9/2 Vestibular System - Crockett Flashcards Preview

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Flashcards in 9/2 Vestibular System - Crockett Deck (25):
1

vestibular system functions

 

1. senses...

  • which way is up
  • direction of movement

2. maintain posture using vestibulospinal and cervical spinal reflexes

  • allows head and trunk to move together or indep

3. control reflexive eye movements (cerebellar and oculomotor pathways)

  • vestibulo-ocular reflexes keep eyes still when head moves
  • vestibulo spinal reflexes enable skeletomotor system to compensate for head movements

4. detects 2 forms of acceleration

  • angular/rotational
  • linear

 

 

functions in conjunction with

  • vision
  • somatosensation

*need at least 2 of the 3 systems to figure out what's going on

2

dynamic and static nature of vestibular system fx

 

other systems worked with

dynamic (sensing movement through space) and static fx (sending head position)

dynamic fx:

  • detecting head movement
  • detecting angular acceleration (semicircular canals)
  • detecting linear accelerations (otolithic organs)

static fx:

  • sense position of head (primarily otolithic organs)

3

symptoms of vestibular disease

  • vertigo
  • nausea, vomiting, pallor, perspiration
  • difficulty walking
  • sensation of spinning or rhythmic movements
  • nystagmus
  • sensation of leaning/being pulled to ground
  • oscillopsia

4

location of vestibular system

  • bony labyrinth
  • membranous labyrinth (within bony labyrinth, surrounded by perilymph fluid)
  • sensory epithelia: receptor sheaths (containing endolymph fluid)

5

perilymph & endolymph

perilymph surrounds membranous labyrinth

  • low in K

endolymph fills membranous labyrinth

  • high in K

difference in potential is imp for hair cell fx

 

volumes of perilymph and endolymph are strictly controlled → overproduction leads to vestibular and auditory sx

6

labyrinthine infarction

inner ear infarct

  • usually occurs with thrombotic narrowing of AICA or basilar artery (at AICA) opening
  • usually associated with infact of brainstem and cerebellum within AICA territory
  • may present with primary vestibular dysfx and/or primary auditory dysfx

7

semicircular canal fx

function in PAIRS

L and R horizontal canals (both angled approx 30 degrees upward)

anterior canal & posterior canal (located in same plane)

8

where are hair cells?

how do hair cells pick up the info they need to?

specialized regions of vestibular labyrinth contain recpetors called hair cells

 

  • ampullae of semicircular canals contain ampullary crest (thickened epithelium populated by hair cells)
  • ampullary crest is covered by cupula (gelatinous mass)
    • cupulas have mass → inertia gives it some lag which ensure that hair cells perceive that lag and sense what's going on
    • better for angular accel than for linear accel

9

types of hair cells

 

  • nerves associated
  • location
  • function

2 kinds of hair cells:

1. type I hair cells

  • have a primary sensory afferent (aka nerve calyx) surrounding the root
  • both afferent and efferent communication with hair cell through primary sensory afferent
  • also has efferent nerve terminal with indirect conection (to nerve calyx → hair cell) which can modulate hair cell fx
  • location: tend to be at peak of crest (prob primary type of cell that detects movement of cupula)

2. type II hair cells

  • have a primary sensory afferent making a single connection
  • have an efferent nerve terminal making a direct connection too
  • location: tend to be at periphery/base of crest

 

hair cell stereocilia show directional polarity with 40-70 stereocilia arranged in size with long kinocilium at one end

  • kinocilium fx: bent one way or another → opening of Fe channels

10

arrangement of vestibular hair cells and relation to fx

afferent nerve is constantly active

  • bending of hair cell stereocilia TOWARDS kinocilium → increased rate of firing in afferent (depolarization)
  • bending of hair cell stereocilia AWAY FROM kinocilium → decreased rate of firing in afferent (hypoerpolarization)

 

11

explain nystagmus

medial vestibular nudles on both sides has neurons that carry signals to cause eye movements that compensate for head movement

  • head rotates left → incr firing on left side → eye rolls right to compensate (and vice versa)
  • brain is constantly comparing signals from these two nuclei to determine which way to move the eyes

IF RIGHT SIDE IS INJURED,

  • L firing > R firing → brain reacts as if head moving to L → eyes roll right (slow component), then snap back to left (fast component)
    • aka leftward nystagmus

12

response to linear accel 

utricle contains macula : thickened epithelium oriented in the horiz plane containing hair cells

  • covered with gelatinous material embedded w crystals of CaCO3
  • head tilt results in bending of sterocilia
  • best response to linear acceleration in horiz plane

saccule contains macula : thickened epithelium oriented in vertical plane

  • same setup
  • best response to linear accel in vertical plane

13

directional sensitivity in utricle, saccule

within utricle, hair cells are polarized towards the striola (indentation)

implication : any given head tilt activates some hair cells but not others!

 

within saccule, hair cells are polarized away from striola

14

benign paroxysmal positional vertigo

caused by otoconia detaching from otolithic membrane of utricle and getting stuck in a semicircular canal

subtypes: (generally uni but sometimes bilateral)

  • depend on which semicirc canal involved
    • 80-90% post. canal
  • free-floating (canalithiasis)
  • attached to cupula (cupulolithiasis)

 

15

vestibular nuclei

several nuclei in the brainstem are associated with the vestibular system

  • medial vestibular nucleus
    • vestibulo-ocular reflex
    • vestibulo-cervical reflex
  • lateral vestibular nucleus
    • control of posture
    • vestibulo spinal reflex
  • superior vestibular nucleus
    • vestibulo-ocular reflex
    • vestibulo-cervical reflex
  • descending (inf) vestibular nucleus
    • integration of inputs from vestibular labyrinth and cerebellum

diff fx reflecting diff afferent inputs

16

cerebellum

has primary afferent projections to flocculonodular lobe (nodulus and uvula)

important for cerebellar control of posture

vetibular system is only system that projects DIRECTLY to cerebellum

  • most vestibulocerebellar vibers con from superio and medial vestibular nuclei (and other brainstem nuclei)

17

vestibulocerebellum

  • direct and indirect input from labyrinth
  • major focus of output is medial (anterior) descending motor systems

18

descending projections from vestibular pathway

vestibulospinal tracts

  • lateral vestibulospinal tract
    • principal pathway by which otolithic organs can regulate the body's extensor organs
    • projects from lateral vestibular nucleus (major source) and SpVN on ipsilateral side
  • medial vestibular tract
    • mainly cervical → vestibulocervical reflexes
    • projects form MVN (major) and LVN, SpVN (minor) bilaterally, through MLF

19

ascending projections from vestibular nuclei

vestibulothalamic projections

  • ventroposterolateral (VPL), pars oralis
    • bilateral projections, mostly from MVN

other targets may include VP, VL, and intralaminar nuclei

 

vestibular thalamocortical projections

  • parietoinsular vestibular cortex
  • parts of Brodmann's 3a and 2V; few cells of Brodmann's 7a, 7b

20

Meniere's disease

overproduction of endolymph → excessive fluid in inner ear → expansion of membrane

  • severe vertigo, dizziness
  • tinnitis (roar of seashore)

21

tumors of CN VIII

Schwannoma: benign tumor of nerve sheath cells of CN VIII

  • operable
  • hearing, vertigo
  • growth of tumor can affect CN VII, V  →pain

22

nystagmus

involuntary rhythmic movements of eyes consisting of a rapid and a slow movement

  • named for fast saccade component, typically to side contralateral to dysfx vestibular apparatus

 

 

23

how can you induce nystagmus?

rotation test : rotate while bent forward 30 degrees - brings horiz system parallel to ground

  • should see fast saccade in direction of spin

caloric test : irrigate external auditory meatus with warm or cold water to set up convection currents in endolymph and displace crista ampullaris

  • nystagmus towards warm water, away from cold water
  • connection: cupula absorbs alcohol faster than endolymph. when you lie down drunk, cupula starts to float, displacing hair cells → SPINS

24

testing for vestibular dysfx

patient with dysfx labyrinth or nerve?

  1. will not respond to caloric stimulation → canal paresis
  2. display past-pointing → when asked to close eyes and "point", will point in direction of lesioned side
  3. turn or fall to dysfx side when marching in place with eyes closed
  4. display gaze nystagmus (contralateral to dysfx side) when looking up to 30% away from forward
  5. display positional nystagmus when head is put in particular positions (tested with Nylen-Barany maneuver)

25

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