Neuro 11 - Vestibular Systems Flashcards
Where is the vestibular labyrinth located
Petrous temporal bone —> filled with perilymph
Describe the organelle structure of the vestibular labyrinth
- Saccule - connected to utricle and cochlea
- Utricle
- 3x semicircular canals. Posterior (/inferior) semicircular canal, anterior (/superior) semicircular canal and lateral semicircular canal. All canals meet at utricle
Vestibular hair cell cilia are embedded in?
Gelatinous matrix containing CaCO3 crystals - otoconia
Vestibular hair cells are mechanical transducers that detect static tilt and acceleration (i.e. if you move at constant speed, fluid doesn’t move, eyes don’t move). Describe their basic structure
Sensory hairs have stereo cilia - arranged in rows of varying highs, with a single long kinocilium
The nerve endings are 2 types:
Type 1 - chalice like endings forming ribbon synapses
Type 2 - simple nerve terminals
Briefly describe the transduction mechanism
(Similar to cochlea)
Hair cell receptor potential produced –> if hair cell moves towards kinocilium, depolarisation. If away then hyper polarisation
Ganglion cell discharge also depends on direction –> if toward kinocilium, increased frequency, if away from kinocilium, decreased frequency
What does the striola do?
and static labyrinth
Striola - divides hair cells into distinct populations with opposing hair bundle polarities
Saccular and utricular maculae lie PERPENDICULAR to each other meaning movement in any direction will stimulate a distinct subset of cells
i.e. head tilt to one side has opposite effects on corresponding hair cells of the other side (CaCO3 crystals involved as well)
Linear acceleration is dealt with by?
2 otolith organs - i.e. utricle and saccule
Angular acceleration is dealt with by?
3 semicircular canals (horizontal, anterior, posterior)
Describe the kinetic labyrinth
Semi-circular canals (SCC) involved
- In SCC, crista of ampulla (lots of hairs) detect head rotation in 3 orthogonal planes
- Cilia of hair cells embedded in gelatinous projection - “cupula”
- Hair cells stimulated by differential movement of endolymph and crest
- Kinocilia all oriented in same direction on each side of the head
Which artery supplies the inner ear
Anterior inferior cerebellar artery - also supplies cerebellum/other parts of the brain
Vestibular nerve primary afferents (superior and inferior) end in..?
Vestibular nuclei of brainstem and in cerebellum
What are the 4 vestibular nuclei, and how are they organised?
Superior, inferior, medial and lateral
Static labyrinth (otoliths) —> lateral and inferior
Kinetic labyrinth (SCC) goes to superior and medial
Where do vestibular nuclei project to?
- Spinal cord
- Nuclei of extra ocular muscles
- Cerebellum (ventral posterior and ventral lateral nuclei of thalamus)
- Centres of CVS + respiratory control
3 things which vestibulo-cerebellar pathway regulates
- Movement coordination
- Posture regulation
- VOR modulation
Vestibular system has resting discharge (nerve fibres giving info at a basal rate). What are the 3 states of the hair cells
- Resting
- Excitation
- Inhibition
Linear acceleration and tilt (and gravity) are caused by otolith movement. Explain linear acceleration
Utricle = horizontal plane movement
Saccule = vertical plane movement
Tilting causes movement of CaCO3 crystals which applies different weight/pressure on the hair cells, so hair cells move in appropriate direction relative to kinocilium.
Semi-circular canals are used for angular acceleration. Explain how
Works via endolymph inertia –> cupula moves and displaces hair cells
If cupula to utricle: depolarisation, if cupula away from utricle: hyperpolarisation
Endolymph flow is different direction to head motion
SCC ONLY RESPOND TO ACCELERATION/DECELERATION, NOT CONSTANT VELOCITY
In both static and kinetic movement (i.e. otolith and SCC), the same movement generates opposite …….
Opposite effects in each ear –> but both interpreted in a complementary way by the brain
Explain how the SCC work in pairs
Both horizontal SCC work together
Lt Ant and Rt Post (LARP)
Rt Ant and Lt Post (RALP)
i.e. one ant and one posterior
What are the 2 vestibular reflexes
- VSR
2. VOR
Describe the VSR
VSR = vestibulospinal reflex
Consists of:
1. Lateral vestibulospinal tract - ipsilateral, motor neurons to limb muscles (lumbar spinal cord)
- Medial vestibulospinal tract - bilateral, motor neurons to neck and back muscles (doesn’t go to lumbar area - stops at cervical and upper thoracic spinal cord). Although bilateral –> has a more dense ipsilateral projection
Describe Vestibulo-ocular pathways
Has about 8ms latency.
Functions to keep images fixed, works on connection between vestibular nuclei and oculomotor nuclei. Maintains our gaze on focussed objects.
Eye movement in opposite direction to head movement
Superior and medial vestibular neurones project to motor neurones supplying
Extraocular muscles
Axons ascend in MLF -(–> excite ipsilateral oculomotor and contralateral abducens muscles)
Explain nystagmus
- One side vestibular labyrinth doesn’t work
- So brain only has one input (from other sides basal firing rate)
- Brain thinks that youre turning your head but you’re not
- VOR triggered and eyes drift towards side that is lesioned. Then you move your eye back to centre as you realise. Rinse repeat
How are vestibular disorders diagnosed?
- Anamnesis
- Balance and gait assessment
- Cerebellum
- Gaze assessment - eye movements
- Vestibular tests
- Imaging
- Subjective assessment
What are symptoms of vestibular dysfunction
- Vertigo - illusion of movement
- Dizziness, giddiness
- Unsteadiness
- Self motion perception
What are the balance disorder types?
Peripheral and central vestibular disorders
Where are central vestibular disorders occuring
CNS - e.g. stroke, MS, Tumours
Where are the peripheral vestibular disorders occuring
Labyrinth and CN8 - e.g. vestibular neuritis, BPPV, Menieres disease, BVF, UVF
Vestibular disorders can be classified into 4 types (evolution)
- Acute: vestibular neuritis (labyrinthitis)
- Intermittent - Benign Paroxysmal Positional Vertigo (BBPV)
- Recurrent - Menieres disease - rare; or migraine (common)
- Progressive - Acoustic neuroma (CN8)
