Vestibular system

The peripheral vestibular apparatus in each inner ear contains three semicircular canals and two otolith organs. The canals are arranged in roughly different planes and detect angular head movement. Each contains fluid and sensory hair cells; when the head rotates, fluid motion deflects the hair-cell structures and changes signals sent along the vestibular nerve.

The utricle and saccule detect linear acceleration and the orientation of the head relative to gravity. Their sensory hair cells are associated with a gelatinous layer containing small calcium-carbonate crystals called otoconia. Movement of this weighted layer bends the hair cells, producing signals about changes in head position and motion.

Vestibular signals travel to brainstem and cerebellar processing centres, which integrate them with visual information and proprioceptive signals from muscles, joints and other body tissues. The combined information helps the nervous system estimate the body's orientation and movement.

The vestibulo-ocular reflex helps keep the visual field stable when the head moves by coordinating eye movements in the opposite direction. Vestibulospinal pathways contribute to postural responses by influencing muscles that help keep the body upright. These systems operate together with other sensory and motor processes; balance is not produced by the inner ear alone.

The system contributes to balance while standing, walking and moving, and helps maintain steady visual focus during head motion. Its signals also provide information about self-motion and orientation. Because the brain integrates multiple sensory sources, a mismatch between vestibular, visual and body-sense signals can contribute to disorientation or motion-related symptoms.

Vestibular function is assessed through clinical history and tests selected for the question being investigated. Different disorders can affect peripheral inner-ear organs, the vestibular nerve or central neural pathways, and dizziness has many possible causes. A symptom alone does not identify a specific vestibular disorder.

The three semicircular canals are commonly described as superior (anterior), posterior and lateral (horizontal). They respond especially to rotational head movements. The otolith organs complement them: the utricle is particularly responsive to horizontal linear acceleration, while the saccule is especially responsive to vertical acceleration. These are functional tendencies rather than isolated one-direction sensors; real movements stimulate combinations of organs and are interpreted centrally.