The Vestibular System

Abstract

The peripheral sensory components of the vestibular system include the hair cells and structures that enclose those cells, the semicircular canals and the otolithic organs, the saccule and utricle. The central components residing within the brainstem are four vestibular nuclei (medial, lateral, superior, and inferior) that are responsible for integrating incoming sensory signals with specific output circuits that form the vestibuloocular network, the vestibulospinal network, and the vestibulothalamic network. These networks are responsible for controlling reflex and voluntary eye movements as well as limb and trunk movements in response to vestibular sensory stimulation (head movement). Vestibular hair cells exhibit cilia that respond to head movements by depolarization if movement of the stereocilia is toward the kinocilium or by hyperpolarization if the stereocilia bend away from the kinocilium. Increased depolarization of hair cells is the result of inward movement of potassium ions through mechanically gated cilia membrane channels. Hair cell afferent fibers provide input centrally to the vestibular nuclei, which then project to (1) horizontal and vertical gaze centers to influence eye movements or to (2) reticular formation cells, which connect with spinal motor neurons that control head, trunk, and limb movements. Nystagmus is a pathological condition resulting from an interruption of central vestibular circuits. Afflicted patients present with involuntary, alternating eye movements that are slow in one direction and fast in the opposite direction. Dizziness and vertigo are additional common symptoms of vestibular system damage. Cerebral cortical representation for the vestibular system is diffuse and involves prefrontal, parietal, and insular regions.