vestibular system
Summary
The vestibular system, located in the inner ear, detects head position, motion, and acceleration to maintain balance, posture, and gaze stability. It comprises the semicircular canals (angular/rotational acceleration), utricle and saccule (linear acceleration and static head position), and vestibular nerve (CN VIII) pathways to the brainstem and cerebellum. Dysfunction causes vertigo, nystagmus, and imbalance.
Detail
The vestibular system consists of peripheral and central components. Peripherally, the semicircular canals (anterior, posterior, lateral) contain endolymph and hair cells with cupulae that detect angular acceleration via bending of stereocilia during head rotation. The utricle and saccule contain otoliths (calcium carbonate crystals in a gelatinous matrix) that respond to linear acceleration and gravity, providing information about head tilt and static position. Hair cells transduce mechanical movement into neural signals via depolarization when stereocilia bend toward the kinocilium (excitatory) or away (inhibitory), following Ewald's laws.
Signals travel via the vestibular ganglion (Scarpa's ganglion) through CN VIII to the vestibular nuclei in the brainstem (pons/medulla), which connect to: (1) oculomotor nuclei via the medial longitudinal fasciculus (MLF) for the vestibulo-ocular reflex (VOR), stabilizing gaze during head movement; (2) spinal cord via vestibulospinal tracts for postural control; (3) cerebellum (flocculonodular lobe) for coordination and calibration; and (4) cortex for conscious spatial orientation.
Clinically, vestibular dysfunction presents with vertigo, nystagmus, nausea, and imbalance. Peripheral causes include BPPV (canalithiasis, treated with Epley maneuver), vestibular neuritis (viral, spares hearing), labyrinthitis (viral/bacterial, affects hearing), and Ménière's disease (endolymphatic hydrops causing episodic vertigo, tinnitus, hearing loss, aural fullness). Central causes (brainstem/cerebellar stroke, MS) are distinguished by direction-changing nystagmus, lack of latency/fatigability, and associated neurological signs (HINTS exam differentiates peripheral vs. central vertigo). Bilateral vestibular loss (e.g., ototoxic aminoglycosides) causes oscillopsia and ataxia without vertigo since damage is symmetric.
Sources
- Kandel's Principles of Neural Science
- First Aid for the USMLE Step 1
- Ganong's Review of Medical Physiology
- Harrison's Principles of Internal Medicine
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