2Anatomy and Physiology of the Balance Organs

FFive organs, three planes

Each vestibular labyrinth has five sensory organs: three semicircular canals (superior/anterior, horizontal/lateral, posterior) detecting angular acceleration, plus utricle and saccule (otolith organs) detecting linear translation, gravity and head tilt Semicircular canals drive the vestibulo-ocular reflex that stabilizes vision during head movement Otolith organs drive vestibulo-collic and vestibulo-spinal reflexes for postural stability and are heavily involved in gross-motor milestones such as sitting, standing and walking.[2004]

TTonic firing and rate coding

Like the cochlea, which has a nonzero baseline 'code for silence', the vestibular organs have a tonic resting firing rate; motion raises or lowers this baseline (rate coding) This baseline lets a prosthetic implant up- or down-modulate from rest to encode motion Canal hair cells encode motion about a single axis, whereas otolith hair cells are oriented in many directions, making otolith coding far more complex.[2004]

TPush-pull pairing of the canals

Canals work in coplanar push-pull pairs (mnemonics LARP and RALP): left anterior with right posterior, right anterior with left posterior, and the two horizontal canals together Paired organs encode the same plane in opposite directions This push-pull redundancy is the physiologic reason a single unilateral vestibular implant can theoretically restore motion sensing in all planes even in bilateral disease.[2007]

FBalance is redundant

Balance draws on three inputs — vestibular, proprioceptive and visual — and loss of one is usually tolerated until a second is impaired Symptoms surface on uneven or soft surfaces or in the dark, when proprioceptive and visual cues are degraded Two key vestibular jobs are postural maintenance via vestibulospinal pathways and gaze stabilization via the vestibulo-ocular reflex.[2004]