This document summarizes the physiology of equilibrium and balance. It describes the structures involved, including the utricle, saccule, and semicircular canals. It explains how the utricle and saccule detect linear acceleration and orientation using hair cells and otoliths, while the semicircular canals detect rotational movement through endolymph flow. Working together with visual input, neck proprioceptors, and the central nervous system, these structures maintain balance through vestibular pathways and reflexes that stimulate appropriate postural muscles.

1. By Sanjay George
PHYSIOLOGY OF EQUILIBRIUM

2. STRUCTURES INVOLVED IN
MAINTENANCE OF BALANCE IN THE EAR
• Utricle and Saccule
• Semicircular Canals

3. PHYSIOLOGY OF VESTIBULAR SYSTEM
• Peripheral – membranous labyrinth and
vestibular nerve
• Central – made up of nuclei and fiber tracts
in CNS

5. 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.

8. 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.

9. 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.

11. 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.

12. • 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.

14. • 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.

15. 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.

16. 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.