Vestibular System

Question 1

Peripheral receptor apparatus

Answer 1

In the inner ear, responsible for transducing head motion/position

Question 2

Central vestibular nuclei (brainstem)

Answer 2

Integrating and distributing info that controls motor activities and spatial orientation

Question 3

Vestibuloocular network (vestibular nuclei)

Answer 3

Involved in the control of eye movements

Question 4

Vestibulospinal network

Answer 4

Coordinates head movements, axial musculature and postural reflexes

Question 5

Vestibulothalamocortical network

Answer 5

Conscious perception of movement/spatial orientation

Question 6

Receptor cells in vestibular organs are innervated by

Answer 6

Primary afferents of the vestibular ganglion
Central processes of these bipolar cells enter the brainstem and terminate in the ipsilateral vestibular nuclei and cerebellum

Question 7

Blood supply to labyrinth

Answer 7

Via labyrinthine artery- branch of AICA
Enters temporal bone via internal auditory meatus
Interruption of this blood supply compromises vestibular and cochlear function
-Vertigo, nystagmus, and/or unstable gait

Question 8

Meniere’s disease

Answer 8

Excessive endolymph, causing hydrops

Fluctuating hearing, vertigo, BPPV, nausea

Question 9

Type I and type II hair cells

Answer 9

CN VIII innervation

Movement of stereocilia towards kinocilia causes depolarization

Question 10

Ampullae

Answer 10

Semicircular ducts
Hair cells in cristae, extends across base of ampulla
Extends into cupula
Rotational/angular accelerations displace endolymph which bends cupula to one side and displaces stereocilia

Question 11

Maculae

Answer 11

Utricle and saccule
Stereocilia extend into gelatinous otolith membrane covered by otoconia
Gravity/linear accelerations displace otoconia which bend underlying hair cell stereocilia

Question 12

Vestibular afferents

Answer 12

CN VIII from semicircular ducts (ampullae) or otolith organs (maculae) enter at pontomedullary junction
Project to vestibular nuclei- superior, medial, lateral, inferior
Secondary neurons target CN III, IV, VI nuclei and vestibulocerebellum, SC, reticular formation and thalamus

Question 13

Vestibulocerebellar (VC) fibers

Answer 13

Only sensory organ in body that sends direct projections to cerebellum
Fibers course through the juxtarestiform body, part of the inferior cerebellar peduncle
Primary VC fibers target dentate nucleus and terminate as mossy fibers
Secondary VC fibers target flocculonodular lobe and fastigial and dentate nuclei

Question 14

Reciprical cerebellovestibular fibers

Answer 14

Target vestibular nuclei (juxtarestiform body)

Provide regulatory mechanisms for control of eye movements, head movements and posture

Question 15

BPPV

Answer 15

Otoconial crystals from utricle separate from otolithic membrane and become lodged in cupula of a semicircular canal

Question 16

Vestibular schwannoma

Answer 16

Benign schwann cell tumor
Typically within cerebellopontine angle
Impinges of structures traversing internal acoustic meatus- CN VII, VIII, labyrinthine artery
May present with hearing loss, gait difficulty, tinnitus

Question 17

Vestibular neuritis

Answer 17

Severe vertigo, nausea, vomiting, no hearing loss or CNS deficits
Involves edema of vestibular nerve maybe from viral infection

Question 18

Lateral vestibulospinal tract

Answer 18

Arises from lateral and inferior vestibular nuclei
Topographical projection to ipsilateral SC
Anterorostral areas- cervical cord
Posterocaudal area- lumbosacral cord
Powerful control of extensors for postural control

Question 19

Medial vestibulospinal tract

Answer 19

Arises from medial vestibular nucleus
Integrated input from vestibular receptors, cerebellum and PCMLS
Fibers descend bilaterally through MLF and terminate in cervical SC- stabilize neck flexors/extensors
Critical in the vestibulocolic reflex, stabilizes head via activation of neck musculature

Question 20

Thalamocortical pathways

Answer 20

All vestibular nuclei project to ventral posterior nuclear complex
Thalamic nuclei function to relay vestibular input needed for processing of motion and body orientation
They target primary somatosensory cortex, parietoinsular vestibular cortex and posterior parietal cortex

Question 21

Lesions to parietoinsular vestibular cortex and posterior parietal cortex

Answer 21

PIVC- vertigo, unsteadiness, loss ‘visual vertical’

Posterior parietal cortex- lesions result in confusion in spatial awareness

Question 22

Vestibulooccular reflex

Answer 22

Stabilizes retinal images during head movements through vestibular input
Head rotations stimulate semicircular canals- provides vestibular input about head motion- drives counter rotation of the eyes

Question 23

Sequence of events in VOR

Answer 23

Rightward head rotation
Fluid movement in right semicircular duct
Activation of right afferent fibers
Right vestibular nuclei
Activation of left CN VI nucleus and right CN III nucleus (Via MLF) - inhibitory neurons from vestibular nuclei also target ipsilateral CN VI
Left lateral rectus linked with right medial rectus

Question 24

Nystagmus during VOR

Answer 24

VOR directs eyes opposite to head motion in slow phase

Eyes reach limit of how far they can turn and rapidly spring back to central position during fast phase

Question 25

Spontaneous nystagmus

Answer 25

Unilateral damage to vestibular system, brainstem or cerebellum Silencing of output from damaged side= net differences in firing rates of CN VIII when head is stationary Causes spontaneous nystagmus, may include vertigo, falls, vomiting

Question 26

Spontaneous nystagmus damage

Answer 26

Peripheral damage: Vestibular labyrinth and/or CN VIII Imbalance b/w left and right vestibular input causes nystagmus Does not affect voluntary saccades and smooth pursuit Central damage to brainstem/cerebellum: Nystagmus with impaired voluntary saccades and/or smooth pursuit Involvement of multiple pathways or cortical areas

Question 27

Caloric test

Answer 27

Tests vestibular labyrinth function/VOR without moving head Uses water to alter convection currents in endolymph which alters CN VIII firing rate Warm water causes nystagmus that beats toward irrigated ear Cold water causes nystagmus that beats away from irrigated ear COWS- Cold-opposite, Warm-same

Question 28

Oculocephalic reflex- Dolls eyes

Answer 28

Head thrust test: rotating the head back and forth horizontally induces compensatory eye movements that are dependent on visual and vestibular function Reflex occurs when eyes move in opposite direction of the head movements-- dolls eyes Comatose patient-- reflex assesses brainstem function Intact brainstem- dolls eye reflex present Brainstem lesion- reflex absent

Question 29

What reflexes will be absent in comatose patient with dysfunction at the brainstem

Answer 29

Dolls eyes and ice water calorics test