Vestibular system
This article may be too technical for most readers to understand.(June 2023) |
The vestibular system, in
As movements consist of rotations and translations, the vestibular system comprises two components: the
The brain uses information from the vestibular system in the head and from
Semicircular canal system
The semicircular canal system detects rotational movements. Semicircular canals are its main tools to achieve this detection.
Structure
Since the world is three-dimensional, the vestibular system contains three
- 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 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 the cupula which contains hair cells that transduce the mechanical movement to electrical signals.[1]
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.[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.
Vestibulo-ocular reflex (VOR)
The vestibular-ocular reflex (VOR) is a
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]
Mechanics
The mechanics of the semicircular canals can be described by a damped oscillator.[citation needed] If we designate the deflection of the cupula with , and the head velocity with , the cupula deflection is approximately[citation needed]
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[citation needed]. 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.
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. Signals sent to the cerebellum are relayed back as muscle movements of the head, eyes, and posture.
- To nuclei of cranial nerves III, IV, and VI. Signals sent to these nerves cause the vestibular-ocular reflex. They allow the eyes to fix on a moving object while staying in focus.
- To the 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. Signals sent to the spinal cord allow quick reflex reactions to both the limbs and trunk to regain balance.
- To the thalamus. Signals sent to the thalamus allow for head and body motor control as well as being conscious of body position.[3]
Otolithic organs
While the semicircular canals respond to rotations, the
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.[5] 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.[6]
Experience from the vestibular system
Experience from the vestibular system is called
Pathologies
Diseases of the vestibular system can take different forms and usually induce
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.[11]
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 - 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.[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.[12]
Vestibular dysfunction has also been found to correlate with cognitive and emotional disorders, including depersonalization and derealization.[13]
Other vertebrates
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.[14]
Invertebrates
A large variety of vestibular organs are present in invertebrates. A well-known example is the halteres of
See also
References
- )
- ^ Gold, Daniel. "Vestibular neuritis with and head impulse test and unidirectional nystagmus". Neuro-Ophthalmology Virtual Education Library (NOVEL): Daniel Gold Collection. Spencer S. Eccles Health Sciences Library. Retrieved 20 November 2019.
- ^ OCLC 799004854.
- ^ Vilis, Tutis (13 November 2018). "Balance" (PDF). The Physiology of the Senses.
- ^ Williams, S. Mark; McNamara, James O.; Lamantia, Anthony-Samuel; Katz, Lawrence C.; Fitzpatrick, David; Augustine, George J.; Purves, Dale (2001). "The Otolith Organs: The Utricle and Sacculus". NCBI Bookshelf - Neuroscience.
- PMID 18338968.
- ^ Lawson, B. D., & Riecke, B. E. (2014). The Perception of Body Motion. Handbook of Virtual Environments, CRC Press, 163-196.
- S2CID 16225002.
- PMID 24319930.
- ^ "Vertigo". University of Maryland Medical Center. Retrieved 13 November 2015.
- ^ "Ears May Hold Answers to the Mysteries of Sudden Infant Death Syndrome".
- S2CID 19090536.
- PMID 24194706.
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
- S2CID 54458839.
- S2CID 4046361.
- S2CID 1922751.
- S2CID 30019383.
Further reading
- Brandt, Thomas (2003). Vertigo: It's Multisensory Syndromes. Berlin: Springer. OCLC 52472049. (Comment: For clinicians, and other professionals working with dizzy patients.)
- Brill, Christopher; Hancock, Peter A.; Gilson, Richard D. (2003). "Driver Fatigue: Is Something Missing?" (PDF). University of Central Florida. (Comment: Research on driver or motion-induced sleepiness aka 'sopite syndrome' links it to the vestibular labyrinths.)
- Cullen, Soroush; Cullen, Kathleen (2008). "Vestibular system". Scholarpedia. 3 (1): 3013. .
- Highstein, S. M.; Fay, R. R.; Popper, A. N., eds. (2004). The vestibular system. Berlin: Springer. OCLC 56068617. (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 Archived 2007-05-20 at the Wayback Machine, which contains animations of all sensory systems, and additional links.
- Dizzytimes.com Archived 2010-08-05 at the Wayback Machine Online Community for people with vertigo and dizziness.
- Vestibular System, Neuroscience Online (electronic neuroscience textbook)