The Vestibular System Flashcards
What are the 5 components of the Vestibular system?
- peripheral receptor apparatus: in inner ear, transduces head motion/position
- central vestibular nuclei (brainstem): integrates and dist. info that controls motor activities and spacial orientation
- Vestibuloocular network (vestibular nuclei): involved in control of eye movements
- vestibulospinal network: coordinates head movements, axial musculature, and posture reflexes
- vestibulothalamocortical network: conscious perception of mvts/spatial orientation
What are the parts of the vestibular labyrinth?
3 semicircular canals
2 otolith organs = utricle and sacule
bony labyrinth surrounding membranous labyrinth
Where is perilymph?
in the space btw membranous labyrinth and bony labyrinth
similar to CSF
bathes the vestibular portion of CN 8
Where is endolymph?
in the membranous labyrinth
covers sensory receptors of both the vestibular and auditory systems
What is important about the compositions of endolymph and perilymph?
they have different ionic concentrations
disturbances in dist or ionic content –> vestibular disease
balance is maintained by special secretory cells in membranous labyrinth and endolymphatic sac
What do the semicircular canals sense?
rotational head movements (angular accelerations)
What do the otolith organs sense?
translational head movements = linear accelerations
What innervates receptor cells in vestibular organs?
primary afferent fibers of the vestibular ganglion
central processes of these bipolar cells –> enter brainstem and terminate in ipsi vestibular nuclei and cerebellum
How are the horizontal canal and the utricle oriented?
slightly tilted anterodorsally relative to horizontal plane (nasooccipital plane)
when you walk or run, head normally declined so that you look down at something a few meters in front of you –> makes the plane of horizontal canal and utricle parallel w/ the earth
How are the saccule and the ant and post canals arranged?
vertically in the head
orthogonal to the horizontal canal and utricle
What is the blood supply to the vestibular system?
to the labyrinth: labyrinthine A = branch of AICA; enters temporal bone via internal auditory meatus
sylomastoid A also provides branches to labyrinth, mainly to semicircular canals
what occurs if there is an interruption of blood supply to the labyrinth?
will compromise vestibular and cochlear fxn
Sx: vertigo, nystagmus, unstable gait
What is the ampulla?
ending of semicircular canal duct
sensory receptors for the canals reside in base of each ampulla
How are recpetors in the utricle and saccule oriented?
in utricle: oriented longitudinally allong base
in saccule: oriented vertically along medial wall
Where does endolymph drain?
into the endolympatic sinus via small ducts –> endolymphatic duct = located adjacent to the dura mater
How is the saccule connected to the coclea?
by the ductus reuniens
What is Meniere’s disease?
too much endolymph –> endolymphatic hydrops = abnormal distention of the membranous labyrinth
Sx: fluctuating hearing loss, vertigo, positional nystagmus, and nausea; unpredictable attacks of sx and inability to make head movements
Tx: diuretic and salt-restricted diet to reduce hydrops; implantation of a small shunt into swollen endolymphatic sac
What are the vestibular sensory receptors?
hair cells w/ stereocilia that project from apical surface
60-100 hexavonally arranged stereocilia and a single longer kinocilium
receive synapses from vestibular efferent fibers that control receptor sensitivity
What are the different types of vestibular hair cells?
type I: chalice-shaped, surrounded by afferent terminal (nerve calyx)
type II: hair are cylindrical and innervated by synaptic boutons
Where are efferent vestibular cell bodies and what do they do?
lie rostral to vestibular nuclei and lateral to abducens nucleus
activated by behaviorally arousing stimuli or CN V stimulation –> innervate type I and II hair cells
How is the ampulla structured?
hair cells and supporting cells embedded in the crista, extend across base of ampulla
type I = central regions
type II = peripheral areas
enveloped in cupula = genatinous structure, same spec density as endolymph
rotational head movement –> endolymph displaced –> pushes cupula to one side or another –> displaces stereocilia in same direction
How is the macula structured?
hair cell stereocilia extend into otolith membrane = gelatinous coating
otolith covered by calcium carbonate crystals = otoconia
otoconia not displaced by normal endolymph mvts; move w/ changes in head position relative to gravity/linear acceleration –> displacement bends underlying hair cell stereocilia
Where is the superior vestibular nucleus?
superiorlateral in central pons
bordered by restiform body and 4th ventricle
Where is the medial vestibular nucleus?
lateral floor of 4th ventricle –> rostrocaudal extent
from pons to medulla (longest nucleus)
Where is the lateral vestibular nucleus?
lateral to medial V nucleus
from pons to upper medulla
contains large neurons = Deiters cells
Where is the inferior vestibular nucleus?
lateral to medial V nucleus
in medulla
In general, how do primary afferent fibers enter the vestibular nuclei?
enter at pontomedullary jxn
traverse resiform body and branch into ascending and descending fibers
Where do afferents form the ampulla/semicircular canals project?
to superior and medial vestibular nuclei
minor inputs to lateral and inferior vestibular nuclei
Where do afferents from otolith organs project?
to lateral, medial, and inferior vestibular nuclei
saccular afferents also go to contra oculomotor nucleus –> influence vertical eye movements
What is significant about the vestibular organ and the cerebellum?
it is the only sensory organ in the body that sends direct primary afferent projections to the cerebellar cortex and nuclei
How does the vestibular system communicate w/ the cerebellum?
primary vestibulocerebellar fibers: from fastigial –> thru juxtarestiform body (inf cerebellar peduncle) –> collaterals to dentate nucleus –> terminate as mossy fibers
secondary vestibulocerebellar projections: from vestibular nuclei to fastigial and dentate nuclei
What are vestibulovestibular fibers?
form reciprocal connections w/ analogous contralateral nucleus
most prominent from superior and medial nuclei
What are spinovestibular fibers?
arise from all levels of spinal cord
provide proprio input
primarily to medial and lateral vestibular nuclei
What is the function of the vestibuloocular network?
permits keeping a fixed gaze on an object while head is moving
achieved through vestibuloocular reflex: compensatory (equal in magnitude and opposite direction to the head mvt perceived)
reflex goes in multiple directions, can be suppressed
How, in general, does the rotational/horizontal vestibuloocular reflex occur?
horizontal canals and utricle receive info –> each of the six pairs of eye muscles must be controlled in unison to produce appropriate response
If the head turns left, what muscles must be activated to keep looking at same spot?
want to turn both eyes right
left eye: medial rectus m (CN 3)
right eye: lateral rectus m (CN 6)
When you turn your head left, how do you turn your eyes to keep looking at the same thing?
primary afferents from left horizontal canals –> medial and lateral vestibular nuclei –> medial longitudinal fasciculus (MLF) –> contra abducens nucleus (Right) –> CN 6 to ipsi lateral rectus muscle (right eye)
abducens interneurons –> excitatory impulse –> contra oculomotor nucleus (left eye) –> medial rectus m.
2nd set of vestib neurons: excitatory signals directly to ipsi (left) CN3 nucleus –> left med rectus m
3rd set of vestib neurons: inhib signas to ipsi (left) abducens nucleus
During a leftward head turn, how are signals from the right vestibular nuclei inhibited?
decreased via commissural neurons from left vestibular nuclei
What is a torsional eye movement?
when you roll tilt your head –> compensatory eye mvt = counter-roll = torsional mvt
How does nystagmus occur
With large head rotations, compensatory eye mvts are active:
- vestibuloocular reflex: eyes move slowly opposite to head motion = slow phase
- eye reaches limit of turn
- will rapidly spring back to center (same direction as head) = fast phase
- Another slow phase begins
How do you name nystagmus?
direction of fast return phase = direction of head turn
What is the caloric test?
Test for vestibular labyrinth in pts w/ dizziness or vertigo
COWS
Cold water –> nystagmus beating to Opposite side
Warm water –> nystagmus beating to Same side
both ears should give equal response
unilateral lesion of vestibular path –> reduced or absent nystagmus ipsi
What does vestibulospinal network do?
influences m tone and reflexive postural adjustments
lateral and medial tracts
How does the lateral vestibulospinal tract (LVST) run?
neurons in lateral and inf vestibular nuclei –> project to all levels of ipsi SC (anterorostral –> cervical; posterocaudal –> lumbosacral) –> lat medulla and ant funiculus –> alpha and gamma motoneurons and interneurons in laminae 7 to 9 –> collaterals in diff segments
How does the medial vestibulospinal tract (MVST) run?
input from vestibular receptors, cerebellum, and PCMLS from SC –> medial vestibular nucleus (mainly) –> descend bilaterally thru MLF and terminate in laminae 7-9 of cervical SC –> neck flexor and extensor Ms
seen in vestibulocolic reflex (stabilizes neck)
How do vestibular neurons project to the thalamus?
neurons in all nuclei –> bilaterally to thalamus (most go contra) –> VPL, VPI, and Anterior Pulvinar (dorsal to VPI) of thalamus –> to areas of the cortex
Where are vestibular signals sent in the cortex?
(go here after thalamus)
- 2v and 3a = primary somatosensory cortex
- Area 7 = parietal cortex
- Insular areas of lateral sulcus and PIVC
- Prefrontal cortex and superior frontal gyrus
What vestibular info is processed in areas 2v and 3a of SI?
2v –> sensations of whole body motion
3a –> motor control of head and body
What vestibular info is processed in area 7 of the parietal cortex?
spatial coding, visual, and vestibular motion signals
integrates cues of body motion in space
lesions result in confusion in spatial awareness
What vestibular info is processed in Insular areas of lateral sulcus and PIVC?
cells respond to body motion, somatosensory, proprio, and visual motion stim
PIVC lesions –> vertigo, unsteadiness, and loss of perception for visual vertical
What vestibular info is processed in the prefrontal cortex and superior frontal gyrus?
vestibular signals related to frontal eye field
cotrols saccades and smooth pursuit eye mvts
What is vertigo?
an illusion of body motion when no real motion is taking place
What is benign paroxysmal positional vertigo?
one of most common vestibular disorders
brief episodes of vertigo in particular changes of body position
otoconial crystals from utricle separate from otolith membrane –> lodged in cupula of canal = cupulolithiasis –> abnormal cupula deflections
what is vestibular Schwannoma?
slow growing, benign tumor that originates from Schwann cells of vestibular root (90% of cases)
typically in cerebellopontine angle –> impinges on structures traversing internal acoustic meatus (CN 7, 8, labyrinthine A)
hearing loss, gait difficulties, and tinnitus
what is vestibular neuritis?
Edema of vestibular nerve from viral infection
see severe vertigo, nausea and vomiting, but no deafness
Tx: antiemetics, vestibular suppressants, steroids, antivirals
