By Sanjay George

PHYSIOLOGY OF EQUILIBRIUM

STRUCTURES INVOLVED IN MAINTENANCE OF BALANCE IN THE EAR

• Utricle and Saccule

• Semicircular Canals

PHYSIOLOGY OF VESTIBULAR SYSTEM

• Peripheral – membranous labyrinth and vestibular nerve

• Central – made up of nuclei and fiber tracts in CNS

UTRICLE

• Stimulated by linear acceleration and deceleration or gravitational pull during head tilts.

• Macula of utricle lies in horizontal plane on the inferior surface of utricle.

• Hair cells stimulated by displacement of otolithic membrane.

• Hair cells oriented in different directions.

• Determines orientation of the head when the head is upright.

SACCULE

• Function similar to utricle.

• Macula of saccule located in a vertical plane and signals head orientation when person 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 rotation stimulated most.

• Stimulus – flow of endolymph displacing the cupula.

• Stimulation of semicircular canals produce nystagmus.

MAINTENANCE OF EQUILIBRIUM

• Function of Utricle and Saccule :

• Hair cells in maculae oriented in different directions.

• With different position of head, stimulation of different hair cells apprise the brain of the position of the head.

• In turn Vestibular, cerebellar and reticular motor systems excite appropriate postural muscles.

• Detection of linear acceleration and deceleration by the utricle and saccule:

• Otoliths fall back on hair cells during acceleration and deceleration.

• Information of disequilibrium causes person to shift.

• Detection of head rotation by semicircular ducts

• Predictive function of semi circular canals:

• Predicts disequilibrium is going to occur and thereby causes equilibrium centers to make appropriate anticipatory preventive adjustments.

OTHER FACTORS CONCERNED WITH EQUILIBRIUM

• Visual Input

• Neck Proprioceptors

• Proprioceptive and exteroceptive information from other parts of the body.

• Neuronal connections of vestibular apparatus 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 brain stem as well as down the spinal chord by way of vestibulospinal and reticulospinal tracts.

• Antigravity muscles may be stimulated or inhibited.

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