Physiology of Balance Flashcards
What is the structure of the inner ear?
thin walled membranous labyrinth floats inside the outer bony wall/labyrinth
perilymph/periotic fluid - separates bony labyrinth from membranous labyrinth
endolymph/otic fluid - fills the membranous labyrinth
What is the structure of hair cells in the inner ear?
single motile kinocilium surrounded by numerous non-motile stereocilia
What happens when stereocilia deflect towards the kinocilium?
depolarisation - increased firing of CN VIII
What happens when stereocilia deflect away from the kinocilium?
hyperpolarisation - decreased firing of CN VIII
Where is the vestibular system located?
bony labyrinth of the inner ear
continuous with the cochlea
What are the names of the three semi-circular canals?
posterior
anterior
lateral
How are the semi-circular canals orientated?
at 90 degrees to each other
How are semi-circular canals paired?
horizontal ducts (in the same plane) anterior canal of one side with the posterior canal of the other (parallel)
What are ampulla?
bulbous expansion at the base of each semi-circular canal
What do ampulla contain?
sensory epithelium/crista that contains hair cells
What do semi-circular ducts sense?
rotary movement/angular acceleration of the head
Where do the hair bundles from the crista extend into?
cupula
What is the cupula?
gelatinous mass that bridges the width of the ampulla forming a fluid barrier through which endolymph can’t circulate
What happens when there is movement of head in the plane of one of the semi-circular canals (angular acceleration)?
inertia of endolymph distends the cupula away from the direction of head movement
What is inertia?
desire to stay the same
What happens in the semi-circular canals when their is linear movement of the head?
equal force on both sides of cupula - no displacement of hair bundles
What are the utricle and saccule?
two large chambers of the vestibular system
What is the function of the utricle and saccule?
sense forward and backward (linear acceleration) movement of the head
What is the macula?
small flat epithelial patch in the utricle and saccule that contains supporting cells and sensory hair cells
What do sensory hair cells in the macula synapse with?
sensory endings of the vestibular nerve
Where are hair cells located in the macula?
embedded in the otolithic membrane
What is the otolithic membrane?
gelatinous mass
What are otoliths?
calcium carbonate crystals
Where are otoliths found?
otolithic membrane
What is the function of otoliths?
increase membrane weight and its resistance to change in motion
What happens when the head is tilted?
otolithic membrane shifts its position due to gravity
bends the cilia of the hair cells
What is the neural pathway for the vestibular system?
information from the vestibular system goes directly to the vestibular nuclei in the brainstem or cerebellum
What is the vestibular nuclei?
main integrative centre for balance
Where do the vestibular nuclei receive inputs from?
vestibular system
visual receptors
somatic receptors - particularly stretch receptors in the neck muscles that report the angle or inclination of the head
Where do the vestibular nuclei send impulses too?
brainstem centres that control the extraocular eye movements and reflex movements of the neck, limb and trunk muscles
What are the vestibulo-ocular reflexes?
keep the eyes still as the head moves
What are the vestibulospinal reflexes?
allow the body to maintain or regain balance
What are nystagmus?
involuntary eye movements that preserve eye fixation on stable objects in the visual field during angular and rotational movements of the head
Give an example of nystagmus
as the head rotates the vestibulo ocular reflexes cause a slow compensatory drifting of eye movement in the opposite direction - stabilises binocular fixation point - pattern of slow-fast movement is nystagmus
How is the direction of nystagmus clinically names?
for the direction of the fast phase
When is nystagmus pathological?
spontaneous nystagmus that occurs without head movement
