Vestibular Sytem DLA

Question 1

Describe vestibular labyrinth functional anatomy

Answer 1

• Vestibular labyrinth embedded in the petrous part of the temporal bone
• Two types of vestibular organs:
  
  • Otolith organs (detecting linear acceleration)
    
    • Utricle
    • Saccule
  • Semicircular canals (detecting angular acceleration)
    * Anterior semicircular canal
    * Posterior semicircular canal
    * Horizontal (lateral) semicircular canal
• Fluid compartment filled with endolymph, which is rich in potassium

Question 2

Explain mechano-electric coupling

Answer 2

• Opening or closing of mechanically gated cation channels (TRPA1 channels) causes graded potential changes in hair cells, which range from depolarization (excitation) to hyperpolarization (inhibition).
• Mechanical force produced by a “tip link” between neighboring cilia directly opens the cation channels during deflection of the cilia towards the tallest cilium.
• The inward current of potassium ions depolarizes the hair cells.

Question 3

Describe linear acceleration of the otolith organ?

Answer 3

Deflection of the cilia towards the largest of the cilia (kinocilium) causes the largest signal (excitation); deflection towards the smallest causes the smallest signal (inhibition)

Depending on the direction of the movement, different groups of hair cells get either excited or inhibited

Question 4

Why does orientation matter for semicircular canals?

Answer 4

Hair cells all oriented the same, inhibition to one side of hair cell turning, excitation in the other

Question 5

What is the essence of the vestibular pathways?

Answer 5

Vestibular signal transduction takes place in hair cells located in
otolith organs and in the ampullae of semicircular canals
• Hair cells transmit their signal on bipolar cells, that have their cell bodies located in the vestibular ganglion; their axons form the vestibular portion of CN VIII
• The CN VIII axons enter the brainstem at the ponto-medullary junction and target neurons in the vestibular nuclei and the cerebellum
• Review the location of vestibular nuclei in the brainstem and their blood supply helps with the interpretation of vascular brainstem lesions
• Descending pathways from the vestibular nuclei via vestibulospinal tracts to motor neurons in the spinal cord contribute to adjustments of head and body
• Vestibular input to nuclei of CN III, CN IV and CN VI drives extraocular muscle reflexes, coordinating eye movements with movements of the head
• Vestibulocortical pathways that lead to conscious perception of vestibular sensations are poorly understood