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The vestibular system, in
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s, is a
sensory system The sensory nervous system is a part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular bio ...
that provides the leading contribution to the
sense of balance The sense of balance or equilibrioception is the perception of balance and spatial orientation. It helps prevent human Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposa ...
and
spatial orientation Image:Change of axes.svg, Changing orientation of a rigid body is the same as rotation (mathematics), rotating the axes of a frame of reference, reference frame attached to it. In geometry, the orientation, angular position, attitude, or direction ...
for the purpose of coordinating
movement Movement may refer to: Common uses * Movement (clockwork), the internal mechanism of a timepiece * Motion (physics), commonly referred to as movement Arts, entertainment, and media Literature * Movement (short story), "Movement", a shor ...
with balance. Together with the
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the Organ of Cort ...

cochlea
, a part of the
auditory system The auditory system is the sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neur ...
, it constitutes the labyrinth of the inner ear in most mammals. As movements consist of rotations and translations, the vestibular system comprises two components: the
semicircular canals The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The se ...
, which indicate rotational movements; and the
otolith An otolith ( grc-gre, ὠτο-, ' ear + , ', a stone), also called statoconium or otoconium or statolith, is a calcium carbonate Calcium carbonate is a chemical compound with the formula Ca CO3. It is a common substance found in rocks as th ...
s, which indicate
linear accelerations
linear accelerations
. The vestibular system sends signals primarily to the neural structures that control
eye movement Eye movement includes the voluntary or involuntary movement of the eyes, helping in acquiring, fixating and tracking visual stimuli. A special type of eye movement, rapid eye movement, occurs during REM sleep. The eyes are the visual organs ...
; these provide the anatomical basis of the vestibulo-ocular reflex, which is required for clear vision. Signals are also sent to the muscles that keep an animal upright and in general control posture; 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 from
proprioception Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense A sense is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological ...
throughout the body to enable the mammal to understand its body's
dynamics Dynamics (from Greek language, Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power (disambiguation), power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study o ...
and
kinematics Kinematics is a subfield of physics, developed in classical mechanics, that describes the motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon in which an object changes it ...

kinematics
(including its position and acceleration) from moment to moment. How these two perceptive sources are integrated to provide the underlying structure of the
sensorium A sensorium (/sɛnˈsɔːrɪəm/) (plural: sensoria) is the apparatus of an organism's perception considered as a whole, the "seat of sensation" where it experiences and interprets the environments within which it lives. The term originally ente ...

sensorium
is unknown.


Semicircular canal system

The semicircular canal system detects rotational movements. The semicircular canals are its main tools to achieve this detection.


Structure

Since the world is three-dimensional, the vestibular system contains three
semicircular canals The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The se ...
in each
labyrinth In Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A be ...
. They are approximately
orthogonal In mathematics, orthogonality is the generalization of the notion of perpendicularity to the linear algebra of bilinear forms. Two elements ''u'' and ''v'' of a vector space with bilinear form ''B'' are orthogonal when . Depending on the bili ...
(at right angles) to each other, and are the '' horizontal'' (or ''lateral''), the '' anterior semicircular canal'' (or ''superior''), and the '' posterior'' (or ''inferior'') semicircular canal. Anterior and posterior canals may collectively be 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 In dance and gymnastics, a turn is a rotation of the body about the vertical axis. It is usually a complete rotation of the body, although quarter (90°) and half (180°) turns are possible for some types of turns. Multiple, consecutive turns are ty ...

pirouette
. *The anterior and posterior semicircular canals detect rotations of the head in the
sagittal plane In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molec ...

sagittal plane
(as when nodding), and in the
frontal plane A coronal plane (also known as the frontal plane) is any vertical plane that divides the body into ventral and dorsal (belly and back) sections. It is one of the three main planes of the body used to describe the location of body parts in relati ...
, as when
cartwheeling A cartwheel is a sideways rotary movement of the body. It is performed by bringing the hands to the floor one at a time while the body inverts. The legs travel over the body trunk while one or both hands are on the floor, and then the feet retur ...
. Both anterior and posterior canals are oriented at approximately 45° between frontal and sagittal planes. The movement of fluid pushes on a structure called the cupula which contains hair cells that transduce the mechanical movement to electrical signals.


Push-pull systems

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 makes it possible 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 (VOR) is a
reflex In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

reflex
eye movement Eye movement includes the voluntary or involuntary movement of the eyes, helping in acquiring, fixating and tracking visual stimuli. A special type of eye movement, rapid eye movement, occurs during REM sleep. The eyes are the visual organs ...
that stabilizes images on the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well ...

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 observing whether 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 \theta, and the head velocity with \dot q, the cupula deflection is approximately : \theta (s) = \frac \dot (s) 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 maintain 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.


Projection pathways

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. *To the
cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain The hindbrain or rhombencephalon is a developmental Development of the human body is the process of growth to maturity. The process begins with fertilization ...

cerebellum
. Signals sent to the cerebellum are relayed back as muscle movements of the head, eyes, and posture. *To nuclei of cranial nerves
III
III
,
IV
IV
, and VI. Signals sent to these nerves cause the vestibulo-ocular reflex. They allow for the eyes to fix on a moving object while staying in focus. *To the
reticular formation The reticular formation is a set of interconnected nuclei ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organ ...

reticular formation
. Signals sent to the reticular formation signal the new posture the body has taken on, and how to adjust circulation and breathing due to body position. *To the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord, which contain ...

spinal cord
. Signals sent to the spinal cord allow quick reflex reactions to both the limbs and trunk to regain balance. *To the
thalamus The thalamus (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...

thalamus
. Signals sent to the thalamus allow for head and body motor control as well as being conscious of body position.


Otolithic organs

While the semicircular canals respond to rotations, the otolithic organs sense linear accelerations. Humans have two otolithic organs on each side, one called the utricle, the other called the
saccule The saccule is a bed of sensory cells in the inner ear. It translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane. When the head moves vertically, ...
. The utricle contains a patch of
hair cell Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cel ...
s and supporting cells called a
macula The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area near the center of the retina The retina (from la, rete) is the innermost, light-sensitive layer of tissue of the eye of most vertebrate Vertebrates () comprise ...
. Similarly, the saccule contains a patch of hair cells and a
macula The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area near the center of the retina The retina (from la, rete) is the innermost, light-sensitive layer of tissue of the eye of most vertebrate Vertebrates () comprise ...
. Each hair cell of a macula has forty to seventy stereocilia and one true cilium called a
kinocilium A kinocilium is a special type of cilium The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and li ...
. 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. When the head is tilted, however, 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 to other input from the eyes and stretch receptors in the neck, thereby detecting whether the head is tilted or the entire body is tipping. Essentially, these otolithic organs sense how quickly you are accelerating forward or backward, left or right, or up or down. 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, 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 the striola. 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 allow humans to tell which way the head is tilting, 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.


Experience from the vestibular system

Experience from the vestibular system is called
equilibrioception The sense of balance or equilibrioception is the perception Perception (from the Latin ''perceptio'', meaning gathering or receiving) is the organization, identification, and interpretation of Sense, sensory information in order to repr ...
. It is mainly used for the sense of
balance Balance may refer to: Common meanings * Balance (ability) in biomechanics * Balance (accounting) * Balance or weighing scale Arts and entertainment Film * Balance (1983 film), ''Balance'' (1983 film), a Bulgarian film * Balance (1989 film), ''Bal ...
and for
spatial orientation Image:Change of axes.svg, Changing orientation of a rigid body is the same as rotation (mathematics), rotating the axes of a frame of reference, reference frame attached to it. In geometry, the orientation, angular position, attitude, or direction ...
. 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 An elevator (North American English North American English (NAmE, NAE) is the most generalized variety (linguistics), variety of the English language as spoken in the United States and Canada. Because of their related histories and ...

elevator
, with essentially constant visual input, will feel she is descending as the elevator starts to descend. There are a variety of direct and indirect vestibular stimuli which can make people feel they are moving when they are not, not moving when they are, tilted when they are not, or not tilted when they are. Although the vestibular system is a very fast sense used to generate reflexes, including the
righting reflexThe righting reflex, also known as the labyrinthine righting reflex, is a reflex In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
, to maintain perceptual and postural stability, compared to the other senses of vision, touch and audition, vestibular input is perceived with delay.


Pathologies

Diseases of the vestibular system can take different forms, and usually induce
vertigo Vertigo is a condition where a person has the sensation of movement or of surrounding objects moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea Nausea is a diffuse sensation of ...

vertigo
and instability or loss of balance, often accompanied by nausea. The most common vestibular diseases in humans are vestibular neuritis, a related condition called
labyrinthitis Labyrinthitis, also known as vestibular neuritis, is the inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogen In biology Biology is the nat ...
,
Ménière's disease Ménière's disease (MD) is a disorder of the inner ear that is characterized by episodes of vertigo, tinnitus, hearing loss, and a fullness in the ear. Typically, only one ear is affected initially; however, over time both ears may become invo ...
, and
BPPV Benign paroxysmal positional vertigo (BPPV) is a disorder arising from a problem in the inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen and ...
. In addition, the function of the vestibular system can be affected by tumors on the
vestibulocochlear nerve The vestibulocochlear nerve (auditory vestibular nerve), known as the eighth cranial nerve, transmits sound and equilibrium (balance) information from the inner ear to the brain. Through olivocochlear fibers, it also transmits motor and modulat ...

vestibulocochlear 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 called
motion sickness Motion sickness occurs due to a difference between actual and expected motion. Symptoms commonly include nausea Nausea is a diffuse sensation of unease and discomfort, often perceived as an urge to vomiting, vomit. While not painful, it can ...
(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 called space sickness or
space adaptation syndrome Space adaptation syndrome (SAS) or space sickness is a condition experienced by as many as half of all space travelers during their adaptation to weightlessness Weightlessness is the complete or near-complete absence of the sensation of wei ...
. Either of these "sicknesses" usually ceases 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 possibly
nystagmus Nystagmus is a condition of involuntary (or voluntary, in some cases) eye movement, acquired in infancy or later in life, that in extremely rare cases may result in reduced or limited vision. Due to the involuntary movement of the eye, it has be ...
due to the variable viscosity of the blood and the endolymph during the consumption of alcohol. The term for this is positional alcohol nystagmus (PAN): * 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. 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.
Benign paroxysmal positional vertigo Benign paroxysmal positional vertigo (BPPV) is a disorder arising from a problem in the inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen an ...
(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 term ''vertigo'' 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 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. Vestibular dysfunction has also been found to correlate with cognitive and emotional disorders, including
depersonalization Depersonalization can consist of a detachment within the self, regarding one's mind or body, or being a detached observer of oneself. Subjects feel they have changed and that the world has become vague, dreamlike, less real, lacking in significanc ...
and
derealization Derealization is an alteration in the perception of the external world, causing sufferers to perceive it as unreal, distant, distorted or falsified. Other symptoms include feeling as if one's environment is lacking in spontaneity, emotional colori ...
.


Other vertebrates

Though humans as well as most other vertebrates exhibit three semicircular canals in their vestibular systems,
Lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of Agnatha, jawless fish of the order (biology), order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by ...

Lamprey
s and
Hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, a ...

Hagfish
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 saccula found in humans, but rather form one continuous structure referred to as the macula communis.


Other Vestibular systems

Birds Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

Birds
possess a ''second'' vestibular organ in the back, the lumbosacral canals. Behavioral evidence suggests that this system is responsible for stabilizing the body during
walking Walking (also known as ambulation) is one of the main gait Gait is the pattern of movement Movement may refer to: Common uses * Movement (clockwork), the internal mechanism of a timepiece * Motion (physics), commonly referred to as move ...

walking
and
standing Standing, also referred to as orthostasis, is a position in which the body is held in an ''erect'' ("orthostatic") position and supported only by the feet. Although seemingly static, the body rocks slightly back and forth from the ankle The a ...

standing
. The presence of this second system explains how many birds are capable of sleeping on one leg while putting their head under a wing.


Invertebrates

A large variety of vestibular organs are present in invertebrates. A well-known example are the halteres of
flies Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...

flies
(Diptera) which are modified hind wings.


See also

* Dark cell * Migraine-associated vertigo *
Statocyst The statocyst is a balance sensory receptor present in some aquatic invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This in ...

Statocyst


References


Further reading

* (Comment: For clinicians, and other professionals working with dizzy patients.) * (Comment: Research on driver or motion-induced sleepiness aka ' sopite syndrome' links it to the vestibular labyrinths.) * * (Comment: A book for experts, summarizing the state of the art in our understanding of the balance system) * Lawson, Ben D; Rupert, Angus H; Kelley, Amanda M
"Mental Disorders Comorbid with Vestibular Pathology"
A preview of an article on how vestibular disorders can cause symptoms that look like mental disorders.


External links


Vestibular Disorders Association
For more information about vestibular (inner ear balance) disorders.
(Video) Head Impulse Testing site
(vHIT) Site with thorough information about vHIT
SensesWeb
which contains animations of all sensory systems, and additional links.
Dizzytimes.com
Online Community for Sufferers of Vertigo and Dizziness.

''Neuroscience Online'' (electronic neuroscience textbook) {{Authority control Nervous system Sensory systems