vestibular system Flashcards
5 components of the vestibular system
- peripheral receptor apparatus
- central vestibular nuclei
- vestibuloocular network
- vestibulospinal network
- vestibulothalamocortical network
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 spacial orientation
vestibuloocular network
vestibular nuclei - involved in the control of eye movements
vestibulospinal network
coordinates head movements, axial musclulature, and postural reflexes
receptor structures in the vestibular labyrinth
- 3 semicircular canals
- 2 otolith organs
labyrinth components
- bony labyrinth
- membranous labyrinth
bony labyrinth
protective convering/shell that houses the fluid-filled system, the membranous labyrinth
membranous labyrinth
consits of connecting tubes and prominences that house vestibular receptors
perilymph
in the space between membranous labyrinth and bony labyrinth
-similar ionic composition to CSF
endolymph
fills membranous labyrinth
What bathes the vestibular portion of CN VIII?
perilymph
What covers sensory receptors for both vestibular and auditory systems?
endolymph
What does a disturbance in the ionic concentration between perilymph and endolymph cause?
vestibular disease
How is the ionic contents of perilymph and endolymph maintained?
specialized secretory cells in membranous labyrinth and endolymphatic sac
semicircular canals
3: horizontal, anterior, posterior
function: rotational head movements (angular acceleration)
otolith organs
2: utricle, saccule
function: translational head movements (linear accelerations)
What are receptor cells in vestibular organs innervated by?
primary afferent fibers of the vestibular ganglion (Scarpa’s ganglion)
What type of cells are in the vestibular receptor organs?
bipolar cells
Where do the central processes of bipolar cells of the vestibular receptor organs enter and terminate?
brainstem and terminate in the ipsilateral vestibular nuclei and cerebellum
What way is the horizontal semicircular canal oriented?
in a plane slightly tilted anterodorsally. becomes parallel with ground when running and looking a few meters in front of feet.
blood supply to the vestibular system
- labyrinthine artery from the anterior inferior cerebellar artery
- stylomastoid artery branches to the labyrinth semicircular canals (not primary source)
path of labyrinthine artery
enters temporal bone through the internal auditory meatus
What functions will be obstructed by occlusion to labyrinthine artery?
vestibular and cochlear function –> vertigo, nystagmus, unstable gait
What do the ducts of semicircular canals connect to?
utricle
What is the ampulla?
ducts of semicircular canals ending in a single prominent enlargement with sensory receptors for the semicircular canals at the base
Where are the receptors in the utricle?
oriented longitudinally along its base
How are receptors oriented in the saccule?
vertically oriented along medial wall
Where does endolymph from the labyrinth drain?
into endolymphatic sinus via small ducts.
-endolymphatic sinus communicates with endolymphatic duct with the endolymphatic sac
Where is the endolymphatic sac?
adjacent to dura mater
How is the saccule connected to the cochlea?
ductus reuniens
What is Meniere’s disease?
disruption of normal endolymph volume or comparison composition of endolymph vs perilymph resulting in endolymphatic hydrops (an abnormal distention of the membranous labyrinth)
symptoms of meniere’s disease
- fluctuating hearing loss
- vertigo
- postitional nystagmus
- nausea
- unpredictable attacks of auditory and vestibular symptoms including vomiting, tinnitus, and inability to make head movements or stand passively
Meniere’s disease treatment
- diuretic and salt-restricted diet to reduce hydrops
- impantation of a small shunt into swollen endolymphatic sac
What are vestibulosensory receptors?
hair cells with 60-100 hexagonally arranged stereocilia and a single longer kinocilium that project from apical surface
type I hair cell
chalice-shaped (shorter) surrounded by afferent terminal (nerve calyx)
type II hair cell
cylindrical and innervated by synaptic boutons
-longer, efferent, relay hyper- or de-polarization
Where do hair cells receive synapses from?
vestibular efferent fibers that control receptor sensitivity
-efferent cell bodies rostral to vestibular nuclei and lateral to abducens nucleus
How are hair cells activated?
behaviorally arousing stimuli or CN V stimulation - motion/movement of endolymph
Where is the crista?
base of ampulla, embeds hair cells
- type I central
- type II peripheral
What surrounds the ampulla?
cupula
cupula
gelatinous structure that attaches to the roof and walls forming a fluid-tight partition with the same density as endolymph
How does the ampulla work?
rotational head movements produce angular accelerations –> endolymph displaced –> pushes cupula to one side or the other –> displaces stereocilia in the same direction
-look left –> left activated and right inhibited
What motion does the ampulla/cupula sense?
angular acceleration
What motion does the macula/otolith membrane sense?
gravity/linear acceleration
describe the macula
hair cell stereocilia extend into otolith membrane
describe the otolith membrane
gelatinous coating of the macula, covered by otoconia
- more dense than surrounding endolymph
- not displaced by normal endolymph movements
what are otoconia made of?
calcium carbonate crystals
How does the macula work?
changes in head position relative to gravity/linear accelerations (forward-backward, up-down) displace otoconia
-bends underlying hair cell stereocilia
How do vestibular nuclei work?
- process positional and mvoement info for control of visual and postural reflexes
- target oculomotor nuclei, vestibulocerebellum, contralateral vestibular nuclei, SC, reticular formation, and thalamus
vestibular nuclei
- superior vestibular nucleus
- medial vestibular nucleus
- lateral vestibular nucleus
- inferior vestibular nucleus
superior vestibular nucleus
- superolaterally in central pons, bordered by restiform body and 4th ventricle
- afferents from semicircular canals (ampulla)
medial vestibular nucleus
lateral floor of 4th ventricle –> rostrocaudal extent
-afferents from semicircular canals (ampulla) and otolith organs (maculae)
lateral vestibular nucleus
lateral to medial vestibular nucleus and contains large Deiters cells
-afferents from otolith organs (maculae) and minor from semicircular canals (ampulla)
inferior vestibular nucleus
lateral to medial vestibular nucleus, extends through medulla
-afferents from otolith organs (maculae) and minor from semicircular canals (ampulla)
Where do saccular afferents project to?
contralateral oculomotor nucleus and influence vertical eye movements
Where do primary vestibular afferent fibers enter?
pontomedullary junction, transverse restiform body and branch to ascending and descending fibers
What is the only sensory organ that sends direct primary afferent projections to the cerebellar cortex and nuclei?
vestibular organs
primary vestibulocerebellar fibers
-from fastigial through juxtarestiform body (inferior cerebellar peduncle) sends collaterals to the dentate nucleus and terminate as mossy fibers
secondary vestibulocerebellar fibers
vestibular nuclei to fastigial and dentate nuclei
vestibulogvestibular fibers
arise from all vestibular nuclei and send reciprocal connections to analogous contralateral nuclei
-most prominent in superior and medial
spinovestibular fibers
arise from all levels of spinal cord and provide proprioceptive input primarily to medial and lateral vestibular nuclei
Where else do the vestibular nuclei neurons receive input from?
reticular formation, raphe nuclei, thalamus, and several cortical regions
vestibuloocular network
permits keeping a fixed gaze on an object while the head is moving
reflex to stabilize movements
vestibuloocular reflex
vestibuloocular reflex
compensatory = equal in magnitude and opposite in direction to head movement perceived
- any direction or speed
- can be supressed at will to focus on moving target
What vestibular organs are used in the rotational vestibuloocular reflex for
- vertical movements
- horizontal movements
- torsional eye movements
- vertical: vertical semiscircular canals and saccule
- horizontal: horizontal canals and utricle
- torsion: vertical semiscircular canals and utricle
Describe the steps of the rotational vestibuloocular reflex with a left head turn
- left horizontal semicircular canal activates neurons in left vestibular nuclei
- inhibitory signals from right vestibular nuclei are decreased via commissural neurons
- neurons in the left vestibular nuclei excite contralateral abducens motor neurons
- produces contraction in right lateral rectus and left medial rectus muscles
- compensatory rightward eye movement keeps the object of interest in the fovea
Where is the medial longitudinal fasciculus (MLF)?
between abducens nucleus to contralateral abducens nucleus
left head turn: where are excitatory and inhibitory signals?
- abducens interneurons send excitatory impulses to contralateral oculomotor nucleus –> left medial rectus m
- vestibular neurons send excitatory signals to ipsilateral oculomotor nucleus –> left medial rectus m
- vestibular neurons carry inhibitory signals to ipsilateral abducens nucleus –I left lateral rectus
- abducens motor neurons send excitatory impulses via CN VI to ipsilateral (right) lateral rectus m
nystagmus
compensatory eye movement with large head rotations
1. vestibuloocular reflex directs eyes slowly in direction opposite to head motion = slow phase
2. eye reaches limit of how far it can turn
3. will RAPIDLY spring back to central position, moves eye in same direction
4. another slow phase begins
combination of slow phases punctuated by fast return phases
What is nystagmus named for?
FAST return phase (left-beating or right-beating)
caloric test
warm or cold water in external auditory canal
-warm - toward ear
-cold - away
COWS
vestibulospinal network
lateral vestibulospinal tract and medial vestibulospinal tract
lateral vestibulospinal tract (LVST)
- lateral and inferior vestibular nuclei
- topographically to ipsilateral SC
- through lateral medulla and anterior laminae VII and IX
- coordination of postural control
medial vestibulospinal tract (MVST)
- neck muscles
- medial vestibular nucleus, less from inferior and lateral
- input from vestibular receptors, cerebellum, and PCMLS info from SC
- MLF to laminae VII to IX of cervical SC
- neck flexor and extensor motor neurons
- vestibulocolic reflex
vestibular thalamus
neurons from all 4 vestibular nuclei project bilaterally to thalamus, but most terminate in contralateral thalalmic nuclei
VPL and VPI and anteror pulvinar
areas 2v and 3a (primary somatosensory cortex)
2v - sensations of whole body motion
3a - motor control of head and body
area 7 (parietal cortex)
houses neurons involved in spatial coding, visual (optic flow) and vestibular motion signals
- integrates cues of body motion in space
- lesions: confusion in spatial awareness
insular areas of lateral sulcus and parietoinsular vestibular cortex (PIVC)
- cells respond to body motion, somatosensory, proprioceptive, and visual motion stimuli
- PICV lesions cause episodes of vertigo, unsteadiness, and loss of perception for ‘visual vertical;
preforntal cortex and superior frontal gyrus
receive vestibular signals and are related to the frontal eye field (FEF)
-controls of saccades and smooth pursuit eye movements
What are the areas of the vestibular cortex?
- areas 2v and 3a in primary somatosensory cortex
- area 7 of parietal cortex
- insular areas of lateral sulcus and parietoinsular vestibular cortex (PIVC)
- prefrontal cortex and superior frontal gyrus
dizziness
nonspecific term that generally means a spatial disorientation that may or may not involve feelings of movement
- may be accompanied by nausea or postural instability
- not exclusively vestibular in origin
vertigo
an illusion of body motion, often spinning or turning, experienced when no real motion is taking place
subjective vertigo
patient experiences the sensation of spinning while things in the environment are not moving
objective vertigo
sensation is one of the objects spinning while the patient is not moving
-nystagmus beats in opposite to the original direction of rotation
benign paroxysmal positional vertigo
- common vestibular disorder
- brief episodes of vertigo that coincide with particular changes in body position
- triggered by turning over in bed, getting up in the morning, bending over, or rising from a bent position
- otoconial crystals from the utricle separate from the otolith membrane and become lodged in the cupula of a semicircular canal (cupulolithiasis) producing abnormal cupula deflections when the head changes position relative to gravity
vestibular schwannoma
- benign tumor that originates from the Schwann cells of the vestibular root (90% of cases), rarely from cochlear root
- represents 5-10% of intracranial tumors
- typically located in the cerebellopontine angle where it impinges on structures traversing the internal acoustic meatus (CN VII, CN VIII, labyrintine artery)
- slow growing and pt may present with hearing loss (almost all), gait difficulties (70%), and tinnitus (70%)
vestibular neuritis
- pt present with severe vertigo, nausea, and vomiting, but no hearing loss or other CNS deficits
- thought to involve edema of the vestibular nerve or ganglion: acute viral infection (herpes simplex virus)
- some report history of URI, cold, influenza
- treatment: antiemetics, vestibular suppressants, corticosteroids, and antiviral agents
