Vestibular Physiology

An interactive teaching atlas of vestibular physiology and the assessment of vertigo. Content synthesized from Dr Prahlada N.B's Physiology of the Vestibular System and peer-reviewed sources.

Dr Prahlada N.B · Karnataka ENT Hospital and Research Centre (R) · Champions Educational and Medical Society (R) · Amogh Foundation

1. Introduction & Overview

The vestibular system is an evolutionarily preserved apparatus for balance, posture, spatial orientation, and the coordination of eye movement. Understanding its physiology is the foundation for diagnosing and managing vertigo.

Deep in each inner ear sits a set of tiny sensors that detect how the head moves and where it is in space. They feed the brainstem and cerebellum, which respond by steadying the eyes and adjusting posture so the world stays stable as you move.

The vestibular apparatus combines peripheral and central inputs to perceive head motion and to align gaze, most visibly through the vestibulo-ocular reflex. The peripheral sensors are the three semicircular canals, which respond to angular acceleration, and the two otolith organs — utricle and saccule — which detect linear acceleration and head tilt. Their signals reach the brainstem and cerebellum, which generate reflexive responses to stabilize gaze and maintain equilibrium ²⁸.

A working grasp of vestibular physiology underpins the diagnosis of benign paroxysmal positional vertigo, vestibular neuritis, bilateral vestibulopathy, and central vestibular syndromes. The same physiology explains the diagnostic tests — the video head impulse test, caloric irrigation, and vestibular evoked myogenic potentials — each of which probes a specific limb of the system¹³.

The vestibular apparatus sits within the inner ear, alongside the cochlea. Sound is conducted through the external canal, tympanic membrane, and ossicles, while the labyrinth is bathed in inner-ear fluids that transduce head motion independently of hearing.

The membranous labyrinth: three semicircular canals arranged near-orthogonally, the vestibule housing the utricle and saccule, and the cochlea. Endolymph fills the entire space; ampullae mark the canal bases where the sensory cristae sit.

How this atlas is organised

Modules move from evolution and embryology through fluid homeostasis, hair-cell transduction, the canal and otolith sensors, afferent coding, the vestibulo-ocular and vestibulospinal reflexes, cerebellar modulation, and visual–vestibular integration, closing with clinical correlates. Every module is layered for Foundation, Trainee, and Clinician readers.

2. Evolution & Embryology of the Labyrinth

The labyrinth’s sealed, fluid-filled form is an evolutionary solution to a physical problem: how to transduce motion reliably once an animal leaves the water.

The balance organ grows from a small patch of embryonic skin that folds inward to form a hollow ball, which then develops into the coiled cochlea, the three canals, and the two otolith organs — all enclosed and filled with fluid.

The vestibular apparatus arises from the ectodermal otic placode, which invaginates to form the otic pit and then pinches off as the otic vesicle. The vesicle matures into the membranous labyrinth: the cochlea, the semicircular canals, the utricle, and the saccule².

Mechanosensory hair cells most likely first evolved in water-dwelling vertebrates to detect water current. Packaging them inside a labyrinth created a shielded endolymphatic compartment, and that compartmentalization permitted the ionic regulation that reliable transduction depends on — an indispensable adaptation for non-aquatic life, where environmental water ions are not available for direct sensory interfacing ³.

The ectodermal otic placode invaginates to form the otic pit, which closes off as the otic vesicle. The vesicle then differentiates into the full membranous labyrinth — three canals, two otolith organs, and the cochlea.

Five sensors per side

The mature peripheral apparatus holds five vestibular sensors in each temporal bone — three semicircular canals and two otolith organs — all bathed in endolymph. The modules that follow take each in turn.

3. Labyrinth Fluid Spaces & Ionic Homeostasis

Two fluids, two ionic worlds. The chemistry of endolymph and perilymph is what makes hair-cell transduction fast and sensitive.

The inner ear holds two different fluids. The one bathing the tips of the hair cells is unusually rich in potassium. That special chemistry is what lets the hair cells generate a strong, quick signal when they are bent.

The labyrinth is compartmentalized into endolymph and perilymph. Endolymph, bathing the apical hair-cell surface, is rich in potassium and low in sodium; perilymph resembles ordinary extracellular fluid, high in sodium. The potassium-rich environment generates the depolarizing current that drives hair-cell excitation. The gradient is maintained actively by non-sensory “dark cells” near the sensory epithelium, whose ATP-dependent pumps hold potassium in the endolymph against its gradient ⁴.

Beyond ionic concentration there is a positive endolymphatic potential relative to perilymph, most marked over the maculae and cristae. The resulting electrochemical gradient increases the driving force for cation influx when stereocilia deflect. Both the high potassium and the positive potential are required for sharp mechanoelectrical transduction ⁵. Disturbances of endolymph homeostasis are central to the pathophysiology of endolymphatic hydrops.

Two electrochemically distinct fluids fill the labyrinth: K⁺-rich endolymph inside the membranous compartment, and Na⁺-rich perilymph outside. Dark cells actively pump K⁺ into the endolymph against its gradient. The combined chemical gradient and positive endolymphatic potential drive the current that depolarises hair cells.

Why homeostasis matters clinically

Because transduction depends on a precisely maintained chemical environment, conditions that disrupt endolymph volume or composition translate directly into sensory dysfunction — the link explored further in the clinical correlates module.

4. Hair Cells & Mechanoelectric Transduction

Vestibular hair cells are the elementary sensory receptors that convert the mechanical deflection of head movement into an electrical signal. Everything the vestibular system reports begins here.

Each hair cell carries a tuft of fine hairs called stereocilia, with one taller hair, the kinocilium, at one edge. Bending the tuft towards the kinocilium switches the cell on — it sends more signals to the brain. Bending it the other way switches it down. At rest the cell already fires steadily, so it can report movement in both directions.

Hair cells come in two types. Flask-shaped Type I cells sit in the central zones of the cristae and maculae and are enclosed by a single afferent calyx; cylindrical Type II cells lie more peripherally and synapse with multiple bouton terminals⁶. Each bundle holds roughly 40–200 stereocilia of graded height alongside one kinocilium, which defines the cell’s axis of directional sensitivity. Deflection towards the kinocilium opens mechanically-gated channels at the stereocilia tips; potassium flows in from the K⁺-rich endolymph, the cell depolarizes, and afferent firing increases. Deflection away closes the channels and the cell hyperpolarizes ⁷.

Directional sensitivity follows a cosine tuning curve: response amplitude scales with the cosine of the angle between the stimulus direction and the cell’s polarity axis. The receptor potential reaches roughly 20 mV, and transduction current can peak near 200 pA at maximal deflection, giving high-resolution encoding of both angular and linear acceleration depending on hair-cell location⁸. Type I cells, with their calyceal synapse and predominantly irregular afferents, support the fast, high-frequency channel that clinical tests such as VEMP and the video head impulse test depend upon ¹⁶.

Two morphologies. Type I cells, flask-shaped, are enclosed by a single calyceal afferent and dominate the irregular, dynamically sensitive channel. Type II cells are cylindrical and contact multiple bouton terminals, driving the regular, tonic channel.

Resting. Deflection towards the kinocilium opens mechanically-gated channels; K⁺ influx depolarizes the cell and increases afferent firing. Deflection away closes them.

Mechanoelectric transduction

Transduction is fast and graded rather than all-or-none. Because the cell holds a resting discharge, the afferent can encode the direction of head motion as an increase or decrease from baseline — the foundation of the bidirectional, push-pull signalling seen across the labyrinth¹.

5. Semicircular Canals

The semicircular canals are the sensors of angular acceleration. Their geometry and hydrodynamics let the labyrinth report head rotation about any axis in space.

There are three canals on each side, set at roughly right angles to one another, so between them they cover every direction a head can turn. When the head rotates, the fluid inside a canal lags behind and pushes on a flexible flap, the cupula. Bending the cupula bends the hair cells beneath it, and the canal reports the turn.

The three canals — horizontal, anterior, and posterior — lie in approximately orthogonal planes. The horizontal canal is tilted about 30° up from the true horizontal, which is why the head is pitched forward for caloric testing ⁹. At the base of each canal the crista ampullaris carries the hair cells, and the cupula seals the ampullary lumen above them. Canals are paired coplanarly between the ears, so each works against a partner on the opposite side, enabling push-pull signalling: excitation on one side is mirrored by inhibition on the other ¹.

Canal dynamics are described by two time constants — a short constant for the cupula’s immediate deflection and a longer constant, roughly 5–10 s, for its return to rest. Central processing in the vestibular nuclei and nodulus extends the response through the velocity-storage mechanism, so per-rotational nystagmus can persist for 18–30 s after a sustained turn¹⁰. Velocity storage is itself a clinically relevant target: it shapes caloric responses and is modulated by the cerebellar nodulus.

The three canals lie in approximately orthogonal planes. The horizontal canal is tilted ~30° above the true horizontal — the reason the head is pitched forward 30° to align it with gravity for caloric testing.

The three coplanar pairings across the two ears: the two horizontal canals together, the left anterior with the right posterior, and the left posterior with the right anterior. Excitation on one side mirrors inhibition on the other — the push-pull foundation of vestibular signalling.

Inside each ampulla, hair cells of the crista project into the gelatinous cupula, which seals the ampullary lumen. Endolymph movement bends the cupula and, with it, the hair-cell stereocilia.

Coplanar canals signal as a push-pull pair. A head turn excites one ampulla and inhibits its contralateral partner; the brainstem reads the difference. At rest both fire near 90 spikes/s.

Two time constants describe canal mechanics. T₁ is the short constant of cupular deflection at rotation onset — fast enough to follow rapid head movement. T₂ is the long constant of passive return to rest, roughly 5–10 seconds. Central velocity storage extends the effective response well beyond T₂.

Canal hydrodynamics & velocity storage

Because the cupula has the same density as the surrounding endolymph, it responds to angular acceleration rather than to gravity. The velocity-storage mechanism then integrates and prolongs that signal, improving spatial orientation during sustained motion and extending the window over which gaze can be stabilized ¹⁰.

6. Otolith Organs

The utricle and saccule sense linear acceleration and head tilt. Where the canals report rotation, the otolith organs report gravity and translation.

Two small organs in each ear sense straight-line movement and tilt. They carry tiny crystals that lag behind when you accelerate, tugging on hair cells underneath. One organ is tuned to side-to-side and forward motion, the other to up-and-down motion.

The utricular and saccular maculae are specialized sensory epithelia for linear acceleration and head attitude relative to gravity. The utricle lies roughly horizontally and responds mainly to fore-aft and lateral motion; the saccule lies roughly vertically and responds to up-down acceleration and head tilt. Each macula contains a striola, a curved band where hair-cell polarity reverses, giving the organ multidirectional sensitivity within its plane ²⁰.

Macular hair cells sit beneath a gelatinous membrane loaded with otoconia — calcium carbonate crystals with a density near 2.7 g/cm³. Their inertia lags during linear acceleration, shifting the membrane and flexing the stereocilia. The system responds near-linearly up to about 1 g, with gel damping that limits post-acceleration oscillation ¹⁵. Displaced otoconia entering a semicircular canal are the mechanical basis of benign paroxysmal positional vertigo.

The utricular macula lies roughly horizontally — best at sensing fore-aft and lateral linear motion — while the saccular macula lies roughly vertically, ideal for vertical acceleration and head tilt. Together they cover linear motion in all three dimensions.

Calcium-carbonate otoconia sit on a gelatinous membrane above the macular hair cells. Head tilt or linear acceleration shifts the otoconial mass, dragging the gel and bending the hair cells beneath. The striola is a curved central line where hair-cell polarity reverses.

The striola is a curved central band where hair-cell polarity reverses. Cells on either side point their kinocilia toward the striola, so any single linear acceleration excites one population and inhibits the other — giving the macula bidirectional sensitivity across its plane.

Macular afferent coding

As in the canals, regular afferents signal sustained tilt with a steady rate, while irregular afferents respond to dynamic linear acceleration, their gain rising with frequency — together encoding both static orientation and rapid translation ¹.

7. Vestibular Afferent Neurons

Vestibular afferents fire continuously, even at rest. That baseline is what lets a single fibre report motion in either direction.

The nerve fibres leaving the balance organ are never silent — they tick along steadily all the time. When the head moves, that rate speeds up or slows down, and the brain reads the change.

Primary vestibular afferents originate at the hair cells of the canals and otolith organs and project to the vestibular nuclei and cerebellum. They are spontaneously active, firing at roughly 10–200 spikes per second even under static conditions. That resting discharge makes vestibular output bidirectional: the rate rises or falls with the direction of head movement¹¹.

Afferents fall into two physiological classes. Regular afferents have a stable inter-spike interval and low variability, preferentially innervate Type II hair cells, and faithfully convey low-frequency, steady-state motion. Irregular afferents fire variably, innervate Type I cells, and are dynamically sensitive with a large phase lead at high frequencies, making them well suited to brief, high-speed head motion ⁸. The irregular channel is the substrate for VEMP and high-acceleration head-impulse responses.

The vestibular branch of cranial nerve VIII splits into two divisions. The superior carries afferents from the anterior and lateral canals and the utricle; the inferior carries the posterior canal and the saccule. Cell bodies sit in Scarpa’s ganglion before entering the brainstem.

Regular afferents fire with a near-constant inter-spike interval, faithfully conveying steady-state or low-frequency motion. Irregular afferents fire variably, with high-frequency phase lead — well suited to detecting brief, high-acceleration head movements. Together they form complementary tonic and phasic channels into the vestibular nuclei.

The four vestibular nuclei sit beneath the floor of the fourth ventricle. The superior and medial nuclei dominate VOR output to the ocular muscles via the medial longitudinal fasciculus, the medial nucleus feeds the MVST for head and neck control, the lateral nucleus drives the LVST for postural tone, and the inferior nucleus shares heavy traffic with the cerebellum.

A complementary pair

Regular and irregular afferents are not redundant: the regular channel transmits prolonged or slowly varying stimuli, while the irregular channel delivers short, high-gain responses to sharp perturbations¹³.

8. Vestibulo-Ocular Reflex

The vestibulo-ocular reflex keeps vision steady during head movement. It is among the fastest reflexes in the body and one of the most clinically informative.

When your head turns, your eyes automatically roll the opposite way by the same amount, so whatever you are looking at stays in focus. This happens in a few thousandths of a second, without any conscious effort.

The VOR stabilizes gaze by generating eye movements opposite to head movement, keeping visual targets on the fovea. It operates with a latency of only 10–12 milliseconds, achieved through a three-neuron arc — vestibular hair cell, vestibular nucleus, ocular motor nucleus. It can compensate for head rotations up to about 350° per second ¹⁷.

The horizontal VOR routes through the abducens nucleus and medial longitudinal fasciculus; an MLF lesion produces internuclear ophthalmoplegia. The vertical and torsional components are driven by the anterior and posterior canals via specific extraocular muscle pairs. A separate translational VOR, driven by the otolith organs, compensates for linear motion and is vergence-dependent, with gain scaled to target distance ²¹. The reflex’s short latency and clear anatomy make it an excellent diagnostic target, assessed by the video head impulse test ¹⁸.

Horizontal VOR. A leftward head turn excites the left horizontal canal, which drives the contralateral abducens nucleus and, via the MLF, the ipsilateral medial rectus — moving both eyes rightward to keep gaze on target.

Vertical VOR pairings. The anterior canal drives the ipsilateral superior rectus and the contralateral inferior oblique to elevate gaze; the posterior canal drives the ipsilateral superior oblique and contralateral inferior rectus to depress it.

The vestibulo-ocular reflex is one of the fastest reflexes in the body — at 10–12 ms it is more than an order of magnitude faster than a voluntary saccade or visual reaction. That speed lets it cancel head motion before any visual blur can develop.

The three-neuron arc

The brevity of the VOR comes from its economy: just three synapses separate the labyrinth from the eye muscles. Each canal pushes its agonist muscles while inhibitory interneurons relax the antagonists, producing smooth, conjugate compensatory movement ¹⁹.

9. Vestibulospinal Reflexes

While the VOR steadies the eyes, the vestibulospinal reflexes steady the body — adjusting muscle tone in the neck, trunk, and limbs to keep you upright.

The same balance organ that moves your eyes also helps you stand without falling. It sends signals down the spinal cord that tense or relax muscles in the neck, back, and legs to keep you steady.

The vestibulospinal tracts — the medial and lateral vestibulospinal tracts — integrate vestibular input with spinal proprioception to control posture. They regulate muscle tone in the neck, trunk, and limbs in response to destabilizing forces. The vestibulocollic reflex uses vestibular feedback to stabilize head position through neck muscle activation ²².

Polysynaptic connections between the vestibular nuclei and the cervical spinal cord support rapid, adaptable postural responses. These reflexes are critical for gait stability and fall prevention, especially in older adults and in patients with vestibular impairment³². Loss of vestibulospinal function contributes to the unsteadiness seen in bilateral vestibulopathy.

The medial vestibulospinal tract (MVST) projects to the cervical cord, stabilising head and neck position. The lateral vestibulospinal tract (LVST) descends the length of the cord, regulating extensor tone for trunk and limb posture against gravity.

Vestibulocollic reflex. A perturbation that tips the head triggers an opposing contraction of the sternocleidomastoid and other neck muscles, driven from the vestibular nuclei through the medial vestibulospinal tract — restoring head position within milliseconds.

Posture as an integrated output

Postural control is never vestibular alone: the vestibulospinal system blends otolith, semicircular, proprioceptive, and visual cues into a single coordinated motor output ³⁴.

10. Cerebellar Modulation & Central Integration

The vestibular system does not run open-loop. The cerebellum continuously recalibrates its reflexes, and central neuron classes transform raw signals into timed motor commands.

The balance reflexes can be re-tuned. If they start to drift — after an illness, or with new glasses — the cerebellum, a control centre at the back of the brain, gradually corrects them using feedback from the eyes.

The flocculus, nodulus, and vermis receive vestibular input and shape the VOR. The flocculus sends inhibitory Purkinje-cell signals back to the vestibular nuclei, adjusting VOR gain — the ratio of eye to head movement — according to retinal slip, the drift of the visual image during head motion ²³.

Cerebellar plasticity is driven by climbing fibres from the inferior olive, which signal retinal slip and trigger adaptive change in Purkinje-cell output, recalibrating the VOR after unilateral vestibular loss ³⁰. Floccular lesions abolish this gain adaptation ²⁵. Converting head-velocity signals into eye-position signals requires a distributed neural integrator — prepositus hypoglossi, vestibular nuclei, cerebellum — whose failure produces gaze-evoked nystagmus²⁷.

The cerebellar adaptation loop. Retinal slip — the error signal — is carried by climbing fibres from the inferior olive to Purkinje cells in the flocculus, which inhibit the vestibular nuclei and adjust VOR gain. The corrected reflex reduces the slip on the next head movement.

Central vestibular neuron types

Central vestibular neurons split into firing classes. PVP cells encode head velocity but pause during saccades; burst-tonic cells generate a rapid burst followed by a sustained tonic discharge that holds eye position; eye-head velocity neurons integrate the two; vestibular-only neurons carry head signal without eye coupling; pause cells fall silent between saccades.

Position-vestibular-pause neurons encode head velocity and pause during saccades; burst-tonic neurons integrate velocity and position; eye-head-velocity and floccular target neurons fine-tune the VOR and smooth pursuit; vestibular-only neurons serve posture and perception²⁶.

11. Visual–Vestibular Integration & Eye–Head Coordination

Stable perception is a team effort. Vestibular signals are continuously cross-checked against vision and proprioception, and the brain suppresses or re-engages the VOR as a gaze shift demands.

Your sense of balance works together with your eyes and your body’s position sense. When you deliberately look somewhere new, the brain briefly switches off the automatic eye reflex so it does not fight the movement, then switches it back on.

A mismatch between expected and actual visual input — retinal slip — is a powerful stimulus for adjusting VOR gain, letting the brain recalibrate motor output from visual feedback. During large voluntary gaze shifts the eyes saccade first and the head follows; the vestibular system briefly suppresses the VOR at gaze-shift onset, then re-engages it to stabilize the new view ³³.

This recalibration is governed by the flocculus and paraflocculus, where convergent vestibular and visual inputs meet. Such plasticity is essential during sensorimotor learning — adapting to prism glasses or virtual reality — and during functional recovery after vestibular injury ²⁹. Eye-head coordination itself integrates semicircular, otolith, cervical proprioceptive, and visual signals within the brainstem, superior colliculus, and cerebellum ³².

When the head turns but the eyes need to fix on a moving target that moves with the head, the smooth-pursuit system cancels the VOR — the eyes move with the head, not against it. Visual and vestibular inputs compete and cooperate so that the brain stabilises on whichever reference matters in context.

Balance is never vestibular alone. Three sensory streams — vestibular, visual, and somatosensory — converge on brainstem and cerebellar integrators that produce coordinated motor output to the eyes, neck, and limbs. A mismatch among the streams underlies many forms of chronic dizziness.

Implication for rehabilitation

Because the system is plastic, vestibular rehabilitation therapy can exploit it: gaze-stabilization drills that deliberately drive retinal slip promote central adaptation and functional recovery³¹.

Gaze-stabilisation exercises (VOR×1) ask the patient to keep eyes on a target while turning the head. Any residual retinal slip is the error signal the cerebellum uses to retune the reflex, gradually restoring gain after a vestibular injury.

12. Clinical Correlates & Vestibular Testing

Each diagnostic test probes a specific limb of vestibular physiology. Reading a test result well means knowing which structure it interrogates.

Doctors test the balance system in a few different ways — checking how the eyes respond to quick head turns, to warm or cool water in the ear, and to sound. Each test looks at a different part of the system.

The video head impulse test assesses VOR performance at high frequencies and identifies a deficient semicircular canal. Caloric testing evaluates the horizontal canal and reveals side-to-side asymmetry. Vestibular evoked myogenic potentials assess otolith function — cervical VEMP for the saccule, ocular VEMP for the utricle ¹³.

Mapping physiology to disease: benign paroxysmal positional vertigo reflects displaced otoconia within a canal; vestibular neuritis is an acute unilateral afferent hypofunction; endolymphatic hydrops disturbs the fluid homeostasis of Module 3; central syndromes implicate the brainstem and cerebellar circuits of Module 10. Treatment leans on plasticity — vestibular rehabilitation therapy recalibrates reflex pathways through tailored gaze-stabilization exercises³⁴.

Nystagmus. A slow vestibular drift in one direction is interrupted by a rapid corrective saccade. The direction the fast phase beats names the nystagmus. Pattern and behaviour with gaze localise the cause.

Downbeat nystagmus — points to a cerebellar / craniocervical lesion (often Chiari, or floccular).

Head impulse test. Normally the eye velocity mirrors the head, keeping gain near 1 with no corrective saccade. When the canal is deficient the eye lags, and a visible catch-up saccade appears once the head impulse ends — a localising sign of peripheral hypofunction.

Caloric test. Warm and cool irrigations of each ear induce nystagmus whose peak slow-phase velocity reflects horizontal-canal function. A symmetric butterfly indicates intact canals; a small-amplitude side signals a unilateral peripheral weakness.

Pure-tone audiogram. All thresholds at or below 25 dB HL.

cVEMP pathway. A loud acoustic click excites saccular hair cells; the signal travels via the inferior vestibular nerve and medial vestibulospinal tract to inhibit the ipsilateral sternocleidomastoid, generating the biphasic P13-N23 myogenic response.

The Dix-Hallpike provokes posterior-canal BPPV. The patient sits, the examiner turns the head 45° toward the suspect ear, then lays the patient back rapidly with the head hanging extended. A latent, fatigable upbeat-torsional nystagmus confirms the diagnosis.

Disease mechanisms at a glance

Acute vestibular neuritis. Inflammation of one vestibular nerve silences afferent input on the affected side, leaving the intact side unopposed. The resting asymmetry drives a unidirectional spontaneous nystagmus beating away from the lesion, the head impulse test is abnormal toward the lesion, and hearing is spared.

Canalithiasis. Otoconia displaced from the utricular macula enter a semicircular canal — most often the posterior canal — and settle gravitationally. When the head moves, the debris drifts through the endolymph, abnormally deflecting the cupula and producing the brief, position-triggered vertigo of BPPV.

Two pathologies underlie BPPV. In canalithiasis, free-floating otoconia drift through the endolymph and briefly deflect the cupula — characteristically with a latency and a vertigo lasting under a minute. In cupulolithiasis, debris adheres to the cupula itself, making it gravity-sensitive and producing a sustained, persistent nystagmus on positioning.

The Epley manoeuvre treats posterior-canal BPPV by sequentially rotating the head so otoconia migrate around the canal and exit into the utricle. Each position is held until nystagmus settles before the next step.

Endolymphatic hydrops. When endolymph homeostasis is disturbed the membranous compartment distends, raising pressure on both cochlear and vestibular sensors. The resulting fluctuating hearing, tinnitus, fullness, and episodic vertigo define Ménière’s syndrome.

A vestibular schwannoma arises on the vestibular branch of CN VIII within the internal auditory canal. Because it grows slowly, the brain compensates for vestibular asymmetry, so the dominant complaints are unilateral progressive hearing loss and tinnitus rather than vertigo.

Superior canal dehiscence is a bony gap in the roof of the superior semicircular canal. The third mobile window allows sound and pressure to deflect the canal endolymph, producing the Tullio phenomenon (sound-induced vertigo) and an autophony of internal sounds.

Vestibular migraine does not destroy the vestibular periphery. Instead, cortical spreading depression and trigeminovascular activation sensitise central vestibular pathways, producing episodic vertigo that may occur with, before, or apart from headache.

Bilateral vestibulopathy. With both sides equally reduced there is no asymmetry to drive a spontaneous nystagmus, but the VOR fails on every head movement. Vision bounces with the head — oscillopsia — and balance becomes visually dependent, sharply worse in the dark.

Medial longitudinal fasciculus. Internuclear ophthalmoplegia — failed adduction, abducting-eye nystagmus.

From physiology to the bedside

The diagnostic toolkit is rooted in VOR dynamics and eye-head interaction; interpreting it well is simply applied vestibular physiology ¹⁸. The clinical cases and conditions sections work through these patterns in detail.

Glossary

Afferent neurons: Sensory neurons transmitting signals from hair cells to the brain, classified into regular (tonic) and irregular (phasic) types by firing pattern and sensitivity.
Angular acceleration: A change in rotational velocity, sensed by the semicircular canals of the vestibular system.
Burst-tonic (BT) neurons: Central neurons encoding both eye velocity and position, with a phasic burst followed by a sustained tonic discharge.
Caloric test: A vestibular diagnostic that stimulates the horizontal canal with warm or cold irrigation of the external auditory canal, inducing nystagmus.
Climbing fibers: Afferents from the inferior olive that synapse with Purkinje cells, carrying error signals such as retinal slip for motor learning in the cerebellum.
Cosine tuning curve: The directional response profile of hair cells and afferent neurons, with maximal response when deflection aligns with the cell's polarity axis.
Dark cells: Specialized cells in the vestibular labyrinth that maintain high potassium levels in endolymph, crucial for mechanoelectric transduction.
Endolymph: Potassium-rich fluid within the membranous labyrinth that deflects the cupula or otolithic membrane during head motion.
Hair cells: Sensory receptors of the vestibular system in the ampullae and maculae, bearing stereocilia and a kinocilium that detect fluid motion.
Head impulse test (HIT): A bedside test of VOR function that detects semicircular canal deficits by observing corrective saccades during rapid head turns.
Inferior olive: A brainstem nucleus involved in motor learning that sends climbing fibers to the cerebellum for VOR recalibration.
Irregular afferents: Primary vestibular neurons with variable discharge and high dynamic sensitivity, connected to Type I hair cells and tuned to rapid motion.
Neural integrator: A network in the prepositus hypoglossi, vestibular nuclei, and cerebellum that converts eye-velocity signals into position commands to hold gaze.
Otoconia: Calcium carbonate crystals resting on the otolithic membrane; their inertia drives hair-cell deflection during linear motion.
Otolith organs: The utricle and saccule, which sense linear acceleration and head tilt with respect to gravity.
Purkinje cells: The principal inhibitory neurons of the cerebellar cortex, modulating vestibular nucleus output based on cerebellar inputs.
Retinal slip: The motion of an image across the retina, detected as an error signal used to adapt and recalibrate the VOR.
Semicircular canals: Three orthogonal fluid-filled tubes — horizontal, anterior, posterior — that sense angular acceleration of the head.
Velocity storage mechanism: A central circuit that prolongs the VOR response beyond the physical stimulus, mediated by the vestibular nuclei and cerebellum.
Vestibular nuclei (VN): Brainstem nuclei — superior, medial, lateral, inferior — that receive labyrinthine afferent input and modulate vestibular reflexes.
Vestibulo-ocular reflex (VOR): A reflex that stabilizes images on the retina during head movement by producing compensatory eye movements.
Vestibulospinal reflex (VSR): A reflex that controls postural muscles in response to vestibular input to maintain balance and orientation.

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