54 Vestibular System

Question 1

describe hair cells and afferent fiber activities

Answer 1

Question 2

describe what happens during depolarization of hair cells

Answer 2

• voltage-gated Ca channels embedded in the hair cell membrane are opened during depolarization, resulting in an inward current of Ca ions through these channels and in turn, the release of excitatory transmitter release

Question 3

describe the otolith organs

Answer 3

• the saccule and the utricle are endolymph filled pockets that contain in their walls a patch of sensory hair cells
• their cilia support (at their tips) calcium carbonate crystals glued together with a jelly like material
• when the cilia of the sensory hair cells are tilted, either by gravity forces or by linear acceleration, the cilia bend which leads to depolarization/hyperpolarization of the hair cells

Question 4

describe semicircular canals

Answer 4

• the semicircular canals are endolymph filled pipes which contain a patch of sensory hair cells (crista) with their cilia inserted into a jelly-like structure (cupula)
• when the endolymph moves relative to the walls of the canals, induced by angular acceleration (rotation) of the head around one of the axes of the semicircular canals, the cupula with the inserted cilia bends, which leads to de/hyperpolarization of hte hair cells

Question 5

the otolith organs detects ____ acceleration

while

the semilcircular canals detect ____ acceleration

Answer 5

the otolith organs detects linear acceleration

while

the semilcircular canals detect angular acceleration

Question 6

describe what happens when the excited semicircular canals induce the vestibulo-ocular reflex

Answer 6

• the semicircular canals relay the info. of head rotation to the vestibular nuclei in the brainstem
• fibers originating in the excited labyrinth, which form the afferent limb of the VOR, enter the brainstem at the ponto-medually junction and synapse in the vestibular nucleus
• axons of neurons located in the vestibular nucleus cross the body’s midline and synapse in the contralateral abducens nucleus
• the abducens activates the LR muscle of the right eye, and via fibers that cross the body’s midline, ascend in the MLF and activate the MR muscle of the left eye

Question 7

describe nystagmus

Answer 7

• rhythmically alternating eye movements
• 2 distinct phases:
  
  • slow phase: driven by a reflex (VOR)
  • fast phase: resets the position of the eye through saccadic movements (this is the direction of the nystagmus)

Question 8

describe caloric testing to test VOR

Answer 8

• caloric testing: injection of warm or cold water into the outer ear of a comatose patient induces conjugate eye movements (VOR intact) or stay fixed relative to the head (lesion affecting the VOR)
  
  • warm water: the eyes move away during the slow phase from the stimulated ear (driven by the VOR)
  • cold water: the eyes move towards during the slow phase the stimulated ear (driven by the VOR)

Question 9

describe what test is being done

Answer 9

when cold water is used, slow phase of nystagmus is toward the same side while fast phase is toward the opposite side

Question 10

describe what test is being done

Answer 10

when warm water is used, the slow phase of nystagmus goes to the opposite side while the fast phase goes to the same side