicircular canal. The cupula then becomes inappropriately sensitive to gravity and a sensation of turning is brought on by a change of head position with respect to gravity. There are other examples of clinical problems arising from vestibular failure, many of which cause great disturbance to the sense of the body in space.

— Graham Barnes

See also motion sickness; nystagmus; vection.

The noun has one meaning:

Meaning #1: organs mediating the labyrinthine sense; concerned with equilibrium
  Synonym: vestibular apparatus

The vestibular system, which contributes to our balance and our sense of spatial orientation, 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 accelerations. 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.

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.

Contents 1 Semicircular canal system 1.1 Structure 1.2 Push-pull systems 1.3 Vestibulo-ocular reflex (VOR) 1.4 Mechanics 1.5 Central processing 2 Otolithic organs 3 Experience from the vestibular system 3.1 Vestibular/somatogyral illusions 4 Pathologies 5 BPPV 6 See also 7 Footnotes 8 References 9 External links

Semicircular canal system

The semicircular canal system detects 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 (right angles) 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.

• 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 canals 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 transduct the mechanical movement to electrical signals ^[1]

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.

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.

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. The otoconia crystals in the otoconia 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 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.

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.

Vestibular/somatogyral illusions

See Sensory illusions in aviation.

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.

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.

See also

• Migraine-related vestibulopathy

Footnotes

1. ^ Medical Physiology, Walter Boron & Emile 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 labyrinths.)

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.
• Dizzytimes.com Online Community for Sufferers of Vertigo and Dizziness.

v • d • e Nervous system: Sensory systems / senses (TA A15) Special senses Visual system/sight Auditory system/hearing Chemoreception (Olfactory system/smell • Gustatory system/taste) Touch Pain • Heat • Balance (Equilibrioception) • Mechanoreception (Pressure, vibration, proprioception) Other Sensory receptor

v • d • e Sensory system: Auditory and Vestibular systems (TA A15.3, GA 10.1029) Outer ear Pinna (Helix, Antihelix, Tragus, Antitragus, Earlobe) • Ear canal • Auricular muscles Eardrum (Umbo, Pars flaccida) Middle ear Tympanic cavity Labyrinthine wall/medial: Oval window · Round window • Secondary tympanic membrane • Prominence of facial canal • Promontory of tympanic cavity Membranous wall/lateral Mastoid wall/posterior: Mastoid cells • Aditus to mastoid antrum • Pyramidal eminence Carotid wall/anterior Tegmental wall/roof: Epitympanic recess Jugular wall/floor Ossicles Malleus (Neck of malleus, Superior ligament of malleus, Lateral ligament of malleus, Anterior ligament of malleus) · Incus (Superior ligament of incus, Posterior ligament of incus) · Stapes (Anular ligament of stapes) Muscles Stapedius · Tensor tympani Eustachian tube Bony part of pharyngotympanic tube · Cartilage of pharyngotympanic tube (Torus tubarius) Inner ear/Labyrinth/ (Vestibule, membranous labyrinth, bony labyrinth) Auditory system Cochlear labyrinth General cochlea Scala vestibuli • Helicotrema • Scala tympani • Modiolus • Cochlear cupula Perilymphatic space Perilymph • Vestibular aqueduct • Cochlear aqueduct Cochlear duct / scala media Reissner's/vestibular membrane • Basilar membrane Endolymph • Stria vascularis • Spiral ligament Organ of Corti: Stereocilia • Tectorial membrane • Sulcus spiralis (externus, internus) • Spiral limbus Vestibular system/ Vestibular labyrinth Static/translations: Utricle (Macula) · Saccule (Macula, Endolymphatic sac, Endolymphatic duct) · Kinocilium · Otolith Kinetic/rotations: Semicircular canals (Superior, Posterior, Horizontal) • Ampullary cupula • Ampullae (Crista ampullaris) ear navs: anat/pathways/physio/dev, noncongen/congen, eponymous signs, proc

v • d • e Auditory and vestibular pathways Auditory Hair cells → Spiral ganglion → Cochlear nerve VIII → Cochlear nuclei (Anterior, Dorsal) → Trapezoid body → Superior olivary nuclei → Lateral lemniscus → Inferior colliculi → Medial geniculate nuclei → Acoustic radiation → Primary auditory cortex Vestibular Vestibular nerve VIII → Vestibular nuclei, Flocculonodular lobe Vestibular nuclei → Vestibulospinal tract, Ventral posterolateral nucleus → Vestibular cortex ear navs: anat/pathways/physio/dev, noncongen/congen, eponymous signs, proc

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