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 · Vestibule · Vestibulocochlear nerve (VIII) · Pons · Medulla oblongata · Cerebellum
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.
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.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Functions of the cerebellum
- Hearing: middle ear, cochlea and auditory pathway
- The visual pathway, pupillary reflexes and eye movements
- Motor cortex and the corticospinal tract
Related conditions
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 (OpenStax (Rice University), 2022); Physiology, Vestibular System (StatPearls (NCBI Bookshelf), 2024); Benign Paroxysmal Positional Vertigo (StatPearls (NCBI Bookshelf), 2024); Meniere Disease (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).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.