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Thevestibular system, invertebrates, is asensory system that creates thesense of balance andspatial orientation for the purpose of coordinatingmovement with balance. Together with thecochlea, a part of theauditory system, it constitutes thelabyrinth of the inner ear in mostmammals.
As movements consist of rotations and translations, the vestibular system comprises two components: thesemicircular canals, which indicaterotational movements; and theotoliths, which indicatelinear accelerations. The vestibular system sends signals primarily to the neural structures that controleye movement; these provide the anatomical basis of thevestibulo-ocular reflex, which is required for clear vision. Signals are also sent to the muscles that keep an animal upright and in general controlposture; these provide the anatomical means required to enable an animal to maintain its desired position in space.
The brain uses information from the vestibular system in the head, and fromproprioception throughout the body to enable an understanding of the body'sdynamics andkinematics (including its position and acceleration) from moment to moment. How these two perceptive sources are integrated to provide the underlying structure of thesensorium is unknown.
The semicircular canal system detects rotational movements. Semicircular canals are its main tools to achieve this detection.
Since the world is three-dimensional, the vestibular system contains threesemicircular canals in eachlabyrinth. They are approximatelyorthogonal (at right angles) to each other, and are thehorizontal (orlateral), theanterior semicircular canal (orsuperior), and theposterior (orinferior) semicircular canal. Anterior and posterior canals may collectively be calledvertical semicircular canals.
The movement of fluid pushes on a structure called thecupula which contains hair cells that transduce the mechanical movement to electrical signals.[1]
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 apush-pull fashion: when one canal is stimulated, its corresponding partner on the other side is inhibited, and vice versa.[citation needed]
This push-pull system makes it possible to sense all directions of rotation: while theright horizontal canal gets stimulated during head rotations to the right (Fig 2), theleft 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.
Thevestibular-ocular reflex (VOR) is areflexeye movement that stabilizes images on theretina 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 theRapid head impulse test orHalmagyi-Curthoys-test, in which the head is rapidly and forcefully moved to the side while observing whether the eyes keep looking in the same direction.[2]
The mechanics of the semicircular canals can be described by a damped oscillator. If we designate the deflection of the cupula with, and the head velocity with, the cupula deflection is approximately[3]
α is a proportionality factor, ands corresponds to the frequency. For fluid simulations, the endolymph has roughly the same density and viscosity as water. The cupula has the same density as endolymph,[3] and it is a jelly mostly made ofpolysaccharides withYoung's modulus.[4]
T1 is the characteristic time required for the cupula to accelerate until it reaches terminal velocity, and T2 is the characteristic time required for the cupula to relax back to neutral position. The cupula has a small inertia compared to the elastic force (due to the jelly) and the viscous force (due to the endolymph), so T1 is very small compared to T2. For humans, the time constants T1 and T2 are approximately 5 ms and 20 s, respectively.[5] 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 to maintain clear vision.
Signals from the vestibular system also project to the cerebellum (where they are used to keep the VOR effective, a task usually referred to aslearning oradaptation) 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.
The vestibular nuclei on either side of the brainstem exchange signals regarding movement and body position. These signals are sent down the following projection pathways.
While the semicircular canals respond to rotations, theotolithic organs sense linear accelerations. Humans have two otolithic organs on each side, one called theutricle, the other called thesaccule. The utricle contains a patch ofhair cells and supporting cells called amacula. Similarly, the saccule contains a patch of hair cells and amacula. Each hair cell of a macula has forty to seventy stereocilia and one true cilium called akinocilium. The tips of these cilia are embedded in an otolithic membrane. This membrane is weighted down with protein-calcium carbonate granules called otoconia. These otoconia add to the weight and inertia of the membrane and enhance the sense of gravity and motion. With the head erect, the otolithic membrane bears directly down on the hair cells and stimulation is minimal. However, when the head is tilted, the otolithic membrane sags and bends the stereocilia, stimulating the hair cells. Any orientation of the head causes a combination of stimulation to the utricles and saccules of the two ears. The brain interprets head orientation by comparing these inputs to each other and other input from the eyes and stretch receptors in the neck, thereby detecting whether the head is tilted or the entire body is tipping.[6] Essentially, these otolithic organs sense how quickly you are accelerating forward or backward, left or right, or up or down.[7] 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 constant linear acceleration, one somehow has to distinguish otolith signals that are caused by linear movements from those caused by gravity. Humans can do that quite well, but the neural mechanisms underlying this separation are not yet fully understood.Humans can sense head tilting and linear acceleration even in dark environments because of the orientation of two groups of hair cell bundles on either side of thestriola. Hair cells on opposite sides move with mirror symmetry, so when one side is moved, the other is inhibited. The opposing effects caused by a tilt of the head cause differential sensory inputs from the hair cell bundles allowing humans to tell which way the head is tilting.[8] Sensory information is then sent to the brain, which can respond with appropriate corrective actions to the nervous and muscular systems to ensure that balance and awareness are maintained.[9]
Experience from the vestibular system is calledequilibrioception. It is mainly used for the sense ofbalance and forspatial 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 sense that they have turned to the left if the chair is turned to the left. A person in anelevator, with essentially constant visual input, will sense they are descending as the elevator starts to descend. There are a variety of direct and indirect vestibular stimuli which can make people sense they are moving when they are not, not moving when they are, tilted when they are not, or not tilted when they are.[10] Although the vestibular system is a very fast sense used to generate reflexes, including therighting reflex, to maintain perceptual and postural stability, compared to the other senses of vision, touch and audition, vestibular input is perceived with delay.[11][12]
Diseases of the vestibular system can take different forms and usually inducevertigo[citation needed][13] and instability or loss of balance, often accompanied by nausea. The most common vestibular diseases in humans arevestibular neuritis, a related condition calledlabyrinthitis,Ménière's disease, andBPPV. In addition, the vestibular system's function can be affected by tumours on thevestibulocochlear nerve, an infarct in the brain stem or in cortical regions related to the processing of vestibular signals, and cerebellar atrophy.
When the vestibular system and the visual system deliver incongruous results, nausea often occurs. When the vestibular system reports movement but the visual system reports no movement, the motion disorientation is often calledmotion sickness (or seasickness, car sickness, simulation sickness, or airsickness). In the opposite case, such as when a person is in a zero-gravity environment or during a virtual reality session, the disoriented sensation is often calledspace sickness orspace adaptation syndrome. Either of these "sicknesses" usually cease once the congruity between the two systems is restored.
Alcohol can also cause alterations in the vestibular system for short periods and will result in vertigo and possiblynystagmus due to the variable viscosity of the blood and the endolymph during the consumption of alcohol. The term for this ispositional alcohol nystagmus (PAN):
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.[citation needed]
Benign paroxysmal positional vertigo (BPPV) is a condition resulting in acute symptoms of vertigo. It is probably caused when pieces that have broken off otoliths 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.
A similar condition to BPPV may occur in dogs and other mammals, but the termvertigo cannot be applied because it refers to subjective perception. Terminology is not standardized for this condition.
A common vestibular pathology of dogs and cats is colloquially known as "old dog vestibular disease", or more formally idiopathic peripheral vestibular disease, which causes a sudden episode of loss of balance, circling head tilt, and other signs. This condition is very rare in young dogs but fairly common in geriatric animals, and may affect cats of any age.[14]
Vestibular dysfunction has also been found to correlate with cognitive and emotional disorders, includingdepersonalization andderealization.[15]
Though humans as well as most other vertebrates exhibit three semicircular canals in their vestibular systems,lampreys andhagfish are vertebrates that deviate from this trend. The vestibular systems of lampreys contain two semicircular canals while those of hagfish contain a single canal. The lamprey's two canals are developmentally similar to the anterior and posterior canals found in humans. The single canal found in hagfish appears to be secondarily derived.
Additionally, the vestibular systems of lampreys and hagfish differ from those found in other vertebrates in that the otolithic organs of lampreys and hagfish are not segmented like the utricle and saccule found in humans, but rather form one continuous structure referred to as the macula communis.[16]
Birds possess asecond vestibular organ in the back, the lumbosacral canals.[17][18] Behavioral evidence suggests that this system is responsible for stabilizing the body duringwalking andstanding.[19]
A large variety of vestibular organs are present in invertebrates. A well-known example is thehalteres offlies (Diptera) which are modified hind wings.
{{cite book}}: CS1 maint: multiple names: authors list (link)Patients with vestibular disorders have been reported to experience other personality changes that suggest that vestibular sensation is implicated in the sense of self. These are depersonalization and derealization symptoms such as feeling "spaced out", "body feeling strange" and "not feeling in control of self". We propose in this review that these symptoms suggest that the vestibular system may make a unique contribution to the concept of self through information regarding self-motion and self-location that it transmits, albeit indirectly, to areas of the brain such as the temporoparietal junction
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