Editing Sense of balance

{{Short description|Physiological sense regarding posture}}
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[[File:Balance.JPG|thumb|200px|Balance skill development in children]]
[[File:Plank on two medicine balls.JPG|thumb|Balance training using [[medicine ball]]s]]
The '''sense of balance''' or '''equilibrioception''' is the [[perception]] of [[balance (ability)|balance]] and [[Orientation (geometry)|spatial orientation]].<ref name="wol">{{Cite book |last=Wolfe |first=Jeremy |title=Sensation & perception |last2=Kluender |first2=Keith |last3=Levi |first3=Dennis |publisher=[[Sinauer Associates]] |year=2012 |isbn=978-0878935727 |edition=3rd |page=7}}</ref> It helps prevent [[human]]s and nonhuman [[animal]]s from falling over when standing or moving. Equilibrioception is the result of a number of [[sensory systems]] working together; the eyes ([[visual system]]), the inner ears ([[vestibular system]]), and the body's sense of where it is in space ([[proprioception]]) ideally need to be intact.<ref name="wol" />

The vestibular system, the region of the inner ear where three semicircular canals converge, works with the visual system to keep objects in focus when the head is moving. This is called the [[Vestibulo-ocular reflex|vestibulo-ocular reflex (VOR)]]. The balance system works with the visual and [[skeletal system]]s (the muscles and joints and their sensors) to maintain orientation or balance. Visual signals sent to the [[brain]] about the body's position in relation to its surroundings are processed by the brain and compared to information from the vestibular and skeletal systems.

==Vestibular system==
{{Main|Vestibular system}}
[[File:Balance Disorder Illustration A.png|thumb|right|Diagram of [[vestibular system]]]]
In the vestibular system, equilibrioception is determined by the level of a [[fluid]] called [[endolymph]] in the [[labyrinth (inner ear)|labyrinth]], a complex set of tubing in the inner ear.

==Dysfunction==
{{Main|Balance disorder}}
[[File:Balance Disorder Illustration C.png|thumb|left|This figure shows nerve activity associated with rotational-induced [[physiologic nystagmus]] and spontaneous nystagmus resulting from a lesion of one labyrinth. Thin straight arrows show direction of slow components, thick straight arrows show direction of fast components, and curved arrows show direction of [[endolymph]] flow in the horizontal [[semicircular canals]]. The three semicircular canals are marked AC (anterior canal), PC (posterior canal), and HC (horizontal canal).]]When the sense of balance is interrupted it causes [[dizziness]], [[Orientation (mental)|disorientation]] and [[nausea]]. Balance can be upset by [[Ménière's disease]], [[superior canal dehiscence syndrome]], an [[inner ear infection]], by a bad [[common cold]] affecting the head or a number of other medical conditions including but not limited to [[vertigo]]. It can also be temporarily disturbed by quick or prolonged [[acceleration]], for example, riding on a [[merry-go-round]]. Blows can also affect equilibrioreception, especially those to the side of the head or directly to the ear.

Most [[astronaut]]s find that their sense of balance is impaired when in orbit because they are in a constant state of [[weightlessness]]. This causes a form of [[motion sickness]] called [[space adaptation syndrome]].

==System overview==
[[File:Comprehensive List of Relevant Pathways for the Balance & Acceleration Systems.png|thumb|This diagram linearly (unless otherwise mentioned) tracks the projections of all known structures that allow for balance and acceleration to their relevant endpoints in the human brain.]]
[[File:Vestibular balance system.jpg|thumb|Another diagram showing neural pathway of [[Vestibular system|vestibular]]/balance system. Arrows show the direction of information relay.]] This overview also explains acceleration as its processes are interconnected with balance.

=== Mechanical ===
There are five [[sensory organs]] [[innervated]] by the [[vestibular nerve]]; three [[semicircular canals]] ([[Semicircular canals|Horizontal SCC]], [[Semicircular canals|Superior SCC]], [[Semicircular canals#Posterior semicircular canal|Posterior SCC]]) and two [[otolith]] organs ([[saccule]] and [[Utricle (ear)|utricle]]). Each semicircular canal (SSC) is a thin tube that doubles in thickness briefly at a point called [[osseous ampullae]]. At their center-base, each contains an [[ampullary cupula]]. The cupula is a gelatin bulb connected to the [[stereocilia]] of hair cells, affected by the relative movement of the [[endolymph]] it is bathed in.{{cn|date=January 2025}}

Since the cupula is part of the [[bony labyrinth]], it rotates along with actual head movement, and by itself without the endolymph, it cannot be stimulated and therefore, could not detect movement. Endolymph follows the rotation of the canal; however, due to [[inertia]] its movement initially lags behind that of the bony labyrinth. The delayed movement of the endolymph bends and activates the cupula. When the cupula bends, the connected stereocilia bend along with it, activating chemical reactions in the hair cells surrounding [[crista ampullaris]] and eventually create [[action potentials]] carried by the vestibular nerve signaling to the body that it has moved in space.{{cn|date=January 2025}}

After any extended rotation, the endolymph catches up to the canal and the cupula returns to its upright position and resets. When extended rotation ceases, however, endolymph continues, (due to inertia) which bends and activates the cupula once again to signal a change in movement.<ref>Seeley, R., VanPutte, C., Regan, J., & Russo, A. (2011). ''Seeley's Anatomy & Physiology (9th ed.). New York: [[McGraw Hill]]{{ISBN?}}{{page?|date=August 2022}}</ref>

Pilots doing long banked turns begin to feel upright (no longer turning) as endolymph matches canal rotation; once the pilot exits the turn the cupula is once again stimulated, causing the feeling of turning the other way, rather than flying straight and level.

The horizontal SCC handles head rotations about a vertical axis (e.g. looking side to side), the superior SCC handles head movement about a lateral axis (e.g. head to shoulder), and the posterior SCC handles head rotation about a [[Anatomical terms of location#Cranial and caudal|rostral-caudal]] axis (e.g. nodding). SCC sends adaptive signals, unlike the two otolith organs, the saccule and utricle, whose signals do not adapt over time.{{Citation needed | date = May 2019 | reason = Purves et al.'s 2008 ''Neuroscience'' p. 346 indicates all vestibular hair cells rapidly adapt to maintain sensitivity to accelerations despite constant input from gravitational forces that are million times larger. Or please clarify.}}

A shift in the [[otolithic membrane]] that stimulates the cilia is considered the state of the body until the cilia are once again stimulated. For example, lying down stimulates cilia and standing up stimulates cilia, however, for the time spent lying the signal that you are lying remains active, even though the membrane resets.

[[Otolithic organs]] have a thick, heavy gelatin membrane that, due to inertia (like endolymph), lags behind and continues ahead past the [[Macula of utricle|macula]] it overlays, bending and activating the contained cilia.

[[Utricle (ear)|Utricle]] responds to linear accelerations and head-tilts in the horizontal plane (head to shoulder), whereas [[saccule]] responds to linear accelerations and head-tilts in the vertical plane (up and down). Otolithic organs update the brain on the head-location when not moving; SCC update during movement.<ref>Albertine, Kurt. Barron's Anatomy Flash Cards</ref><ref>"How Does Our Sense of Balance Work?" How Does Our Sense of Balance Work?[[U.S. National Library of Medicine]], January 12, 2012.</ref><ref>"Semicircular Canals." Semicircular Canals Function, Definition & Anatomy. Healthline Medical Team, January 26, 2015.</ref><ref>Tillotson, Joanne. McCann, Stephanie. ''Kaplan's Medical Flashcards''. April 2, 2013.</ref>

[[Kinocilium]] are the longest stereocilia and are positioned (one per 40-70 regular cilia) at the end of the bundle. If stereocilia go towards kinocilium, [[depolarization]] occurs, causing more [[neurotransmitter]]s, and more vestibular nerve firings, as compared to when stereocilia tilt away from kinocilium ([[Hyperpolarization (biology)|hyperpolarization]], less neurotransmitter, less firing).<ref>Spoor, Fred, and Theodore Garland, Jr. "The Primate Semicircular Canal System and Locomotion."  May 8, 2007.</ref><ref>Sobkowicz, H.M., and S.M. Slapnick. "The Kinocilium of Auditory Hair Cells and Evidence for Its Morphogenet." Ic Role during the Regeneration of Stereocilia and Cuticular Plates. Sept. 1995.</ref>

=== Neural ===
First order [[vestibular nuclei]] (VN) project to [[lateral vestibular nucleus]] (IVN), [[medial vestibular nucleus]] (MVN), and [[superior vestibular nucleus]] (SVN).{{Clarify|date=July 2022}}

The [[inferior cerebellar peduncle]] is the largest center through which balance information passes. It is the area of integration between [[proprioceptive]], and vestibular inputs, to aid in unconscious maintenance of balance and posture.

The [[inferior olivary nucleus]] aids in complex [[Motor skill|motor tasks]] by encoding coordinating timing sensory information; this is decoded and acted upon in the [[cerebellum]].<ref>Mathy, Alexandre, and Sara S.N. Ho. "Encoding of Oscillations by Axonal Bursts in Inferior Olive Neurons." [[ScienceDirect|''Science Direct'']]. May 14, 2009. Web. March 28, 2016.</ref>

The [[cerebellar vermis]] has three main parts. The [[vestibulocerebellum]] regulates eye movements by the integration of visual info provided by the [[superior colliculus]] and balance information. The [[spinocerebellum]] integrates visual, auditory, proprioceptive, and balance information to act out body and limb movements. It receives input from the [[trigeminal nerve]], dorsal column (of the [[spinal cord]]), [[midbrain]], [[thalamus]], [[reticular formation]] and vestibular nuclei ([[Medulla oblongata|medulla]]) outputs{{Clarify|reason=What outputs?|date=July 2022}}. Lastly, the [[cerebrocerebellum]] plans, times, and initiates movement after evaluating sensory input from, primarily, motor cortex areas, via [[pons]] and cerebellar [[dentate nucleus]]. It outputs to the thalamus, [[motor cortex]] areas, and [[red nucleus]].<ref>Chen, S.H. Annabel, and John E. Desmond. "Cerebrocerebellar Networks during Articulatory Rehearsal and Verbal Working Memory Tasks." ''Science Direct''. January 15, 2005. Web. March 28, 2016.</ref><ref>Barmack, Neil H. "Central Vestibular System: Vestibular Nuclei and Posterior Cerebellum." ''Science Direct''. June 15, 2003. Web. March 28, 2016.</ref><ref>Akiyama, K., and S. Takazawa. "Bilateral Middle Cerebellar Peduncle Infarction Caused by Traumatic Vertebral Artery Dissection." [[The Journal of Neuroscience|''JNeurosci'']]. March 1, 2001. March 28, 2016.</ref>

The [[flocculonodular lobe]] is a cerebellar lobe that helps maintain body equilibrium by modifying [[muscle tone]] (the continuous and passive muscle contractions).

MVN and IVN are in the medulla, LVN and SVN are smaller and in pons. SVN, MVN, and IVN ascend within the [[medial longitudinal fasciculus]]. LVN descend the spinal cord within the [[lateral vestibulospinal tract]] and ends at the [[sacrum]]. MVN also descend the spinal cord, within the [[medial vestibulospinal tract]], ending at [[Lumbar vertebrae|lumbar 1]].<ref>Gdowski, Greg T., and Robert A. McCrea. "Integration of Vestibular and Head Movement Signals in the Vestibular Nuclei During Whole-Body Rotation. 01 July 1999. Web. 28 Mar. 2016.</ref><ref>Roy, Jefferson E., and [[Kathleen E. Cullen]]. "Dissociating Self-Generated from Passively Applied Head Motion: Neural Mechanisms in the Vestibular Nuclei." ''JNeurosci''. March 3, 2004. Web. March 28, 2016.</ref>

The [[thalamic reticular nucleus]] distributes information to various other thalamic nuclei, regulating the flow of information. It is speculatively able to stop signals, ending transmission of unimportant info. The thalamus relays info between pons (cerebellum link), motor cortices, and [[Insular cortex|insula]].

The insula is also heavily connected to motor cortices; the insula is likely where balance is likely brought into perception.

The [[Oculomotor nucleus|oculomotor nuclear complex]] refers to fibers going to [[Midbrain tegmentum|tegmentum]] (eye movement), red nucleus ([[gait]] (natural limb movement)), [[substantia nigra]] (reward), and [[cerebral peduncle]] (motor relay). Nucleus of Cajal are one of the named oculomotor nuclei, they are involved in eye movements and reflex gaze coordination.<ref>Takagi, Mineo, and David S. Zee. "Effects of Lesions of the Oculomotor Cerebellar Vermis on Eye Movements in Primate: Smooth Pursuit." April 1, 2000</ref><ref>Klier, Eliana M., and Hongying Wang. "Interstitial Nucleus of Cajal Encodes Three-Dimensional Head Orientations in Fick-Like Coordinates." Articles, January 1, 2007.</ref>

The [[abducens nerve]] solely innervates the [[lateral rectus muscle]] of the eye, moving the eye with the [[trochlear nerve]]. The trochlear solely innervates the [[superior oblique muscle]] of the eye. Together, trochlear and abducens contract and relax to simultaneously direct the pupil towards an angle and depress the globe on the opposite side of the eye (e.g. looking down directs the pupil down and depresses (towards the brain) the top of the globe). The pupil is not only directed, but often rotated, by these muscles. (See [[visual system]])

The thalamus and superior colliculus are connected via the [[lateral geniculate nucleus]]. The superior colliculus (SC) is the [[Topographic map (neuroanatomy)|topographical map]] for balance and quick orienting movements with primarily visual inputs. SC integrates multiple senses.<ref>May, Paul J. "The Mammalian Superior Colliculus: Laminar Structure and Connections." ''Science Direct''. 2006.</ref><ref>Corneil, Brian D., and Etienne Olivier. "Neck Muscle Responses to Stimulation of Monkey Superior Colliculus. I. Topography and Manipulation of Stimulation Parameters." October 1, 2002. Web. March 28, 2016.</ref>[[File:Balance Disorder Illustration B.png|thumb|Illustration of the flow of fluid in the ear, which in turn causes displacement of the top portion of the hair cells that are embedded in the jelly-like cupula. Also shows the [[Utricle (ear)|utricle]] and [[saccule]] organs that are responsible for detecting linear acceleration, or movement in a straight line.]]

==Other animals==
Some animals have better equilibrioception than humans; for example, a [[cat]] uses its [[inner ear]] and [[tail]] to walk on a thin [[fence]].<ref>{{Cite web |title=Equilibrioception |url=https://www.sciencedaily.com/articles/e/equilibrioception.htm |url-status=live |archive-url=https://web.archive.org/web/20110518202058/http://www.sciencedaily.com/articles/e/equilibrioception.htm |archive-date=May 18, 2011 |access-date=January 15, 2011 |website=[[ScienceDaily]]}}</ref>

Equilibrioception in many marine animals is done with an entirely different organ, the [[statocyst]], which detects the position of tiny [[calcareous]] stones to determine which way is "up".

==In plants==
{{main|Gravitropism}}

Plants could be said to exhibit a form of equilibrioception, in that when rotated from their normal attitude the stems grow in the direction that is upward (away from gravity) while their roots grow downward (in the direction of gravity). This phenomenon is known as [[gravitropism]] and it has been shown that, for example, [[Populus|poplar]] stems can detect reorientation and inclination.<ref>{{Cite journal |last=Azri |first=W. |last2=Chambon |first2=C. |last3=Herbette |first3=S. P. |last4=Brunel |first4=N. |last5=Coutand |first5=C. |last6=Leplé |first6=J. C. |last7=Ben Rejeb |first7=I. |last8=Ammar |first8=S. |last9=Julien |first9=J. L. |last10=Roeckel-Drevet |first10=P. |year=2009 |title=Proteome analysis of apical and basal regions of poplar stems under gravitropic stimulation |journal=[[Physiologia Plantarum]] |volume=136 |issue=2 |pages=193–208 |bibcode=2009PPlan.136..193A |doi=10.1111/j.1399-3054.2009.01230.x |pmid=19453506}}</ref>

== See also ==
* [[Proprioception]]
* [[Vertigo]]

==References==
{{Reflist}}

==External links==
{{Sensation and perception}}

{{Authority control}}

[[Category:Vestibular system]]
[[Category:Sensory systems]]
[[Category:Motor control]]