Module 2 of 9

Anatomy & physiology of the VOR

Semicircular canals, otoliths, vestibular nerve divisions, brainstem nuclei, cerebellar circuits — the three-neuron arc that produces every HINTS finding.

The peripheral labyrinth

cochleautriclesacculehorizontal canalanterior canalposterior canalsuperior vestibular n.inferior vestibular n.
Horizontal semicircular canal — Detects rotation in the horizontal plane. Tilted ~30° upward from the orbitomeatal line; this is why the head is pitched forward 30° during the head impulse test, to bring the canal into the testing plane.
Click any structure to surface its description. The colour-coded sections show the membranous labyrinth (canals + otoliths + cochlea) and the bipartite vestibular nerve — the anatomical basis for superior vs inferior vestibular neuritis.

Each labyrinth contains three semicircular canals (horizontal, anterior, posterior) oriented in roughly orthogonal planes, and two otolith organs (utricle and saccule). Angular acceleration of the head deflects the cupula of the canal aligned with the plane of motion, generating an asymmetric firing pattern between paired canals on either side. Linear acceleration and head tilt deflect the otolith maculae.

The vestibular nerve

The vestibular nerve has two anatomically and functionally distinct divisions:

  • Superior division — innervates the horizontal and anterior canals, and the utricle.
  • Inferior division — innervates the posterior canal and the saccule.

This anatomical split matters clinically: superior vestibular neuritis is far more common than inferior, and the pattern of head-impulse abnormality and oVEMP / cVEMP findings follows the division involved.

The three-neuron VOR arc

brainstemhorizontal canalafferentvestibular nucleus(medial)abducens nucleus(contralateral)lateral rectus(eye muscle)via MLF → medial rectus
The horizontal VOR is a three-neuron arc. A pulse travels along it: from the canal afferent into the vestibular nucleus, across the midline to the contralateral abducens nucleus, and on to the lateral rectus — with an MLF branch to the medial rectus of the opposite eye. A peripheral lesion breaks link 1; a brainstem lesion breaks link 2 or 3 (and produces additional central signs).

The vestibulo-ocular reflex stabilises gaze during head movement. Its shortest pathway has just three neurons:

  1. Primary afferent from the canal → vestibular nucleus
  2. Vestibular nucleus → contralateral abducens nucleus
  3. Abducens → medial rectus (via the medial longitudinal fasciculus) and ipsilateral lateral rectus

The horizontal VOR arc is the substrate of the head-impulse test. A unilateral peripheral lesion produces a deficient VOR for ipsilesional head rotations, so the eyes lag behind, and the patient generates a corrective saccade to re-fixate the target.

Trainee

Central VOR connections relevant to HINTS

The vestibular nuclei project to the cerebellar flocculus and nodulus, which calibrate VOR gain and adapt it to context. Lesions of these structures — common in posterior circulation stroke — can produce direction-changing gaze-evoked nystagmus and abnormal smooth pursuit while leaving the brainstem VOR arc intact. This is why a normal head impulse in acute vestibular syndrome is a central red flag.

The interstitial nucleus of Cajal and rostral interstitial nucleus of the medial longitudinal fasciculus generate the vertical and torsional components of the VOR; lesions here produce vertical and torsional nystagmus, also central features.

Clinician

Otolith pathways and the test of skew

Skew deviation reflects an imbalance in the otolith-ocular pathway between the utricle, vestibular nuclei, interstitial nucleus of Cajal, and the ocular motor nuclei. Because this pathway crosses the brainstem, skew is sensitive to brainstem lesions but can occasionally be seen in severe peripheral utricular disease — a recognised pitfall of the test.