Anatomy & Physiology
The membranous labyrinth, the two divisions of the vestibular nerve, and the reflex arcs that turn sound into a measurable muscle potential.
The vestibular labyrinth
The inner ear has two parts. The cochlea handles hearing. The vestibular labyrinth handles balance, and contains two kinds of organ:
- Three semicircular canals — sense rotation.
- Two otolith organs — the saccule and utricle — sense gravity and linear acceleration. These are the organs VEMPs test.
The two divisions of the vestibular nerve
The vestibular nerve splits into two divisions before entering the brainstem — an anatomical fact that makes topographic VEMP diagnosis possible.
| Division | Carries afferents from | VEMP relevance |
|---|---|---|
| Superior | Utricle, superior & horizontal canals | Generates the oVEMP |
| Inferior | Saccule, posterior canal | Generates the cVEMP |
Reflex arcs
The cVEMP travels via the vestibulo-collic reflex: saccular afferents synapse in the inferior vestibular nucleus, descend in the medial vestibulospinal tract, and inhibit motor neurons supplying the ipsilateral sternocleidomastoid.[3]
The oVEMP travels via a crossed vestibulo-ocular reflex: utricular afferents synapse in the superior vestibular nucleus, cross the midline via the medial longitudinal fasciculus, and excite motor neurons supplying the contralateral inferior oblique.
The receptors at the cellular level
Each otolith macula is a thin sheet of hair cells covered by a gelatinous otolithic membrane in which calcium-carbonate otoconia are embedded. A curved line — the striola — runs across the macula where the orientation of the hair-cell kinocilia reverses. Striolar hair cells are predominantly the flask-shaped Type I population innervated by calyceal nerve endings; their afferents fire irregularly at rest, and these irregular afferents are the population that responds to sound and vibration at clinical VEMP parameters.[3,17]
This is why the response is robust to high-frequency, high-intensity transient stimuli but not to sustained head positions — and why species-specific differences in the irregular-afferent population (notable in rodent models) limit how directly small-animal findings translate to human clinical VEMPs.[3]
Central pathways and the brainstem
Both arcs are disynaptic at the brainstem level — a fact reflected in the very short latencies of the diagnostic peaks (~10–25 ms). Conduction along the medial longitudinal fasciculus and reticulospinal tracts is rapid; any process that demyelinates these tracts (multiple sclerosis being the prototype) prolongs latency without necessarily reducing amplitude.[21]