![]() | This article may be too technical for most readers to understand.(June 2023) |
The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, 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, which indicate rotational movements; and the otoliths, which indicate linear accelerations. The vestibular system sends signals primarily to the neural structures that control eye movement; 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 throughout the body to enable the animal to understand its body's dynamics and 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 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 three semicircular canals in each labyrinth. They are approximately orthogonal (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.
The movement of fluid pushes on a structure called the cupula 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 a push-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 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.
The vestibular-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 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, and s 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 of polysaccharides with Young'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 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.
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, the otolithic organs sense linear accelerations. Humans have two otolithic organs on each side, one called the utricle, the other called the saccule. The utricle contains a patch of hair cells and supporting cells called a macula. Similarly, the saccule contains a patch of hair cells and a macula. Each hair cell of a macula has forty to seventy stereocilia and one true cilium called a kinocilium. 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 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 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 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 sense 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 sense she is 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 the righting 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 induce vertigo [ citation needed ] [13] 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, Ménière's disease, and BPPV. In addition, the vestibular system's function can be affected by tumours on the vestibulocochlear nerve, an infarct in the brain stem or in cortical regions related to the processing of vestibular signals, and cerebellar atrophy.
Since the function of the vestibular hair cells is to detect levels of carbon dioxide in the blood and to transmit such information to the brain, a loss of vestibular hair cells can cause death. [14]
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 (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. 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 possibly nystagmus 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 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 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 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. [15]
Vestibular dysfunction has also been found to correlate with cognitive and emotional disorders, including depersonalization and derealization. [16]
Though humans as well as most other vertebrates exhibit three semicircular canals in their vestibular systems, lampreys and 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 saccule found in humans, but rather form one continuous structure referred to as the macula communis. [17]
Birds possess a second vestibular organ in the back, the lumbosacral canals. [18] [19] Behavioral evidence suggests that this system is responsible for stabilizing the body during walking and standing. [20]
A large variety of vestibular organs are present in invertebrates. A well-known example is the halteres of flies (Diptera) which are modified hind wings.
The inner ear is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. 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:
The sense of balance or equilibrioception is the perception of balance and spatial orientation. It helps prevent humans and nonhuman animals from falling over when standing or moving. Equilibrioception is the result of a number of sensory systems working together; the eyes, the inner ears, and the body's sense of where it is in space (proprioception) ideally need to be intact.
The semicircular canals are three semicircular interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the lateral, anterior and posterior semicircular canals. They are the part of the bony labyrinth, a periosteum-lined cavity on the petrous part of the temporal bone filled with perilymph.
The vestibulo-ocular reflex (VOR) is a reflex that acts to stabilize gaze during head movement, with eye movement due to activation of the vestibular system, it is also known as the Cervico-ocular reflex. The reflex acts to stabilize images on the retinas of the eye during head movement. Gaze is held steadily on a location by producing eye movements in the direction opposite that of head movement. For example, when the head moves to the right, the eyes move to the left, meaning the image a person sees stays the same even though the head has turned. Since slight head movement is present all the time, VOR is necessary for stabilizing vision: people with an impaired reflex find it difficult to read using print, because the eyes do not stabilise during small head tremors, and also because damage to reflex can cause nystagmus.
The utricle and saccule are the two otolith organs in the vertebrate inner ear. The word utricle comes from Latin uter 'leather bag'. The utricle and saccule are part of the balancing system in the vestibule of the bony labyrinth. They use small stones and a viscous fluid to stimulate hair cells to detect motion and orientation. The utricle detects linear accelerations and head-tilts in the horizontal plane.
The saccule is a bed of sensory cells in the inner ear. The saccule is from Latin saccus 'sack'. The saccule 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 sensory cells of the saccule are disturbed and the neurons connected to them begin transmitting impulses to the brain. These impulses travel along the vestibular portion of the eighth cranial nerve to the vestibular nuclei in the brainstem.
An ear is the organ that enables hearing and body balance using the vestibular system. In mammals, the ear is usually described as having three parts: the outer ear, the middle ear and the inner ear. The outer ear consists of the pinna and the ear canal. Since the outer ear is the only visible portion of the ear in most animals, the word "ear" often refers to the external part alone. The middle ear includes the tympanic cavity and the three ossicles. The inner ear sits in the bony labyrinth, and contains structures which are key to several senses: the semicircular canals, which enable balance and eye tracking when moving; the utricle and saccule, which enable balance when stationary; and the cochlea, which enables hearing. The ear canal is cleaned via earwax, which naturally migrates to the auricle. The ears of vertebrates are placed somewhat symmetrically on either side of the head, an arrangement that aids sound localization.
Benign paroxysmal positional vertigo (BPPV) is a disorder arising from a problem in the inner ear. Symptoms are repeated, brief periods of vertigo with movement, characterized by a spinning sensation upon changes in the position of the head. This can occur with turning in bed or changing position. Each episode of vertigo typically lasts less than one minute. Nausea is commonly associated. BPPV is one of the most common causes of vertigo.
An otolith, also called statoconium, otoconium or statolith, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of vertebrates. The saccule and utricle, in turn, together make the otolith organs. These organs are what allows an organism, including humans, to perceive linear acceleration, both horizontally and vertically (gravity). They have been identified in both extinct and extant vertebrates.
The vestibular nerve is one of the two branches of the vestibulocochlear nerve. In humans the vestibular nerve transmits sensory information transmitted by vestibular hair cells located in the two otolith organs and the three semicircular canals via the vestibular ganglion of Scarpa. Information from the otolith organs reflects gravity and linear accelerations of the head. Information from the semicircular canals reflects rotational movement of the head. Both are necessary for the sensation of body position and gaze stability in relation to a moving environment.
Human senses are not naturally geared for the inflight environment. Pilots may experience disorientation and loss of perspective, creating illusions that range from false horizons to sensory conflict with instrument readings or the misjudging of altitude over water.
The ampullary cupula, or cupula, is a structure in the vestibular system, providing the sense of spatial orientation.
The otolithic membrane is a fibrous structure located in the vestibular system of the inner ear. It plays a critical role in the brain's interpretation of equilibrium. The membrane serves to determine if the body or the head is tilted, in addition to the linear acceleration of the body. The linear acceleration could be in the horizontal direction as in a moving car or vertical acceleration such as that felt when an elevator moves up or down.
In aviation, a graveyard spiral is a type of dangerous spiral dive entered into accidentally by a pilot who is not trained or not proficient in flying in instrument meteorological conditions (IMC). Other names for this phenomenon include suicide spiral, deadly spiral, death spiral and vicious spiral.
The crista ampullaris is the sensory organ of rotation. They are found in the ampullae of each of the semicircular canals of the inner ear, meaning that there are three pairs in total. The function of the crista ampullaris is to sense angular acceleration and deceleration.
The Epley maneuver or repositioning maneuver is a maneuver used by medical professionals to treat one common cause of vertigo, benign paroxysmal positional vertigo (BPPV) of the posterior or anterior canals of the ear. The maneuver works by allowing free-floating particles, displaced otoconia, from the affected semicircular canal to be relocated by using gravity, back into the utricle, where they can no longer stimulate the cupula, therefore relieving the patient of bothersome vertigo. The maneuver was developed by the physician John M. Epley, and was first described in 1980.
The vestibular evoked myogenic potential is a neurophysiological assessment technique used to determine the function of the otolithic organs of the inner ear. It complements the information provided by caloric testing and other forms of inner ear testing. There are two different types of VEMPs. One is the oVEMP and another is the cVEMP. The oVEMP measures integrity of the utricule and superior vestibular nerve and the cVemp measures the saccule and the inferior vestibular nerve.
The righting reflex, also known as the labyrinthine righting reflex, or the Cervico-collic reflex; is a reflex that corrects the orientation of the body when it is taken out of its normal upright position. It is initiated by the vestibular system, which detects that the body is not erect and causes the head to move back into position as the rest of the body follows. The perception of head movement involves the body sensing linear acceleration or the force of gravity through the otoliths, and angular acceleration through the semicircular canals. The reflex uses a combination of visual system inputs, vestibular inputs, and somatosensory inputs to make postural adjustments when the body becomes displaced from its normal vertical position. These inputs are used to create what is called an efference copy. This means that the brain makes comparisons in the cerebellum between expected posture and perceived posture, and corrects for the difference. The reflex takes 6 or 7 weeks to perfect, but can be affected by various types of balance disorders.
The leans is the most common type of spatial disorientation for aviators. Through stabilization of the fluid in the semicircular canals, a pilot may perceive straight and level flight while actually in a banked turn. This is caused by a quick return to level flight after a gradual, prolonged turn that the pilot failed to notice. The phenomenon consists of a false perception of angular displacement about the roll axis and therefore becomes an illusion of bank. This illusion is often associated with a vestibulospinal reflex that results in the pilot actually leaning in the direction of the falsely perceived vertical. Other common explanations of the leans are due to deficiencies of both otolith-organ and semicircular-duct sensory mechanisms.
The ocular tilt reaction (OTR) comprises skew deviation, head tilt and ocular torsion involving structures of the inner ear responsible for maintenance of balance of the body i.e. the semi-circular canals (SCC), utricle and saccule.
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: 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