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[Physiology](https://durgical.com/en/physiology/) · [Neurophysiology](https://durgical.com/en/physiology/#s-neuro)

# The vestibular system, tone and posture

- Mechanism
- Exam priority: medium

Sensors in the inner ear detect head turns and tilts, keep the eyes steady and keep the body upright.

Updated 2026-11-01

Also called: Vestibulo-ocular reflex, Labyrinth

**See it in 3D:** [Semicircular canals](https://durgical.com/en/anatomy/semicircular-canals/) · [Vestibule](https://durgical.com/en/anatomy/vestibule/) · [Vestibulocochlear nerve (VIII)](https://durgical.com/en/anatomy/vestibulocochlear-nerve/) · [Pons](https://durgical.com/en/anatomy/pons/) · [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/) · [Cerebellum](https://durgical.com/en/anatomy/anterior-quadrangular-lobule/)

## Key points

- Semicircular canals sense angular acceleration; utricle (horizontal) and saccule (vertical) sense linear acceleration and head tilt.
- Bending stereocilia toward the kinocilium opens K⁺ channels and depolarizes the hair cell; bending away hyperpolarizes it.
- Vestibulo-ocular reflex: head turns right, eyes move left at the same speed so the image stays still.
- Nystagmus is named by its fast phase. Caloric test: cold water, fast phase to the opposite side; warm, to the same side.
- Benign paroxysmal positional vertigo: loose otoconia in a canal (usually posterior) cause brief vertigo with position changes.

## What it is

The vestibular labyrinth has three semicircular canals for rotation and two otolith organs (utricle and saccule) for linear acceleration and gravity. Hair cells bathed in potassium-rich endolymph turn movement into signals for the vestibular nuclei, which steady the eyes (vestibulo-ocular reflex), keep extensor tone for posture and tell the cortex where up is.

## How it works, step by step

- Head rotation makes the endolymph lag behind because of inertia, bending the gelatinous cupula in the canal's ampulla.
- Stereocilia on hair cells bend; tip links open mechanotransduction channels and K⁺ from endolymph flows in.
- The hair cell depolarizes, releases more glutamate and vestibular nerve fibres raise their resting firing rate.
- The canals on the two sides work as push–pull pairs: one side fires more, the opposite fewer.
- Vestibular nuclei drive the abducens and oculomotor nuclei (via the medial longitudinal fasciculus) to move the eyes opposite to the head.
- Otolith crystals (otoconia) shift with tilt or linear acceleration and bend utricle and saccule hair cells.
- The lateral vestibulospinal tract boosts antigravity extensor tone; the cerebellum (flocculonodular lobe) tunes the reflexes.

## Useful numbers

| Parameter | Value | Note |
| --- | --- | --- |
| Semicircular canals per ear | 3 (anterior, posterior, lateral) | roughly perpendicular planes |
| Endolymph K⁺ | ≈150 mmol/L | similar to intracellular fluid |
| Perilymph K⁺ | ≈5 mmol/L | similar to CSF |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Clinical notes

- Ménière disease: endolymphatic hydrops with episodes of vertigo, fluctuating hearing loss, tinnitus and ear fullness.
- Vestibular neuritis: sudden prolonged vertigo with horizontal nystagmus and normal hearing.
- Absent caloric and oculocephalic responses in coma point to brainstem failure.
- Motion sickness comes from conflict between vestibular and visual signals.

## Memory aid

COWS: Cold Opposite, Warm Same, for the fast phase of caloric nystagmus.

## Test yourself

Think of the answer, then tap the question to see it.

## When the head turns quickly to the right, which way do the eyes move and why?

To the left: the right horizontal canal is excited, and vestibular nuclei drive the left lateral rectus and right medial rectus so the gaze stays fixed (vestibulo-ocular reflex).

## Why does hair cell depolarization depend on potassium entering rather than sodium?

Stereocilia sit in endolymph, which is rich in K⁺ and electrically positive, so opening transduction channels lets K⁺ flow in and depolarizes the cell.

## Cold water in the right ear of an awake person produces nystagmus with the fast phase to which side?

To the left. Cold causes a slow drift toward the irrigated ear and a corrective fast phase away from it (Cold Opposite).

## Why does BPPV cause vertigo only for seconds after lying down or turning in bed?

Loose otoconia move within a semicircular canal when the head changes position, dragging endolymph and bending the cupula until they settle.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/the-vestibular-system-tone-and-posture/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Semicircular canals](https://durgical.com/en/anatomy/semicircular-canals/)
- [Vestibule](https://durgical.com/en/anatomy/vestibule/)
- [Vestibulocochlear nerve (VIII)](https://durgical.com/en/anatomy/vestibulocochlear-nerve/)
- [Pons](https://durgical.com/en/anatomy/pons/)
- [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/)
- [Cerebellum](https://durgical.com/en/anatomy/anterior-quadrangular-lobule/)

## Related topics

- [Functions of the cerebellum](https://durgical.com/en/physiology/functions-of-the-cerebellum/)
- [Hearing: middle ear, cochlea and auditory pathway](https://durgical.com/en/physiology/hearing-middle-ear-cochlea-and-auditory-pathway/)
- [The visual pathway, pupillary reflexes and eye movements](https://durgical.com/en/physiology/the-visual-pathway-pupillary-reflexes-and-eye-movements/)
- [Motor cortex and the corticospinal tract](https://durgical.com/en/physiology/motor-cortex-and-the-corticospinal-tract/)

## Related conditions

- [Benign paroxysmal positional vertigo (BPPV)](https://durgical.com/en/conditions/benign-paroxysmal-positional-vertigo/)
- [Ménière disease](https://durgical.com/en/conditions/meniere-disease/)
- [Vestibular schwannoma (acoustic neuroma)](https://durgical.com/en/conditions/vestibular-schwannoma/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [14.1 Sensory Perception](https://openstax.org/books/anatomy-and-physiology-2e/pages/14-1-sensory-perception/) (OpenStax (Rice University), 2022); [Physiology, Vestibular System](https://www.ncbi.nlm.nih.gov/books/NBK532978/) (StatPearls (NCBI Bookshelf), 2024); [Benign Paroxysmal Positional Vertigo](https://www.ncbi.nlm.nih.gov/books/NBK470308/) (StatPearls (NCBI Bookshelf), 2024); [Meniere Disease](https://www.ncbi.nlm.nih.gov/books/NBK536955/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 56). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/the-vestibular-system-tone-and-posture/