The Vestibular Labyrinth
Three orthogonal canals, two otolith organs, and a single shared rule: hair cells deflected one way fire faster, the other way fire slower. Everything that follows in VNG is built on this asymmetry.
The membranous labyrinth is suspended in and filled with . When the head rotates in a given plane, the endolymph — by inertia — lags briefly, deflecting the in the corresponding canal's .[McCaslin 2013]
That deflection bends the embedded in the cupula, modulating their .[Goldberg & Fernández 1971] The brain reads the difference between left and right vestibular output — never the absolute rate — which is why a unilateral lesion produces such dramatic asymmetry.
The hair cell
Even at rest, vestibular hair cells fire tonically at ~90 spikes/s. This baseline allows the system to encode movement bidirectionally.
Mechanotransduction in vestibular hair cells follows the gradient of stereocilia: deflection toward the kinocilium opens potassium channels via tip links, depolarising the cell and increasing afferent firing.[Hudspeth & Corey 1977] Deflection in the opposite direction closes these channels and hyperpolarises the cell.
Eye movement occurs in the plane of the canal being stimulated. Horizontal canal stimulation → horizontal nystagmus.
In horizontal canals, ampullopetal (toward ampulla) flow produces a stronger response than ampullofugal flow. Excitation outweighs inhibition.
In vertical canals, the reverse holds: ampullofugal flow excites more strongly than ampullopetal. This matters in BPPV interpretation.