10 - Vestibular systems Flashcards
What is meant by the vestibular system?
a sensory system that provides the leading contribution tot he sees of balance and spatial orientation for the purpose of co-ordinating movement with balance
What are the 3 inputs, the main processor and the outputs of the vestibular system?
the eye
vestibular organ
propiceptors (pressure)
brainstem is the main processor
ocular reflex
postural control
What is the organ of balance?
posterior labyrinth/ vestibular apparatus
What are the 5 organs of the vestibule system?
utricle
saccule
(utricle + saccule = otolith organs)
3x semicircle canals
Describe the locations of the perilymph and the endolymph in the inner ear
Perilymph - extracellular fluid. Found within the semicircular canals and the scala tympani and scala vestibuli of the cochlea (between membraneous labyrinth)
Endolymph - contained in the membranous labyrinth of the inner ear.
The main difference between endolymph and perilymph is that the endolymph is the fluid found in the membranous labyrinth whereas the perilymph is the fluid that surrounds the endolymph, located inside the bony labyrinth.
Where do the 3 semicircle canals attach to?
the utricle
What is the cupula?
The ampullary cupula is a gelatinous body overhanging the hair cells of the ampullary crests of the semicircular ducts; sensory hair cells are immersed in it and movement of endolymphatic fluid causes the cupula to move across the hair cells of the ampullary crest.
Describe the nature of the attachments/entrances of semicircle canals to the utricle
5 entrances in total
3 wider entrances (cupula) (they are wider because they contain organs/cells) and 2 smaller ones
Describe how you can differentiate the different semicircular canals from each other
- the anterior and posterior semicircle canals come together
- the lateral semicircular canal is by itself
What are the two types of hair cells involved in transduction?
(similar to that of the cochlea) Type I: More in number Direct afferents, Indirect efferents
Type II: outer hair cells
Direct afferents and efferents
What is meant by the otolith organs?
utricle + saccule
What are the macula of the static labyrinth?
Specifically, where are these found in the otolith organs?
where the cells are and the place where the process of transduction occurs
- bottom of the utricle
- side of the saccule
Give the structure of the static labyrinth i.e. the otolith organs
otoliths are on top of gelatinous matrix, which is above the hair cells in the maculae
(see OneNote)
What is meant by the striola?
in the centre of the macula
opposing hair bundle polarities from this point outwards
What is meant by the kinetic labyrinth and the static labyrinth?
The static labyrinth: Otolith organs (utricle and saccule)
The kinetic labyrinth: Semicircular canals(SCC)
What is the ampulla (in the kinetic labyrinth)?
Hair cells in crista
Name the gelatinous projection in the kinetic labyrinth?
Cupula - this is the wider part of the SCCs at the site where they bind to the utricle
What is the difference between stereocilia and kinocilia?
nonmotile stereocilia and flexible motile kinocilia
kinocilium (true cilium - have a 9+2 microtubule arrangement)
stereocilia (no microtubules, instead they have actin cores)
What is a kinocilia?
a motile cilium that occurs alone at the apex of a sensory hair cell of the inner ear among numerous nonmotile stereocilia
Kinocilia: same direction on each side of the head
Describe the direction of the hair bundles/kinocilia in the static labyrinth when they are deflected
utricle
deflection: towards striola
saccule
deflection: away from the striola
Describe the direction of the hair bundles/kinocilia in the kinetic labyrinth when they are deflected
horizontal canals - kincolila deflect towards the utricle
ventrcial canals - kincolila deflect away from the utricle
Describe the blood supply to the vestibular organs
- anterior inferior cerebellar artery
(arises from the basillare arteries) - labyrinth artery arises from ^, the sole blood supply to the inner ear
Where do the primary afferents end at?
in vestibular nuclei (pons and medulla) and in cerebellum
NOTE: there are 2 vestibular nerves
- superior - to utricle and horizontal canals
- inferior - to saccule and posterior canals
Name the vestibular nuclei
superior
lateral
medial
inferior
Describe the organisation of the nuclei and the nerves of the static and kinetic labyrinth (i.e. which organs have afferents leading to which nuclei?)
Static labyrinth (otoliths) - lateral & inferior nuclei Kinetic labyrinth (SCC) - superior & medial nuclei
Describe which vestibular organs the 2 vestibular nerves supply
Superior vestibular nerve: superior canal, lateral canal, utricle
Inferior vestibular nerve: posterior canal and saccule
Where do the vestibular nuclei project to?
- spinal cord
- nuclei of the extraocular muscles
- Cerebellum
- Centers for cardiovascular + respiratory control
- thalamus
What do the vestibulo-cerebellar pathways control/modulate?
Movement coordination
Posture regulation
VOR modulation
Where do the thalamic nuclei (that play a role/impact the vestibular system) project to?
How do these projections impact the vestibular system?
- to the head region of the primary somatosensory cortex
- to superior parietal cortex: ‘vestibular cortex’ concerned with spatial orientation.
Cortical projections may account for feeling of dizziness during certain kinds of vestibular stimulation
State the functions of the vestibular system
- To detect and inform about head movements.
- To keep images fixed in the retina during head movements.
- Postural control
Describe the discharge from the nerve fibres of the vestibular system
have a ‘resting discharge’ - nerve fibres are normally discharging in their resting state
this increases and decreases with depolarisation/excitation and hyper-polarisation/inhibition respectively
What 2 types of movement of detected by the otoliths (i.e. bending of the stereocilia)?
linear acceleration and tilt (gravity forces)
In what force direction does the utricle and saccule detect and why is this?
Utricule: horizontal
Saccule: vertical
This is due to the orientation of the macula
Describe the orientation of hair cells on either side of the striola and what the indicating effects of this are
on either side of the striola, the hair cells deflect in opposite directions (so half will be polarised and half hyper-polarised)
half will increase in firing rate and half will decrease in firing rate
What type of movement is detected by the semicircle canals?
angular acceleration (constant velocity is not detected like in the otoliths)
Describe the direction of the kinocilia during endolymph movement in the SCCs
depolarisation
- in the superior and inferior SCC - away from the utricle
- in the horizontal SCC - bend towards the utricle
What happens the cupula when the movement os of constant velocity?
the cupola goes back to its original position - SCCs only respond to acceleration
The SCCs also work in pairs (the otoliths have the oppositely oriented hairs either side of the striola). What are these pairs
between both sides of the head
- lateral ventricles are paired
- left anterior and right posterior
- right anterior and left posterior
What are the two main vestibular reflexes?
VSR - Vestibulospinal Reflex
VOR - vestibulo-ocular reflex
Name and describe the two tracts involved in the VSR (Vestibulospinal Reflex)
- on which side are the projections?
- where do the motor neurones project to?
lateral vestibulospinal tract - ipsilateral - motor neurones to limb muscle medial vestibulospinal tract - bilateral - motor neurones to neck and back muscles
What is the function of the vestibulo-ocular reflex?
Between what nuclei does it connect?
To keep images fixed - to do this, eye movement is in opposite direction to head movement.
Conection between vestibular nuclei and oculomotor nuclei.
What is nystagmus?
Nystagmus is an involuntary oscillation of one or both eyes about one or more axes.
One eye will be firing more than the other. This is perceived y the brain as head movement, so they eyes move in the opposite direction (to this perceived movement).
The eyes can be brought back to the middle through conscious movement - but will automatically go back to one side
“dancing eyes”
How can vestibular condition be diagnosed?
Anamnesis Balance and Gait assessment Cerebellum Gaze assessment: eye movements. Vestibular tests: - Caloric test - vHIT - VEMP - Rotational test Imaging: CT Scan, MRI Subjective assessment (questionnaires)
What are the most common symptoms of vestibular conditions?
- Vertigo: Illusion of movement
- —-> usually rotational or ‘true vertigo’
- Dizziness, giddiness: more vague
- Unsteadiness: off balance
- Self –motion perception
Describe how balance disorders can be classified based on the location
Give some examples
Peripheral vestibular disorders: labyrinth and VIII nerve
- eg, vestibular neuritis, BPPV, Meniere’s disease BVF, UVF.
Central vestibular disorders: CNS (brainstem/cerebellum)
- eg, stroke, MS, tumours
