Vestibular

Question 1

How do hair cells encode movement?

Answer 1

• Hair cells - receptor cells; staircase-arranged stereo cilia w/ tallest being kinocilium; synapse on CN VIII afferents
  
  • At rest… some K+ channels are open —>basal neuroT release
  • Bend toward kilocilium —> opens more K+ channels (influx from endolymph into cell)—> depolarization —> Ca++ influx —> more neuroT release —> VIII afferents fire more
  • Bend away from kilocilium —> closes ALL K+ channels —> dec neuroT release —> dec firing

Question 2

Semicircular Canals v Otolith Organs

Answer 2

• 3 Semi-circular canals (lateral superior and posterior)
  
  • Ea canal has central swelling (ampulla) where hair cells are arranged w/ kinocilum pointing to center (utricle) all in a gel (cupula) SO… angular acceleration of head —> fluid lags due to inertia and has affect on stereo cilia (faster = more inertial dampening = more firing)
• 2 vestibular organs (sensory organs in vestibule itself)
• Fluid in vestibule is HEAVY (filled w/ otoconia - heavy calcium carbonate crystals) SO it moves in response to linear acceleration

Question 3

Utriculus v Sacculus

Answer 3

• Utriculus - horizontal plane - hair cells point TOWARD striola (acceleration from side to side or head tilt; also car acceleration)
  
  • Sacculus - vertical plane - hair cells AWAY from striola (elevator)

Question 4

Vestibulo-Ocular Reflex

Answer 4

• Reflex to Keep Eyes on Target While Moving Head (medial vestibular nuclei)
• If rotate head to L …
  
  • EXCITE L horizontal canal —> input to L medial vestibular nucleus
  • L side excites R abducens and L oculomotor
  • Inhibition of R side so NO excitation of L abducens or R oculomotor + Dec commissural inhibition of L side by R side

Abducens excites OPP side
Oculomotor excites SAME side

Question 5

What are the 4 vestibular nuclei and where do they project to?

Answer 5

• All in dorsal brainstem
• Superior, lateral, medial, inferior (descending)
• Where do they project to?
  
  • Ea other (interconnected across sides)
  • CN III, IV, VI
  • Indirectly affect motoneurons via interneurons
    
    • Output to SC
      
      • MVST - bilateral (neck movements)
      • LVST- ipsilateral (postural control)
        - Affects both alpha motoneurons and gamma motoneurons (sensitize the extensor muscles for myotatic reflex)
        - Excite extensors and inhibit flexors
  • Parietal cortex
  • Regions of brainstem that control respiration and circulation

Question 6

Caloric Testing

Answer 6

• Inject warm or cold water into ear canal (7 degrees different than normal)
  
  • Warm water inc firing
  • Cold water dec firing
• Normal reaction = nystagmus so if none then problem
• Quick COWS - cold water —> quick phase in opposite direction; warm water —> quick phase in same direction

Question 7

What causes nystagmus?

Answer 7

• Baseline = balance b/n inhibition of abducens from same side vestibular nucleus and excitation from opposite side vestibular nucleus
• If damage to inner ear of L… no inhibition of L abducens from L side but R side excitation of L abducens persists —> contraction of L lateral rectus when stationary (eyes move to side of lesion)
  
  • Nystagmus = slowly look to side of lesion THEN quick compensatory movement back to neutral
    
    • Slow phase
    • Quick phase

Question 8

Vestibular Compensation

Answer 8

• Corrects self w/in 2 days via flocculonodular lobe of cerebellum
• Recovered when movement is slow but can still see lag in fast movement
  • VOR gain = 1 normally (Vel head = Vel eyes)
  • VOR gain <1 if eyes lag behind head

-Baseline = balance b/n vestibular pathways BUT if lose function of either inner ear canal then corrected by cerebellum so baseline is now not balanced

Question 9

Vestibular Myogenic Potential Test

Answer 9

• Auditory stimulus —> sacculus —> vestibular nucleus —> MVAT neck motoneurons activated
• This is measured by stimulating in ear and recording w/ electrode at neck
• Isolates problem w/ vestibular nuclei or nerves

Question 10

Rotational Testing

Answer 10

• Gradual rotating chair
  - Meas eye movement & record nystagmus
  - Problem = does not isolate which side so not used often

Question 11

Examples of Central v Peripheral Vertigo

Answer 11

• Peripheral Examples
  
  • BPP - otoconia from utriculus gets into posterior semicircular canal; treat by moving patient’s head in specific way to get debris back into utriculus
  • Vestibular Neuritis - ACUTE due to viral infection of vestibular portion of CN VIII; damage due to inflammation so treat w/ steroids; dx via unilateral caloric reduction
  • Meniere Disease - (endolymphatic hydrops) unilateral hearing loss, ear fullness, lateralized tinnitus; tx is diuretic to dec pressure and Na+ restriction
• Central Examples
  
  • Migranous Vertigo - clinical dx based on headache, light/sound sensitivity, visual phenomena
  • Chiari Malformation - posterior tonsil in SC —> vertigo esp when head back; dx via MRI and treat w/ surgery to remove occipital bone to remove pressure
  • Posterior Fossa Stroke - vascular so acute but accompanied by complex of symptoms