Vestibular function Flashcards
What is the vestibular system?
A sensory system essential in the control of posture and balance. Located in the inner ear.
Describe the arrangement chambers in the inner ear.
3 bone lined chambers (bony labyrinth). Inside there are 3 membranous chambers (membranous labyrinth). These are embedded in the petrous part of temporal bone.
Where are perilymph and endolymph?
Perilymph inside body labyrinth, endolymph inside membranous labyrinth.
What is the vestibular apparatus?
- 3 x semi-circular canals (anterior, posterior and middle)
- utricle (semi-circular canals all connect to this)
- saccule
What are the swellings at the base of the semi-circular canals and which motion do these detect?
Ampulla - these contain sensory hair cells.
Detect rotational acceleration.
What are the otolith organs?
Utricle and saccule. These detect linear acceleration:
- saccule: side to side
- utricle: back and forward
What is the function of the semi-circular canals?
To sense motion via sensory hair cells (cilia). Inside the ampulla lie sensory receptors (cristae) contained within gelatinous cupula.
When the head moves, so does endolymph which moves the cupula, which moves the cilia and this activates the cristae –> impulses send via CNVIII.
How does the endolymph move?
It doesn’t want to and so produces drag. This bends the cupula, which bends the cilia in the opposite direction to movement. Endolymph will eventually catch up after a few seconds and move at a constant rate.
What happens if you are spinning and suddenly stop?
Endolymph is moving at a constant velocity - if you stop spinning the endolymph will keep moving as there is momentum
–> dizziness
What are the different types of cilia in the semi-circular canals?
2 types:
- Kinocilium: 1 large hair
- Stereocilia: progressively smaller, many of them
Describe how the brain builds a 3D image of body position via the semi-circular canal cilia.
- Distortion of cilia in the direction of the kinocilium increases discharge of AP’s in the vestibular nerve
- Distortion of cilia away from the kinocilium leads to hyperpolarisation and decreases discharge of AP’s in the vestibular nerve
–> this information is sent to the cerebellum and is integrated (pattern of firing and inhibition) which tells the brain which position the body is in
What are the sensory receptors of the otolith organs and how are these orientated?
Maculae:
- vertical orientation in saccule
- horizontal orientation in utricle
What are the functions of the otolith organs?
- Maculae have a set of cilia: 1 kinocilium and a series of stereocilium
- These protrude into the otolith membrane (gelatinous)
- CaCO3 otolith crystals are embedded in the otolith membrane
The head tilts which moves the otoliths –> distorsion of jelly which moves the cilia.
Backwards tilt moves otolith in direction of kiinocilium –> depolarisation and increased AP discharge (opposite for forwards tilt)
Where do projections via the vestibular nerve go?
- cerebellar centres for balance (control of postural muscles)
- vestibular centres of the medulla –> thalamus –> cerebral cortex for perception of movement and body position (kinaesthesia)
- project to descending motor pathways and extraocular nuclei
Do vestibular nuclei receive any input?
Yes from proprioceptors (limb and body position), neck and eye muscles
What are the main vestibular system reflexes?
1) Tonic labyrinthine reflexes
2) Dynamic righting reflexes
3) Vestibulo-ocular reflexes
What are tonic labyrinthine reflexes?
Keep the axis of the head in a constant relationship with the rest of the body using information from maculae and neck proprioceptors.
What are dynamic righting reflexes?
Rapid postural adjustments that are made to stop you falling when you trip. Long reflexes involving extension of limbs (cats).
What are vestibulo-ocular reflexes?
a) static reflex: when you tilt your head your eyes extort or intort to compensate so that you can see the image the right way
b) dynamic vestibular nystagmus: rapid rotation of the eye against the direction of rotation of the head so that the original direction of gaze is preserved despite head rotation - restricted and when the eyeball is at the end of its range of movement it rapidly flicks back to looking straight ahead
How do you tell the direction of a nystagmus?
It is in the same direction as the rapidflick back eg. R rotation, R nystagmus
How can nystagmus be used to test vestibular function (post-rotary nystagmus)?
Rotate someone in a chair to the left –> left nystagmus. Will then get a right nystagmus when the chair has stopped spinning as the endolymph is playing catch up and is therefore moving the cupula in the opposite direction.
How can nystagmus be used to test vestibular function (caloric stimulation)?
Testing the function of the horizontal semi-circular canal:
- inject warm or cool fluid into the inner ear
- warm fluid (44 degrees): nystagmus towards the affected side
- cold fluid (30 degrees) nystagmus away from the affected side
–> COWS (cold opposite warm same)
(Nystagmus can also be seen with lesions to peripheral or central vestibular pathways)
What is kinetosis and why does this occur?
Motion sickness due to powerful and maintained stimulation of the vestibular system.
More likely to occur if visual and vestibular system are in conflict eg on a bus when its dark. Sickness signal sent to the cerebellum which causes ANS symptoms.
What are the symptoms of kinetosis?
Autonomic symptoms: N/V, low BP, dizziness, sweating and pallor
What is vertigo?
Interference with normal vestibular function as a result of infection (labyrinthitis) can cause vertigo:
- the perception of movement in the absence of movement; may also be nystagmus; impairment of balance and posture; very disabling
What is Meniere’s disease?
Over-production of endolymph causing increased pressure:
- vertigo
- nausea
- nystagmus
- tinnitus
Name an antibiotic that can attack the inner ear.
Streptomycin.
What happens if vestibular impairment becomes chronic?
This can be well compensated by the visual system due to new learning circuits being set up in the cerebellum.
If subjects cannot see this causes problems.
What can lesions of the brain lead to?
Nystagmus at rest.
