Vestibular Anatomy and Physiology: Peripheral End Organs, Central Pathways, and VOR Logic
Peripheral architecture
The labyrinth pairs three semicircular canals (horizontal, anterior, posterior) with two otolith organs (utricle, saccule). Canals sense angular acceleration; the utricle and saccule sense linear acceleration and head tilt via hair cells whose stereocilia and kinocilium define a preferred direction. Each canal is functionally paired with a coplanar partner on the opposite side (horizontal with horizontal; right anterior with left posterior; right posterior with left anterior), so the push-pull arrangement converts head rotation into reciprocal firing changes. The horizontal canal lies roughly 30 degrees off horizontal, which is why the Dix-Hallpike head hang and the supine roll test use specific head rotations.
Blood supply and the VOR
The labyrinth is supplied by the internal auditory artery, usually a branch of the anterior inferior cerebellar artery (AICA); abrupt vertigo with hearing loss and brainstem signs should raise AICA-territory infarction. The vestibulo-ocular reflex stabilizes gaze: canal afferents drive the vestibular nuclei, which project through the medial longitudinal fasciculus to abducens and oculomotor nuclei. For a right horizontal canal excitation, the eyes move left; a rightward head turn produces a leftward slow phase and a rightward corrective quick phase (nystagmus named for the quick phase).
Central pathways
Vestibular nuclei project to the cerebellum (flocculus/nodulus for gain calibration), to the thalamus and cortex (parieto-insular vestibular cortex for perception), and down the vestibulospinal tracts for posture. Velocity storage in the brainstem prolongs the response after rotation stops.
Common traps
Questions conflate ampullofugal versus ampullopetal flow; in the horizontal canal, ampullopetal (utriculopetal) endolymph flow is excitatory, while in vertical canals the opposite is true. Another trap is naming nystagmus by slow phase, or assuming all vertigo is canal-mediated when otolith dysfunction causes tilt illusion.
Rule of thumb
Pair the canals: excitation of one produces slow eye movement in the plane of its partner's inhibition. If a question gives head direction, draw the canal plane before choosing an answer.