Flint_Chapter 163_main.indd

• The vestibular system primarily drives reflexes to maintain stable vision and posture. • By modulating the non-zero baseline firing of vestibular afferent nerve fibers, semicircular canals encode rotation of the head, and otolith organs encode linear acceleration and tilt. • Stimulation of a semicircular canal produces eye movements in the plane of that canal. • A semicircular canal normally is excited by rotation in the plane of the canal bringing the head toward the ipsilateral side. • Any stimulus that excites a semicircular canal’s afferents will be interpreted as excitatory rotation in the plane of that canal. • High-acceleration head rotation in the excitatory direction of a canal elicits a greater response than does the same rotation in the inhibitory direction. • The response to simultaneous canal stimuli is approximately the sum of the responses to each stimulus alone. • Nystagmus due to dysfunction of semicircular canals has a fixed axis and direction with respect to the head. • Brainstem circuitry boosts low-frequency vestibulo-ocular reflex (VOR) performance through velocity storage and neural integration. Failure of these mechanisms suggests a central pathologic process. • The utricle senses both head tilt and translation, but loss of unilateral utricular function is interpreted by the brain as a head tilt toward the opposite side. • Sudden changes in saccular activity evoke changes in postural tone. • The normal vestibular system can rapidly adjust the vestibular reflexes according to the context, but adaptation to unilateral loss of vestibular function may be slow and susceptible to decompensation.