Vestibular System Flashcards
Peripheral receptor apparatus
In the inner ear, responsible for transducing head motion/position
Central vestibular nuclei (brainstem)
Integrating and distributing info that controls motor activities and spatial orientation
Vestibuloocular network (vestibular nuclei)
Involved in the control of eye movements
Vestibulospinal network
Coordinates head movements, axial musculature and postural reflexes
Vestibulothalamocortical network
Conscious perception of movement/spatial orientation
Receptor cells in vestibular organs are innervated by
Primary afferents of the vestibular ganglion
Central processes of these bipolar cells enter the brainstem and terminate in the ipsilateral vestibular nuclei and cerebellum
Blood supply to labyrinth
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
Meniere’s disease
Excessive endolymph, causing hydrops
Fluctuating hearing, vertigo, BPPV, nausea
Type I and type II hair cells
CN VIII innervation
Movement of stereocilia towards kinocilia causes depolarization
Ampullae
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
Maculae
Utricle and saccule
Stereocilia extend into gelatinous otolith membrane covered by otoconia
Gravity/linear accelerations displace otoconia which bend underlying hair cell stereocilia
Vestibular afferents
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
Vestibulocerebellar (VC) fibers
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
Reciprical cerebellovestibular fibers
Target vestibular nuclei (juxtarestiform body)
Provide regulatory mechanisms for control of eye movements, head movements and posture
BPPV
Otoconial crystals from utricle separate from otolithic membrane and become lodged in cupula of a semicircular canal
Vestibular schwannoma
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
Vestibular neuritis
Severe vertigo, nausea, vomiting, no hearing loss or CNS deficits
Involves edema of vestibular nerve maybe from viral infection
Lateral vestibulospinal tract
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
Medial vestibulospinal tract
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
Thalamocortical pathways
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
Lesions to parietoinsular vestibular cortex and posterior parietal cortex
PIVC- vertigo, unsteadiness, loss ‘visual vertical’
Posterior parietal cortex- lesions result in confusion in spatial awareness
Vestibulooccular reflex
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
Sequence of events in VOR
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
Nystagmus during VOR
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
Spontaneous nystagmus
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
Spontaneous nystagmus damage
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
Caloric test
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
Oculocephalic reflex- Dolls eyes
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
What reflexes will be absent in comatose patient with dysfunction at the brainstem
Dolls eyes and ice water calorics test
