Editing Sense of balance (section)

=== 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>