Define the vestibular system?
A sensory system in the inner ear (embedded in temporal bone).
Its made up of fluid (endolymph) filled membranous channels or labyrinths
What makes up the vestibular system?
3 semicircular canals
- Swellings at the base called ampulla
- Connnected to Otolith organs (Utricle –> Saccule)
What does the vestibular system detect?
The semi-circular canals detect rotational acceleration
The otolith organs detect linear acceleration
- Utricle = From/back tilt
- Saccule - Vertical Movement
What is contained within the ampulla at the base of each semicircular canal?
An organ called Cristae made up of:
- A gelatinous cupulu within which sit the cilia of sensory hair cells (they then synapse to the vestibulocochlear nerve)
What types of cilia are there?
Each ampulla and otolith organ contains one large kinocilium and many smaller stereocilia
How do semi-circular canals detect rotational acceleration
As you rotate your head the ampulla moves but the intertia of the endolymph keep it stationary at first
This produces drag on the cupula which bends the cilia in the opposite direction to movement, triggering nervous impulses
Why do you feel dizzy, like your moving, when you stop suddenly?
The endolymphs inertia means it continues to move (and stimulate the cilia) after the ampulla stops
How do we tell which direction were moving in?
Cilia distorted towards the kinocilium –> Depolarisation –> Rise in APs
Cilia distorted away from kinocilium –> Hyperpolarisation –> Loss of APs
The cupula are also all in slightly different orientations so they are stimulated by different movements
Where does the brain integrate all the nervous impulses from the vestibular system?
What do we call the sensory apparatus of the otolith organs?
What plane are the maculae of the otolith organs in?
Utricle - Horizontal
Saccule - Verticle
Describe the structure of the maculae?
Cilia (Kinocilium and stereocilia) project into a gelatinous mass called the otolith membrane
Otoliths (CaCO3 crystals) embedded into the membrane
Why do maculae contain otoliths?
Because they are more effected by gravity than endolymph, so do a better job reacting to linear acceleration
How do the maculae detect linear acceleration?
- Tilt the head back causes the ototlith to move with gravity
- Pulls on the otolith membrane and so the cilia, towards the kinocilium
- Increased APs
- And the opposite for forward tilt
The saccule does a similar thing for verticle movement
Moving your head causes a change in your centre of gravity, how does the body deal with this?
Vestibular nuclei get input from proprioceptos, neck and eye muscles regarding limb/body position.
Then projections from the vestibular nuclei project to the cerebellum via descending motor fibres (to handle posture/balance) and to the cerebral cortex via the thalamus (for conscious perception of movement and body position i.e. kinaesthesia)
What tracts are involved in vestibular reflexes?
Vestibulocortical and vestibulospinal tracts
List the types of vestibular reflexes?
- Tonic Labyrinthine Reflexes
- Dynamic Righting Reflexes
- Vestibulo-ocular Reflexes
(including dynamic vestibular nystagmus & static reflex)
What/how does the tonic labyrinthine reflex do?
It keeps the axis of the head in a constant relationship with the rest of the body
Uses the maculae & neck proprioceptors
What does the dynamic righting reflex do?
Its a rapid postural adjustment to stop you falling when you trip
Explain the nerves involved in the vestibulo ocular reflexes?
Its part of the association between the vestibular apparatus, visual apparatus and postural control.
Afferents from the vestibular nuclei (CN8) travel to the extraocular nuclei (CN3/4/6) to influence eye movement
The static reflex is a type of vestibulo-ocular reflex, how does it work?
When you tilt you head your eyes intort/extory to maintain the image the right way up
Explain the process of dynamic vestibular nystagmus?
Slow Compensation Phase:
Saccadic eye movements rotating the eye against rotation of the head so you maintain your gaze in the same direction
- Eyeball comes to the limit of its movement
- Flicks back to facing straight ahead
The direction of the rapid phase is used when naming the nystagmus
How do we define right vs left nystagmus?
The direction of the rapid phase
So if you turn your head left --> Your eye rotate right to maintain vision --> Reaches the limit --> Flicks back left Therfore Left Nystagmus
What two tests of vestibular function rely on vestibulo-ocular reflexes?
How does the post-rotary Nystagmus test work?
Subject rotated in a barany chair
Then when they stop accelerating and deceleration begins the nystagmus switches sides.
This because as you decelerate the endolymph inertia causes it to move faster than the ampulla so it starts pushing the cupulu in the opposite direction, tricking your brain about direction of movement
How does caloric stimulation test vestibular function?
- The outer ear is washed in cold or warm fluid which sets up convection currents in the endolymph
Warm fluid causes nystagmus towards the effected side, cold opposite.
(COWS - Cold opposite, Warm Same)
What would cause pathological nystagmus?
Lesions of the peripheral or central vestibular pathways e.g. the brainstem
How does motion sickness occur?
The Vestibular and visual inputs to the cerebellum are out of sync triggering a sickness signal from cerebellum to hypothalamus
What are the symptoms of kinetosis?
- Drop in BP
What is labyrinthitis?
Infection interferes with vestibular function
Triggers ANS symptoms and Vertigo
~May also have nystagmus and gross impairment of balance, it can be very disabling
What is Meniere’s Disease?
Excess endolymph production causes a raise in inner ear pressure (Cause unknown)
–> Vertigo, nausea, nystagmus and tinnitus
How can vestibular impairment be compensated for?
By the visual system, this happens over time as learning circuits are set up in the cerebellum
However the person will lose all balance in the dark or with their eyes closed
NAme an ototoxic drug?
How does a brainstem lesion affect the vestibular system?
IT causes pathological nystagmus (At rest)