Vestibular Function Flashcards
Which bone contains the vestibular system?
Temporal bone
What do the three semicircular canals all connect to?
All connect to the utricle
What is the name given collectively to the utricle and the saccule?
Otolith organs
What do the utricle and the saccule do?
They 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 do the semicircular canals detect?
They detect rotational acceleration
Where are the sensory cells of the semicircular canals located?
The sensory cells of the semi-circular canals are embedded in swellings at the base of the bony canals called ampulla.
What are the sensory receptors in the swellings? What are they composed of?
Cristae - consist of flexible gelatinous structure called the cupula
Describe the cupula
Stretches across the entire width of the ampulla and responds to movement of the endolymph fluid within the canals.
How does the cupula make a connection with the vestibular nerve?
Cilia of hair cells are embedded in the gelatinous cupula, these cilia synapse directly with the sensory neurons of the vestibular nerve
What do the hair cells detect?
Rotatinoal acceleration
How is rotational head movement detected?
Endolymph at first does not move during rotation because of inertia, ampull moves instantly because it is embedded in the skull.
Resultant bend in cupula and the cilia embedded in it.
How do you become dizzy?
If rotate at constant velocity, the endolymph catches up and rotates at the same speed, removing the shearing forces, but this takes several seconds.
Sudden stop will cause endolymph to continue to move due to momentum creating a continuing sense of movement and dizziness.
What are the different types of the cilia of the hair cells?
There is a single very large kinocilium and a set of progressively smaller stereocilia
How is the rate of discharge of AP’s in the vestibular nerve increased / decreased?
If the cilia are distorted in the direction of the kinocilium - depolarization and increased discharge of AP’s
If the cilia are distorted away from the direction of the kinocilium - hyperpolarization and decreased discharge of AP’s
What is the sensory apparatus of the utricle and saccule collectively known as?
The maculae
How are the macula orientated in the utricle and the saccule?
The macula in the utricle are orientated on the horizontal plane, those in the saccule on the vertical plane.
What is the structure of the maculae?
Have a set of cilia (one kinocilium and a series of stereocilium) which protrude into a gelatinous mass called the otolith membrane. Embedded in the otolith membrane are CaCO3 crystals called otoliths.
How is the increase rate of action potential firing achieved during a backwards head tilt?
Head tilt detected by utricle
Otoliths have a greater density than endolymph, more affected by gravity
Tilt of the head moves the otoliths and the otolith membrane inwhich they are embedded - distorts jelly and moves the cilia.
Otoliths move in the direction of the kinocilium during backwards head tilt causing depolarisation and increased discharge of APs. Opposite for forward tilt.
What forces does the saccule respond to?
Vertically orientated macula in the saccule respond to vertical forces e.g. movement in a lift, and provide information on orientation of head when lying down.
Where do many of the sensory afferents of the vestibular nerve terminate?
Vestibular centres of the medulla
Vestibular centres of the medulla have strong connections with which brain structure?
Cerebellar centres - which co-ordinate the postural muscles required to maintain balance.
How do projections from the vestibulat nuclei reach the descending motor-pathways?
Projections from the vestibular nuclei on one side project ipsilaterally, bilaterally and contra-laterally to descending motor pathways (and also the extraocular nuclei (extraocular muscles)
Where do proprioceptors signalling limb and body position, neck and eye muscles send their sensory information?
TO vestibular nuclei
How is kinaesthesia achieved?
Vestibular nuclei project via the thalamus to the cerebral cortex - perception of movement and body position
What do vestibular system reflexes involve?
Vestibulocoritcal and vestibulospinal tracts.
What are tonic labyrinthine reflexes?
Keep the axis of the head in a constant relationship with the rest of the body.
What do tonic labyrinthine reflexes use?
Use information from maculae and neck proprioceptors.
What are dynamic righting reflexes?
Rapid postural adjustments that are made to stop you falling when you trip.
They are rapid reflexes and involve extension of all limbs
What allows for the connection between the semi - circular canals and the eye muscles?
Afferents from the semi-circular canals project and connect within the vestibular nuclei to afferent fibres travelling to the extraocular nuclei.
Why is balance lost when you close your eyes?
Because there are powerful descending projections which control posture from the visual system
What is the static vestibulo - occular reflex?
Intorsion and extorsion, image stays the same way up during head tilt
What is dynamic vestibular nystagmus?
When eyeball comes to the end of it’s range of movement, it rapidly flicks back to the zero position i.e. straight ahead.
Nystagmus is the direction of the rapid flick back - so the same as the direction of rotation
What is post rotatory 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?
Washing the outer ear with cold or warm water. Temperature difference from core 37 sets up convection currents which affect the endolymph
What is the direction of nystagmus with cold and warm water?
Cold water - nystagmus away from affected side
Warm water - towards affected side
COWS - (Cold Opposite, Warm Same).
What causes kinetosis?
(motion sickness)
Powerful maintained stimulation of the vestibular system.
Visual and vestibular inputs in to the cerebellum are in conflict
What is responsible for kinetosis?
The cerebellum generates a “sickness signal” to the hypothalamus to bring about the ANS changes.
(Nausea and vomiting, decrease BP and dizziness, sweating and pallor.
All of which are autonomic NS symptoms.)
What is the effect of labyrinthitis?
Acute interference with normal vestibular function as a result of infection
All ANS symptoms and vertigo
What is vertigo?
Vertigo is the perception (hallucination) of movement in the absence of movement. There may also be nystagmus. Gross impairment of posture and balance, very disabling.
What causes Ménière’s disease?
The cause is unknown - associated with over production of endolymph causing an increase in pressure
What is the presentation of menieres disease?
Vertigo, nausea, nystagmus and tinnitus
What drugs affect the inner ear?
Streptomycin
Where abouts do lesions occur which cause nystagmus at rest?
Brainstem
