Vestibular function Flashcards
What is the vestibular apparatus
- Fluid- filled tubes which are found in the temporal bone known as Labiryths
- Connected to the Cochlear.
- They are responsible for posture and balance
What is the vestibular system made up of?
3 semi-circular canals
- At right angles to each other
- Superior, Horizontal, posterior
- At the base of each of these are swellings known as the ampulla
- The ampulla contains sensory cells (detect movement)
- Detect Rotational acceleration
Otolith organs :
- Detect linear accelaration
- Encode information on the position of the head in space
1-Utricle
- Connected to each of the semi-circular canals
- Contains sensory cells
- Detects forward and backward tilt of the head
2-Saccule
- Contains sensory hair cells
- Detects vertical movement of the head
Sensory cells of the SCC
- Embedded in the ampulla
- Also known as Cristae
- Contain a fluid called Cupula= stretches across along whole length of ampulla and detects movement in endolymph
- Within the cupula are cilia of sensitive hair cells which synapse with the sensory neurones of the vestibular neurone
Sensory reception of the SCC
- Detection of rotational accelaration
- Thanks to the cilia of the hair cells of the cupula
- Inertia of Endolymph pushes against cupula (in opposite direction to movement of head)
- In a few seconds- the endolymph moves at the same speed as the rotation of the head- removes shearing forces
- If sudden stop- endolymph will continue to exert inertia due to momentum, the eyes tell you you have stopped: this is what causes dizziness
Sensory reception of the otolith organs
- Apparatus is known as the Maculae
- In the saccule- orientate in vertical plane
- In the Utricle- orientated in the horizontal plane
Cilia of the otolith organs
Have a series of cilia
- One large Kinocilium
- Lots of small stereo cilia
The cilia protrude into the gelatinous material known as the otolith membrane
The otolith membrane contains CaCo3 crystals known as otoliths
Tilting of the head
Detected by the utricle
- the otoliths have a higher density than the endolymph so are more affected by gravity
- when the head tilts- they move the otolith membrane and therefore the cilia
- backwards tilt moves the stereo cilia in the direction of the kinocilium and therefore increases the frequency of action potentials generated.
Verticle force response
Detected by the maculae in the saccule
-e.g movement in a lift, position of the head when lying down.
Cilia of the SCC
Kinocilium: large cilia
Stereo cilia: smaller cilia
Movement of the stereo cilia in the direction of the kinocilium increases discharge and the frequency of action potentials in the vestibular nerve
Movement of the stereo cilia in the direction away from the kinocilium decreases discharge and the frequency of action potentials to the vestibular nerve- leads to hyper polarisation
The cupula are all positioned in different ways- allows them to detect quite a good 3D image based on firing and inhibition.
Relations between the vestibular apparatus, the visual apparatus and the postural muscle
- Projections from the vestibular nuclei ipsilaterally, contra laterally and bi-laterally to the motor pathways and the extra ocular muscles
- Proprioreceptors send impulses to the vestibular nuclei from the limbs, neck and eye muscles : tells you whether the whole body has moved or just the head
- Vestibular nuclei send impulses to the cerebral cortex via the thalamus: makes person aware of where body is in time and space = kinaesthesia
Vestibular system reflexes
Tonic labyrinthine reflex
Dyanamic righting reflex
Vestibule-ocular reflex
Tonic labyrinthine reflex
- Allows you to keep your head straight (axis of the head in line with rest of body)
- Receives input from the proprioreceptors of the neck and the maculae
Dynamic righting reflex
- Postural adjustments which prevent you from tripping
- Long reflexes which involve all limbs
Vestibulo-ocular reflex two types
Static reflex: Eyes introit/extort whilst the head is moving in order to allow the direction of gaze to be maintained
Nystagmus reflex
Vestibulo-ocular reflex principles
strong association between vestibular apparatus, visual apparatus and postural control
- Sensory afferents from the semi-circular canals can connect to afferent fibres travelling to the extra ocular muscles via the vestibular nerve= can control eye movement
- The visual apparatus can send projections to the muscles which control postural movement
- If there is damage to the vestibular apparatus then balance can still be achieved through vision and slow movements
- If vision is lost then balance is lost
Dynamic eye nystagmus
- Series of rapid involuntary eye movements against the direction of head rotation- allows the direction of gaze to be maintained
- When eyeball reaches the end of its range of movement it flicks back to the zero position
- Direction of nyagmus is defined as the direction of the rapid flick back so right rotation of head= right nystagmus
Ways of testing vestibular function via nystagmus
- Post rotatory nystagmus
- Caloric stimulation
Post rotatory nystagmus
Patients are rotated in a barany chair
If they are spun left- during movement there will be a left nystagmus
When the spinning is interrupted- they should present with a right nystagmus (for about 20 s)
This is due to the endolymph catching up and pushing the cupula in the opposite direction
Caloric stimulation
Very simple test for the horizontal semi-circular canal
Water enters into ear and causes convection current in endolymph
When warm water placed in ear- nystagmus in direction of ear
When cold water placed in ear- nystagmus away form ear
Why does nystagmus not occur during swimming
Because both ears are being wet
Diseases which affect the Vestibular function
- Labyrithitis
- Menieres disease
- Lesions of the brain
- Chronic impairment of the vestibular apparatus
