Vestibular Function Flashcards
What is the vestibular system?
A sensory system essential in the control of posture and balance
Where is the vestibular system found and what is it made up of?
In the ear and is a series of fluid filled membranous tubes (labyrinths) which are embedded in the temporal bone
What does the vestibular apparatus consist of?
3 semi-circular canals
Utricle to which the semi-circular cannals all connect
Saccule
What do the semi-circular canals connect to?
Utricle
How are the 3 semi-circular canals positioned in relation to each other?
Right angles
What is found at the bases of the semi-circular canals?
Swellings called ampulla that contain hair cells
Where are hair cells found in the vestibular apparatus?
Ampulla
Utricle
Saccule
What are the utricle and saccule collectively known as?
Otolith organs
What parts of the vestibular apparatus detect linear acceleration?
Otolith organs (utricle and saccule)
What part of the vestibular apparatus detect rotational acceleration?
Semi-circular canals (SCCs)
What do the otolith organs do?
Detect linear acceleration and encode information about the position of the head in space
Back/front tilt is detected by the utricle
Vertical movement is detected by the saccule
What is back/front tilt detected by?
Utricle
What is vertical movement detected by?
Saccule
What does SCC stand for?
Semi circularcanals
What is A?
Superior semicircular canal
What is B?
Horizontal semi-circular canal
What is C?
Posterior semi-circular canal
What is D?
Cristae within ampulla
What is E?
Utricle
What is F?
Saccule
What is G?
Maculae
What is H?
Cochlea
What is the ampulla?
The senosry cells of the semi-circular canals which are embedded in swellings at the base of the bony canals
What are the sensory receptors called that are found with the ampulla?
Cristae
What does cristae consist of?
Flexible gelatinous structure called the capula
What does the capula stretch accross?
Entire width of the ampulla and responds to movement of the endolymph within the canals
What does the capula respond to?
Movement of the endolymph fluid within the canals
What is embedded within the gelatinous capula?
Cilia of hair cells which synapse directly with the sensory neurons of the vestibular nerve (CN VIII)
What do the hair cells detect that are found within the capula of the cristae?
Rotational acceleration
How does the capula detect rotational acceleration?
1) If the skull is rotated left or right from rest the endolymph does not move at first because of its inertia
2) But the ampulla moves instantly because it is embedded within the skull
3) Inertia of endolymph produces drag which bends the capula and consequently the cilia embedded in it in the opposite direction to movement
4) If rotate at constant velocity the endolymph catches up and rotates at the same speed removing the shearing force but this takes several seconds
5) Sudden stop will cause endolymph to continue to move due to momentum creating a sense of movement and dizziness
Why does the endolymph not move at first when the skull is rotated left or right?
Becuase of its inertia
What direction to movement does the capula bend due to the inertia of the endolymph producing drag?
The opposite direction to movement
Why does a sudden stop of skull rotation (left/right) produce the sense of movement and dizziness?
The endolymph carries on moving for a few seconds due to its momentum
What are the 2 types of cilia of the hair cells in the capula?
Single very large kinocilium
Progressively smaller sterocilia
What allows the brain to determine movement in space and time?
Distortion of the cilia in the direction of the kinocilium causes depolarisation and increased discharge of action potentials in the vestibular nerve
Distortion of the cilia away from the kinocilium leads to hyperpolarisation and decreased discharge of action potentials in the vestibular nerve
What does distortion of the cilia in the direction of the kinocilium cause?
Depolarisation and increased discharge of action potentials in the vestibular nerve
What does distortion of the cilia in the opposite direction of the kinocilium cause?
Hyperpolarisation and decreased discharge of action potentials in the vestibular nerve
How does the orientation of the capula compared from one to the next, and what does this allow?
Slightly different to one another
So the brain can build a 3D image of body position using the pattern of firing and inhibition recieved
Where does much of the integration of the sensory information from the capula take place?
Cerebellum
How does the direction of the shearing force on the capula compare to the start of rotation and once the head stops?
At the start is against the direction of head rotation
In the end is in the opposite direction to what it started (so same direction as initial rotation
What is the maculae?
Sensory apparatus of the utricle and saccule are collectively called the maculae
In what plane is the maculae in the utricle orientated?
Horizontal plane
In what plane is the maculae in the saccule orientated?
Vertical plane
Just like the crista, what does the maculae have?
A set of cilia (one kinocilium and a series of sterocilium)
What does the cilia in the maculae protrude?
Into the gelatinous mass called the otolith membrane
What is embedded in the otoloth membrane?
CaCO3 crystals called otoliths
What are otoliths made of?
CaCO3
What do otoliths move in response to?
Gravitational forces
Does maculae in the utricle or saccule detect tilt of the head?
Utricle (horizontal plane)
Explain how tilting the head impacts the utricle?
1) Otoliths have greater density than endolymph and thus are affected more by gravity
2) Tiliting the head moves otoliths and the otolith membrane in the direction which they are embedded
3) This distorts jelly and moves cilia
Are otoliths or endolymph more dense?
Otoliths
Does tilting the head move otoliths and otolith membrane in the direction they are embedded or in the opposite direction?
In the direction they are embedded
In which direction of tilit does the otolith move towards the kinocilium?
Backwards tilit moves otolith towards the kinocolium causing repolarisation and increased dishcarge, opposite for forward tilt
What happens when backwards tilt moves the otolith in the direction of the kinocilium?
Depolarisation and increased discharge of action potentials, opposite for forward tilt
What do the vertical orientated macula of the saccule respond to?
Vertical forces (such as movement in a lift) and provide information on orientation of the head when lying down
What do the distinct pattern of information from the three cristae of the semi-circular canals and the two macuale of the otolith organs to the brain via the vestibular nerve keep it informed of?
Its position in space and any direction of movement
What nerve is used for the three cristae of the semi-circular canals and the two maculae of the otolith organs to transmit information to the brain?
Vestibular nerve
What is a consequence of any movement of the head displacing the centre of gravity in terms of parts of the brain activated?
Strong associations between the vestibular centres of the medulla (where many of the sensory afferents of the vestibular nerves terminate) and the cerebellar centres which co-ordinate postural muscles required to maintain balance
Explain the different associations of the vestibular centres of the medulla and the cerebellar centres which co-ordinate the postural muscles required to maintain balance?
Projections from vestibular nuclei on one side project ipsilaterally, bilaterally and contra-laterally to descending motor pathways (and slo the extraocular nuclei which controls extraocular muscles)
Vestibular nuclei receive input from proprioceptors signalling limb and body position, also from neck and eye muscles
Vestibular nuclei project via thalamus to the cerebral cortex, giving it perceptionof movement and body position (kinaesthesia)
What tracts do vestibular system reflexes involve?
Vestibulocortical and vestibulospinal
What are the 3 vestibular system reflexes?
Tonic labyrinthine reflexes
Dynamic righting reflexes
Vestibulo-ocular reflexes
What is the tonic labyrinthine reflex resonsible for?
Keeping the axis of the head in a constant relationship with the rest of the body
What does the tonic labyrinthine reflex use information from?
Maculae and neck proprioceptors
What are the dynamic righting reflexes responsible for?
Rapid postural adjustment that are made to stop you falling when you trip
What kind of reflexes are dynamic righting reflexes?
Long reflexes, involving extension of all limbs
What is there strong associations between to allow the vestibulo-ocular reflexes to exist?
Vestibular apparatus, visual apparatus and postural control
Explain the relations of the vestibulo-ocular reflexes?
Afferents from semi-circular canals project and connects to afferent fibres travelling to the extraoccular nucleii and thus have strong input to influencing eye movement
Visual system also sends powerful descending projections which control posture
How do people with destruction of the vestibular apparatus still maintain good balance?
If movement is relatively slow and eyes are open, balance is lost immediately on closing eyes
What are some different vestibular-occular reflexes?
Static reflex
Dynamic vestibular nystagmus
What is the static vestibular-occular 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 (vestibular-occular reflex)?
Series of saccadic eye movements that rotate the eye against the direction of rotation of the head and body so that the original directionof gaze is preserved despite head rotating
In dynamic vestibular nystagmus, what happens when the eye comes to the end of its range of movement?
Rapidly flicks back to the zero position (ie straight ahead)
What happens if rotation continues after the eye has reached the end of its range of movement in dynamic vestibular nystagmus?
Rapidly flicks back to the zero position and then slow phase starts again then moves back
In dynamic vestibular nystagmus, what is the direction of nystagmus?
Direction of the rapid flick back
In dynamic vestibular nystagmus, what direction of nystagmus and eye rotation does right rotation produce?
Right rotation produces right nystagmus and the eye rotates left
What is the medical term for involuntary eye movement?
Nystagmus
What is nystagmus used to test?
Vestibular function
What are different ways that nystagmus is used to test vestibular function?
Post-rotary nystagmus (only in research units)
Caloric stimulation
Explain the process of post-rotary nystagmus test?
1) Subjects are rotated in a Barany chair
2) If rotate to the left then during acceleration get a left nystagmus
3) At the end of rotation for about 20s, during deceleration get a right nystagmus
4) This is due to endolymph catching up and now pushing the capula in the opposite direction
In post-rotary nystagmus, if the person is rotated left what direction is the nystagmus in during the acceleration and deceleration phases?
Acceleration - left
Deceleration - right
What is caloric stimulation a test for?
Horizontal semi-circular canals (SCCs)
Explain the process of caloric stimulation?
1) When outer ear is washed with either cold or warm fluid the temperature difference from core 37o gets through the thin bone and sets up convection currents which affect the endolymph
2) Warm fluid (44o) causes nystagmus towards the affected side
3) Cold fluid (30o) causes nystagmus away from the affected side
In caloric stimulation, what direction of nystagmus does warm fluid produce?
Nystagmus towards the affected side
In caloric stimulation, what direction of nystagmus does cold fluid produce?
Nystagmus away from the affected side
Why is it important when irrigating (washing out) the outer ear to use fluid of appropriate temperature?
Stimulation of the vestibular system in the absence of movement can cause nausea and vomiting
What lesions can nystagmus be seen in?
Lesions to periphery or central vestibular pathways
What can powerful maintained stimulation of the vestibular system give rise to?
Kinetosis (motion sickness)
What is the clincal presentation of kinetosis?
Nausea and vomiting
Decreased blood pressure and dizziness
Sweating and pallor
All of the symptoms of kinetosis are symptoms of what nervous system?
They are all autonomic nervous system symptoms
What is the medical term for motion sickness?
Kinetosis
When is motion sickness most likely to occur?
If visual system and vestibular system inputs to the cerebellum are in conflict (such as the vestibular system indicating rotation but the visual system not)
Cerebellum generates a ‘sickness signal’ to the hypothalamus to bring about autonomic nervous system changes
What are examples of diseases and damage that relate to the vestibular system?
Acute interference with normal vestibular function as a result of infection (labyringthitis)
Meniere’s disease
Inner ear is sensitive to attack by drugs such as streptomycin
Lesions of the brainstem lead to nystagmus at rest
What is labyringitis?
Inflammation of one of the two vestibular nerves
What are symptoms of labyrinthitis?
ANS symptoms and vertigo
What are the symptoms of Meniere’s disease?
Vertigo, nausea, nystagmus and tinnitus
What is an example of a drug that can attack the inner ear?
Streptomycin
If vestibular impairment is chronic, what is it quite well compensated by?
Visual system
Learning circuits are set up in the cerebellum but problems occur if subjects cannot use visual information
What is vertigo?
The perception (hallucination) of movement in the absence of movement
What is often associated with vertigo?
Nystagmus
What does vertigo cause gross impairment of?
Posture and balance
