Editing Vestibular system (section)

==Semicircular canal system==
[[File:Cochlea and vestibular system.gif|thumbnail|Cochlea and vestibular system]]
The semicircular canal system detects rotational movements. Semicircular canals are its main tools to achieve this detection.

===Structure===
{{Main|Semicircular canal}}

Since the world is three-dimensional, the vestibular system contains three [[semicircular canals]] in each [[Labyrinth (inner ear)|labyrinth]]. They are approximately [[orthogonal]] (at right angles) to each other, and are the ''[[Horizontal semicircular canal|horizontal]]'' (or ''lateral''), the ''[[anterior semicircular canal]]'' (or ''superior''), and the ''[[posterior semicircular canal|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]].
# The '''anterior''' and '''posterior''' semicircular canals detect rotations of the head in the [[sagittal plane]] (as when nodding), and in the [[coronal plane|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 the [[Ampullary cupula|cupula]] which contains hair cells that transduce the mechanical movement to electrical signals.<ref name="Boron 2005">{{cite book |author=[[Emile Boulpaep|Boulpaep, Emile L.]]; [[Walter Boron|Boron, Walter F.]] |title=Medical physiology: a cellular and molecular approach |publisher=Elsevier Saunders |location=St. Louis, Mo |year=2005 |isbn=978-1-4160-2328-9 |oclc=56963726 }}</ref>

===Push-pull systems===
[[Image:Vestibular PushPull.svg|thumb|right|300px|Push-pull system of the semicircular canals, for a horizontal head movement to the right.]]

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|date=May 2022}}

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)===
{{Main|Vestibulo-ocular reflex}}
[[Image:Simple vestibulo-ocular reflex.PNG|thumb|300px|The vestibulo-ocular reflex. A rotation of the head is detected, which triggers an inhibitory signal to the [[extraocular muscles]] on one side and an excitatory signal to the muscles on the other side. The result is a compensatory movement of the eyes.]]

The '''vestibular-ocular reflex''' ('''VOR''') is a [[reflex]] [[Eye movement (sensory)|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.<ref>{{Cite web|url=https://collections.lib.utah.edu/ark:/87278/s6546h55|title=Vestibular neuritis with and head impulse test and unidirectional nystagmus|last=Gold|first=Daniel|website=Neuro-Ophthalmology Virtual Education Library (NOVEL): Daniel Gold Collection. Spencer S. Eccles Health Sciences Library.|access-date=20 November 2019}}</ref>

===Mechanics===

The mechanics of the semicircular canals can be described by a damped oscillator. If we designate the deflection of the cupula with <math>\theta</math>, and the head velocity with <math>\dot q</math>, the cupula deflection is approximately<ref name=":0">{{Cite journal |last1=Buskirk |first1=W. C. Van |last2=Watts |first2=R. G. |last3=Liu |first3=Y. K. |date=November 1976 |title=The fluid mechanics of the semicircular canals |url=https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/abs/fluid-mechanics-of-the-semicircular-canals/A75DA3D0F571ECF009E1B85B8D755713 |journal=Journal of Fluid Mechanics |language=en |volume=78 |issue=1 |pages=87–98 |doi=10.1017/S0022112076002346 |bibcode=1976JFM....78...87V |issn=1469-7645|url-access=subscription }}</ref>

: <math>\theta (s) = \frac{\alpha s}{(T_1 s+1)(T_2 s+1)} \dot{q} (s)</math>

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,<ref name=":0" /> and it is a jelly mostly made of [[Polysaccharide|polysaccharides]] with [[Young's modulus]] <math>5.4\; \mathrm{Pa}</math>.<ref>{{Cite journal |last1=Selva |first1=Pierre |last2=Oman |first2=Charles M. |last3=Stone |first3=Howard A. |date=2010-04-30 |title=Mechanical properties and motion of the cupula of the human semicircular canal |url=https://www.medra.org/servlet/aliasResolver?alias=iospress&doi=10.3233/VES-2009-0359 |journal=Journal of Vestibular Research |volume=19 |issue=3–4 |pages=95–110 |doi=10.3233/VES-2009-0359|pmid=20448336 |url-access=subscription }}</ref> 

T<sub>1</sub> is the characteristic time required for the cupula to accelerate until it reaches terminal velocity, and T<sub>2</sub> 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 T<sub>1</sub> is very small compared to T<sub>2</sub>. For humans, the time constants T<sub>1</sub> and T<sub>2</sub> are approximately 5 ms and 20 s, respectively.<ref>{{Cite journal |last1=Xu |first1=Mingyu |last2=Tan |first2=Wenchang |date=2000-05-01 |title=The problem of fluid-dynamics in semicircular canal |url=https://doi.org/10.1007/BF02897143 |journal=Science in China Series A: Mathematics |language=en |volume=43 |issue=5 |pages=517–526 |doi=10.1007/BF02897143 |bibcode=2000ScChA..43..517X |issn=1862-2763|url-access=subscription }}</ref> As a result, for typical head movements, which cover the frequency range of 0.1&nbsp;Hz and 10&nbsp;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===
[[File:Vestibular balance system.jpg|thumb|[[Neural pathway]] of the vestibular system]]
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 [[Oculomotor nerve|III]], [[Trochlear nerve|IV]], and [[Abducens nerve|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.<ref name="Saladin 2011">{{cite book |author=Saladin, Kenneth S. |title=Anatomy & Physiology: The Unity of Form and Function |publisher=McGraw-Hill |location=New York |year=2011 |isbn=978-0-07-337825-1 |oclc=799004854  }}</ref>