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NS-1 > The Vestibular System > Flashcards

The Vestibular System Flashcards

1
Q

What are the 5 components of the Vestibular system?

A
  1. peripheral receptor apparatus: in inner ear, transduces head motion/position
  2. central vestibular nuclei (brainstem): integrates and dist. info that controls motor activities and spacial orientation
  3. Vestibuloocular network (vestibular nuclei): involved in control of eye movements
  4. vestibulospinal network: coordinates head movements, axial musculature, and posture reflexes
  5. vestibulothalamocortical network: conscious perception of mvts/spatial orientation
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2
Q

What are the parts of the vestibular labyrinth?

A

3 semicircular canals

2 otolith organs = utricle and sacule

bony labyrinth surrounding membranous labyrinth

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3
Q

Where is perilymph?

A

in the space btw membranous labyrinth and bony labyrinth

similar to CSF

bathes the vestibular portion of CN 8

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4
Q

Where is endolymph?

A

in the membranous labyrinth

covers sensory receptors of both the vestibular and auditory systems

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5
Q

What is important about the compositions of endolymph and perilymph?

A

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

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6
Q

What do the semicircular canals sense?

A

rotational head movements (angular accelerations)

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7
Q

What do the otolith organs sense?

A

translational head movements = linear accelerations

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8
Q

What innervates receptor cells in vestibular organs?

A

primary afferent fibers of the vestibular ganglion

central processes of these bipolar cells –> enter brainstem and terminate in ipsi vestibular nuclei and cerebellum

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9
Q

How are the horizontal canal and the utricle oriented?

A

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

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10
Q

How are the saccule and the ant and post canals arranged?

A

vertically in the head

orthogonal to the horizontal canal and utricle

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11
Q

What is the blood supply to the vestibular system?

A

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

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12
Q

what occurs if there is an interruption of blood supply to the labyrinth?

A

will compromise vestibular and cochlear fxn

Sx: vertigo, nystagmus, unstable gait

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13
Q

What is the ampulla?

A

ending of semicircular canal duct

sensory receptors for the canals reside in base of each ampulla

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14
Q

How are recpetors in the utricle and saccule oriented?

A

in utricle: oriented longitudinally allong base

in saccule: oriented vertically along medial wall

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15
Q

Where does endolymph drain?

A

into the endolympatic sinus via small ducts –> endolymphatic duct = located adjacent to the dura mater

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16
Q

How is the saccule connected to the coclea?

A

by the ductus reuniens

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17
Q

What is Meniere’s disease?

A

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

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18
Q

What are the vestibular sensory receptors?

A

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

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19
Q

What are the different types of vestibular hair cells?

A

type I: chalice-shaped, surrounded by afferent terminal (nerve calyx)

type II: hair are cylindrical and innervated by synaptic boutons

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20
Q

Where are efferent vestibular cell bodies and what do they do?

A

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

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21
Q

How is the ampulla structured?

A

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

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22
Q

How is the macula structured?

A

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

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23
Q

Where is the superior vestibular nucleus?

A

superiorlateral in central pons

bordered by restiform body and 4th ventricle

24
Q

Where is the medial vestibular nucleus?

A

lateral floor of 4th ventricle –> rostrocaudal extent

from pons to medulla (longest nucleus)

25
Q

Where is the lateral vestibular nucleus?

A

lateral to medial V nucleus

from pons to upper medulla

contains large neurons = Deiters cells

26
Q

Where is the inferior vestibular nucleus?

A

lateral to medial V nucleus

in medulla

27
Q

In general, how do primary afferent fibers enter the vestibular nuclei?

A

enter at pontomedullary jxn

traverse resiform body and branch into ascending and descending fibers

28
Q

Where do afferents form the ampulla/semicircular canals project?

A

to superior and medial vestibular nuclei

minor inputs to lateral and inferior vestibular nuclei

29
Q

Where do afferents from otolith organs project?

A

to lateral, medial, and inferior vestibular nuclei

saccular afferents also go to contra oculomotor nucleus –> influence vertical eye movements

30
Q

What is significant about the vestibular organ and the cerebellum?

A

it is the only sensory organ in the body that sends direct primary afferent projections to the cerebellar cortex and nuclei

31
Q

How does the vestibular system communicate w/ the cerebellum?

A

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

32
Q

What are vestibulovestibular fibers?

A

form reciprocal connections w/ analogous contralateral nucleus

most prominent from superior and medial nuclei

33
Q

What are spinovestibular fibers?

A

arise from all levels of spinal cord

provide proprio input

primarily to medial and lateral vestibular nuclei

34
Q

What is the function of the vestibuloocular network?

A

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

35
Q

How, in general, does the rotational/horizontal vestibuloocular reflex occur?

A

horizontal canals and utricle receive info –> each of the six pairs of eye muscles must be controlled in unison to produce appropriate response

36
Q

If the head turns left, what muscles must be activated to keep looking at same spot?

A

want to turn both eyes right

left eye: medial rectus m (CN 3)

right eye: lateral rectus m (CN 6)

37
Q

When you turn your head left, how do you turn your eyes to keep looking at the same thing?

A

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

38
Q

During a leftward head turn, how are signals from the right vestibular nuclei inhibited?

A

decreased via commissural neurons from left vestibular nuclei

39
Q

What is a torsional eye movement?

A

when you roll tilt your head –> compensatory eye mvt = counter-roll = torsional mvt

40
Q

How does nystagmus occur

A

With large head rotations, compensatory eye mvts are active:

  1. vestibuloocular reflex: eyes move slowly opposite to head motion = slow phase
  2. eye reaches limit of turn
  3. will rapidly spring back to center (same direction as head) = fast phase
  4. Another slow phase begins
41
Q

How do you name nystagmus?

A

direction of fast return phase = direction of head turn

42
Q

What is the caloric test?

A

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

43
Q

What does vestibulospinal network do?

A

influences m tone and reflexive postural adjustments

lateral and medial tracts

44
Q

How does the lateral vestibulospinal tract (LVST) run?

A

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

45
Q

How does the medial vestibulospinal tract (MVST) run?

A

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)

46
Q

How do vestibular neurons project to the thalamus?

A

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

47
Q

Where are vestibular signals sent in the cortex?

A

(go here after thalamus)

  1. 2v and 3a = primary somatosensory cortex
  2. Area 7 = parietal cortex
  3. Insular areas of lateral sulcus and PIVC
  4. Prefrontal cortex and superior frontal gyrus
48
Q

What vestibular info is processed in areas 2v and 3a of SI?

A

2v –> sensations of whole body motion

3a –> motor control of head and body

49
Q

What vestibular info is processed in area 7 of the parietal cortex?

A

spatial coding, visual, and vestibular motion signals

integrates cues of body motion in space

lesions result in confusion in spatial awareness

50
Q

What vestibular info is processed in Insular areas of lateral sulcus and PIVC?

A

cells respond to body motion, somatosensory, proprio, and visual motion stim

PIVC lesions –> vertigo, unsteadiness, and loss of perception for visual vertical

51
Q

What vestibular info is processed in the prefrontal cortex and superior frontal gyrus?

A

vestibular signals related to frontal eye field

cotrols saccades and smooth pursuit eye mvts

52
Q

What is vertigo?

A

an illusion of body motion when no real motion is taking place

53
Q

What is benign paroxysmal positional vertigo?

A

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

54
Q

what is vestibular Schwannoma?

A

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

55
Q

what is vestibular neuritis?

A

Edema of vestibular nerve from viral infection

see severe vertigo, nausea and vomiting, but no deafness

Tx: antiemetics, vestibular suppressants, steroids, antivirals

NS-1 (50 decks)

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