STRUCTURES INVOLVED INMAINTENANCE OF BALANCE IN THE EAR• Utricle and Saccule• Semicircular Canals
PHYSIOLOGY OF VESTIBULAR SYSTEM• Peripheral – membranous labyrinth andvestibular nerve• Central – made up of nuclei and fiber tractsin CNS
UTRICLE• Stimulated by linear acceleration anddeceleration or gravitational pull during headtilts.• Macula of utricle lies in horizontal plane on theinferior surface of utricle.• Hair cells stimulated by displacement of otolithicmembrane.• Hair cells oriented in different directions.• Determines orientation of the head when thehead is upright.
SACCULE• Function similar to utricle.• Macula of saccule located in a verticalplane and signals head orientation whenperson is lying down.• Also responds to sound vibrations.
SEMICIRCULAR CANALS• Responds to angular acceleration and deceleration.• 3 canals lie at right angles to each other.• Canal which lies at right angle to axis of rotationstimulated most.• Stimulus – flow of endolymph displacing the cupula.• Stimulation of semicircular canals producenystagmus.
MAINTENANCE OF EQUILIBRIUM• Function of Utricle and Saccule :• Hair cells in maculae oriented in differentdirections.• With different position of head, stimulation ofdifferent hair cells apprise the brain of theposition of the head.• In turn Vestibular, cerebellar and reticular motorsystems excite appropriate postural muscles.
• Detection of linear acceleration anddeceleration by the utricle and saccule:• Otoliths fall back on hair cells duringacceleration and deceleration.• Information of disequilibrium causesperson to shift.
• Detection of head rotation by semicircularducts• Predictive function of semi circular canals:• Predicts disequilibrium is going to occurand thereby causes equilibrium centers tomake appropriate anticipatory preventiveadjustments.
OTHER FACTORS CONCERNED WITHEQUILIBRIUM• Visual Input• Neck Proprioceptors• Proprioceptive and exteroceptiveinformation from other parts of the body.• Neuronal connections of vestibularapparatus with CNS.
PATHWAY FOR EQUILIBRIUM REFLEX• Initiates in vestibular verves.• Then passes to vestibular nuclei and cerebellum.• Next signals are sent to reticular nuclei of brainstem as well as down the spinal chord by way ofvestibulospinal and reticulospinal tracts.• Antigravity muscles may be stimulated orinhibited.