Vestibular Function Flashcards
What is the vestibular system?
A sensory system essential in control of posture and balance. It is a series of fluid filled membranous tubes (labyrinths) which are embedded in the temporal bone
Where is the vestibular system found?
In the inner ear
What bone is the vestibular system found in?
Temporal bone
What are the semi-circular canals?
Organs of balance, involved in maintaining posture
What is the cochlea?
The organ of hearing
What makes up the vestibular apparatus?
3 Semi-circular canals
Utricle
Saccule
What is the utricle?
The swelling at the base of all of the 3 canals
What is the saccule?
The further swelling of endolymph underneath the utricle
How do the 3 semi circular canals lie in relation to each other?
At right angles to each other (3 dimensions)
What is found at the base of the semi circular canals?
Swellings called ampullae
What do ampullae contain?
Sensory hair cells
Cristae
What contains sensory hair cells?
Ampullae
Utricle
Saccule
What are the sensory organs?
Maculae
Function of sensory hair cells
Detect the movement of endolymph
What makes up the otolith organs?
Utricle and saccule together
Function of the otolith organs
Detect linear acceleration and encode information about the position of the head in space
What does the utricle detect?
Back/front tilt
What does the saccule detect?
Vertical movement
What do the semi circular canals detect?
Rotational acceleration
What are found in sensory cells in the ampullae?
Sensory receptors called cristae
What are cristae?
Flexible gelatinous structure called cupula that stretches across the entire width of the ampulla and responds to the movement of the endolymph fluid within the canals
What are found in the cupula?
Cilia of hair cells
Function of cilia of hair cells found in the cupula
Synapse directly with the sensory neurones of the vestibular nerve (CNVIII)
How do the cilia hair cells detect rotational acceleration?
If the skull is rotated to left or right from rest, the endolymph does not move at first because of its inertia. However the ampulla move instantly as they are embedded in the skull
The inertia of the endolymph produces drag which bends the cupula, and consequently the cilia embedded in it, in the opposite direction to the movement.
If rotate at a constant velocity, the endolymph catches up and rotates at the same speed, removing the shearing force, but this takes several seconds
When doing a rotational movement, what will a sudden stop cause?
Cause the endolymph to continue to move due to the momentum creating a continuing sense of movement and dizziness
What makes up the maculae?
Utricle
Saccule
What plane are the macula in the utricle orientated in?
Horizontal plane
What plane are the macula in the saccule orientated in?
Vertical plane
Features of maculae
Have a set of cilia
- one kinocilium
- a series of stereocilia
which protrude into the gelatinous mass called the otolith membrane
Function of the otolith membrane
Protects the cilia
What is embedded in the otolith membrane?
CaCO3 crystals called otoliths
Otoliths vs endolymph
Otoliths are much heavier than the endolymph and therefore are affected by a much greater extent by gravity during movement
What happens when the head is tilted?
The otoliths and the otolith membrane are moved
This distorts the jelly
The cilia then move
What happens when the head is tilted backwards?
Moves the otolith in the direction of the kinocilium causing depolarisation and increased discharge of APs.
What happens when the head is tilted forwards?
Moves the otolith in the opposite direction of the kinocilium causing hyperpolarisation and a decreased discharge of APs
What keeps the brain informed of its position in space and any direction of movement?
Distinct pattern of info sent from
- three cristae of semi circular canals
- two maculae of the otolith organs
Via the vestibular nerve
What do vestibular nuclei receive input from?
Proprioceptors signalling limb and body position, also from neck and eye muscles
What is kinaesthesia?
Perception of movement and body position
What happens once the vestibular nerves end in the brainstem?
Vestibular nerve nuclei still receive input
Two types of cilia of hair cells
Single very large kinocilium
Set of progressively smaller stereocilia
What does distortion of the cilia in the direction of the kinocilium cause?
Depolarisation
Increased discharge of APs in the vestibular nerve
What does distortion of the cilia away from the kinocilium lead to?
Hyperpolarisation
Decreased discharge of APs in the vestibular nerve
Where does much integration of all this sensory information take place?
Cerebellum
What is dynamic vestibular nystagmus?
A series of saccadic eye movements that rotate the eye against the rotation of the head and body so that the original direction of gaze is preserved despite head rotating
What is the direction of nystagmus related to?
The direction of the rapid flick back
R rotation -> R nystagmus
How can nystagmus be used to test vestibular function?
Post rotatory nystagmus
Caloric stimulation
What is post rotatory nystagmus?
Subjects rotated in a barany chair
If rotate left then during acceleration get a left nystagmus
At the end of rotation for about 20 seconds, during deceleration get a right nystagmus (due to endolymph catching up and now pushing the cupula in the opposite direction)
What is caloric stimulation used to test?
Horizontal SCC
What is caloric stimulation?
Outer ear washed with either cold or warm fluid
The temp difference from 37C gets through the thin bone and sets up convection currents which affects the endolymph
What does warm fluid (44C) in caloric stimulation cause?
Causes nystagmus towards the affected side
What does cold fluid (30C) in caloric stimulation cause?
Causes nystagmus away from the affected side
Neumonic to remember caloric stimulation
COWS
- cold opposite
- warm same
What would warm water into the right ear cause?
Right nystagmus
What would cold water into the right ear cause?
Left nystagmus
What can the stimulation of the vestibular system in the absence of movement cause?
Nausea and vomiting
When can nystagmus also be seen?
Lesions to peripheral or central vestibular pathways
What is kinetosis?
Motion sickness
Presentation of kinetosis
Nausea and vomiting
Decrease in BP
Sweating
Pallor
Pathology of kinetosis
Powerful maintained stimulation of the vestibular system
The cerebellum generates a “sickness signal” to the hypothalamus to bring about ANS changes
What is the most likely cause of kinetosis?
If the visual and vestibular system inputs into the cerebellum are in conflict e.g. if the vestibular system indicates rotation but the visual system does not
Presentation of labyrinthitis
ANS symptoms
Vertigo
Possibly nystagmus
Gross impairment of posture and balance
What is vertigo?
The perception (hallucination) of movement in the absence of movement
Presentation of Meniere’s Disease
Vertigo
Nausea
Nystagmus
Tinnitus
What is Meniere’s disease associated with?
Overproduction of endolymph causing increased pressure
Example of a drug that can attack the inner ear
Streptomycin
What is the inner ear very sensitive to?
Attack by drugs
What can happen if vestibular impairment is chronic?
It can be quite well compensated by the visual system
Learning circuits are set up in the cerebellum
What can lead to nystagmus at rest?
Lesions in the brain stem
What do vestibular system reflexes involve?
Vestibulocortical tracts
Vestibulospinal tracts
What are the vestibular system reflexes?
Tonic labyrinthine reflexes
Dynamic Righting Reflexes
Vestibulo-ocular reflexes
Function of the tonic labyrinth reflex
Keeps the axis of the head in a constant relationship with the rest of the body
Where does the tonic labyrinth reflex use information from?
Maculae
Neck proprioceptors
Function of the dynamic righting reflex
Rapid postural adjustments that made you stop falling when you trip
Long reflexes, involving extensions of all limbs
Who is the dynamic righting reflex most evident in?
Cats
How does the vestibulo-ocular reflex show the strong association between the vestibular apparatus and postural control?
Afferents from the semi circular canals project and connect (within the vestibular nuclei) to afferent fibres traveling to extraocular nuclei and thus have strong input into influencing eye movement
The visual system also sends powerful descending projections which controls posture
Types of vestibulo-ocular reflexes
Static reflex
Dynamic vestibular reflex
What is the static reflex?
When you tilt your head, the eyes intort/extort to compensate, so that over a certain range, the image stays the right way up
What is the dynamic vestibular nystagmus?
A series of saccadic eye movements that rotate the eye against the rotation of the head and body so that the original direction of gaze is preserved despite the head rotating
