Vestibular system
Encyclopedia
Balance (ability)
In biomechanics, balance is an ability to maintain the center of gravity of a body within the base of support with minimal postural sway. When exercising the ability to balance, one is said to be balancing....
in most mammals and to the sense of spatial orientation, is the sensory system
Sensory system
A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...
that provides the leading contribution about movement
Motor coordination
thumb|right|Motor coordination is shown in this animated sequence by [[Eadweard Muybridge]] of himself throwing a diskMotor coordination is the combination of body movements created with the kinematic and kinetic parameters that result in intended actions. Such movements usually smoothly and...
and sense of balance. Together with the cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
, a part of the auditory system
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...
, it constitutes the labyrinth of the inner ear in most mammals, situated in the vestibulum
Vestibule of the ear
-Definition:The vestibule is the central part of the osseous labyrinth, and is situated medial to the tympanic cavity, behind the cochlea, and in front of the semicircular canals.The etymology comes from the Latin vestibulum, literally an entrance hall....
in the inner ear
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
(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 otolith
Otolith
An otolith, , also called statoconium or otoconium is a structure in the saccule or utricle of the inner ear, specifically in the vestibular labyrinth of vertebrates. The saccule and utricle, in turn, together make the otolith organs. They are sensitive to gravity and linear acceleration...
s, 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
Vestibulo-ocular reflex
The vestibulo-ocular reflex 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...
, which is required for clear vision; and the projections to the muscles that control our posture are necessary to keep us upright.
Semicircular canal system
The semicircular canal system detects rotational movements. The semicircular canals are its main tools to achieve this detection.Structure
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 horizontalHorizontal semicircular canal
The lateral or horizontal canal is the shortest of the three canals. Movement of fluid within this canal corresponds to rotation of the head around a vertical axis The lateral or horizontal canal (external semicircular canal) is the shortest of the three canals. Movement of fluid within this canal...
(or lateral), the anterior semicircular canal (or superior) and the posterior
Posterior semicircular canal
The posterior semicircular canal is a part of the vestibular system and detects rotations of the head in the sagittal plane.-Structure:It is directed superiorly, as per its nomenclature, and posteriorly, nearly parallel to the posterior surface of the petrous bone...
(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 planeSagittal planeSagittal plane is a vertical plane which passes from front to rear dividing the body into right and left sections.-Variations:Examples include:...
(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
Cupula
The cupula is a structure in the vestibular system, providing proprioception.The cupula is located within the ampullae of each of the three semicircular canals...
, which contains hair cells that transduct the mechanical movement to electrical signals
Push-pull systems
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 (VOR) is a reflex
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
eye movement that stabilizes images on the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
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 thedeflection of the cupula with
, 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 organ
While the semicircular canals respond to rotations, the otolithic organs sense linear accelerations. We have two on each side, one called utricle, the other sacculeSaccule
The saccule is a bed of sensory cells situated in the inner ear. The saccule translates head movements into neural impulses which the brain can interpret. The saccule is sensitive to linear translations of the head, specifically movements up and down...
. 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.
We 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, causing differential sensory inputs from the hair cell bundles allow us 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 equilibrioceptionEquilibrioception
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 , ears and the body's sense of where it is in space ideally...
. It is mainly used for the sense of balance
Balance (ability)
In biomechanics, balance is an ability to maintain the center of gravity of a body within the base of support with minimal postural sway. When exercising the ability to balance, one is said to be balancing....
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
Elevator
An elevator is a type of vertical transport equipment that efficiently moves people or goods between floors of a building, vessel or other structures...
, with essentially constant visual input, will feel she is descending as the elevator starts to descend.
Pathologies
Diseases of the vestibular system can take different forms, and usually induce vertigoVertigo (medical)
Vertigo is a type of dizziness, where there is a feeling of motion when one is stationary. The symptoms are due to a dysfunction of the vestibular system in the inner ear...
and
instability, often accompanied by nausea. The most common ones are Vestibular neuritis, a related condition called Labyrinthitis
Labyrinthitis
Labyrinthitis is an inflammation of the inner ear, and a form of unilateral vestibular dysfunction. It derives its name from the labyrinths that house the vestibular system . Labyrinthitis can cause balance disorders....
, 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
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.External links
- (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.
- Vestibular System, Neuroscience Online (electronic neuroscience textbook)