Vestibular System - Balance Flashcards
What is the vestibular system?
A sensory system essential for control of posture and balance
Found in inner ear, is series of fluid filled membranous tubes (labyrinths) which are embedded in temporal bone
What does the vestibular apparatus consist of?
3 semi-circular canals, the utricle which the semi-circular canals attach to and saccule
Swelling at base of semi-circular canals are the ampulla which contain sensory hair cells
Utricle and saccule contain sensory hair cells
What is the function of the semi-circular canals, utricle and saccule?
SC - rotational acceleration
Saccule - vertical acceleration
Utricle - back/front tilt and horizontal acceleration
What are the otolith organs?
Utricle and saccule
Describe the sensory receptors in the semi-circular canals
Sensory cells are embedded in the ampulla
Inside ampulla are hair cells which act as sensory receptors
Hair cells are embedded in flexible gelatinous structure called cupula which stretches along entire length of ampulla - becomes distorted by endolymph fluid
What do the cilia of hair cells synapse with?
Synapse directly with sensory neurons of vestibular nerve (CNVIII)
How does semi-circular canals detect rotational acceleration?
If skull rotated left or right from rest then endolymph at first does not move due to inertia
Ampulla moves instantly as it is embedded in skull
What does inertia of the endolymph cause?
Produces drag which bends the cupula and the cilia embedded in it, in oppisite direction of movement
If rotation is constant velocity then endolymph catches up and rotates at same speed
What does sudden stop after rotation cause for the endolymph?
Endolymph continues to move die to momentum so continuing a sense of movement and dizziness
What are the 2 types of cilia of hair cells?
Large kinocilium
Small stereocilia
What does distortion of cilia in direction of kinocilium cause?
Depolarisation and increased discharge of APs in vestibular nerve
What does distortion of cilia away from kinocilium lead to?
Hyperpolarisation and decreased discharge of APs in vestibular nerve
What does kinocilium allow brain to do?
Determine movement in time and space
Brain can use can build 3D image of body position using pattern of firing and inhibition received
Most of the sensory info takes place in cerebellum
What is the maculae?
Sensory apparatus of utricle and saccule
How are the macula orientated in the utricle and saccule?
Utricle - on the horizontal plane
Saccule - on the vertical plane
Describe the structure of the maculae
Have a set of cilia - one kinocilium, a series of stereocilium which protrude into gelatinous mass called otolith membrane
Embedded in otolith membrane is CacO3 crystals called otoliths
How is a tilt of head detected in macula in utricle?
In horizontal plane
otoliths have greater density than endolymph so affected more by gravity
Tilting head moves otoliths and membrane
Distorts jelly and moves cilia
What does backwards tilt cause?
Moves otolith in direction of the kinocilium causing depolarisation and increased discharge of APs
Oppisite for forward tilt
What does vertically orientated macula in saccule respond to?
Vertical forces - elevation in lift and provides info on orientation when lying down
What all keeps the brain informed about body’s position in space and direction of movement?
3 ampulla of semi-circular canals and 2 maculae of otolith organs to brain via vestibular nerve
How does vestibular nerve help coordinate postural muscles?
Connect vestibular nerve by neural pathways to cerebral centres without synapsing in the medulla
What stimulates vestibular nuclei in brainstem?
Afferents from semi-circular canals and otolith organs
Proprioceptors in limbs, trunk, neck and head
Where does the vestibular nuclei in brainstem send info?
Descending spinal motor pathways
Cerebral cortex
Extraocular nuclei
Cerebral centres - also afferents can send directly
What do the vestibular system reflexes involve?
Vestibulocortical and vestibulospinal tracts
Tonic labyrinthine reflex, vestibulo-ocular reflex and dynamic righting reflexes
Explain the tonic labyrinthine reflex
Keeps the axis of head in a constant relationship with rest of body
Uses information from maculae and neck proprioceptors
Explain the dynamic righting reflex
Rapid postural adjustments that are made to stop you from falling
Long reflexes involving extension of all limbs
Explain the vestibulo-ocular reflex
Strong association is evident between vestibular apparatus, visual apparatus and postural control
In medullary vestibular nuclei - vestibular system synapse with afferent fibres travelling to extraocular nuclei
What is the function of vestibular nystagmus?
A series of saccadic eye movements that rotate eye against direction of passive rotation and body so original direction of gaze is preserved despite head rotating
Explain the vestibular nystagmus
When eyeball comes to end of range of movement, it rapidly flicks back to starting position
If rotation continues the slow phase starts again, then flicks back
Right rotation then right nystagmus
How can nystagmus be used to check vestibular function?
Caloric stimulation - test for horizontal SCC
Temp effects endolymph
Warm fluid - nystagmus towards effected side
Cold fluid - nystagmus away from effected side
COWS
How does motion sickness occur?
If visual and vestibular system inputs to cerebellum are in conflict
Cerebellum generates signal to hypothalamus - ANS changes so nausea, vomiting, decrease BP, dizziness, sweating and pallor
What is labyrinthitis?
Acute interference with normal vestibular function as result of infection
All ANS symptoms and vertigo
What is vertigo?
Perception of movement in absence of movement
Gross impairment of posture and balance
Describe Meniere’s disease
Vertigo, nausea, nystagmus, and tinnitus
Associated with overproduction of endolymph causing increase pressure
