Vestibular Anatomy and Function

Question 1

Functions of the vestibular system

Answer 1

-postural control
-gaze stability
-provide sensory information for spatial orientation

• postural control, postural stability, postural orientation

Question 2

postural stability

Answer 2

maintenance of the COM within the BOS

Question 3

postural orientation

Answer 3

ability to maintain a relationship between body segments and between body and environment for a task

Question 4

postural control inputs and outputs

Answer 4

INPUT
-visual
-somatosensory
-vestibular

OUTPUT
-movement strategies such as ankle, hip, and stepping
-muscle synergies
-automatic responses are present very early in children learning to walk

Question 5

What is central processing in the vestibular system?

Answer 5

adaptive strategies change with demands of the task and the environment

-anticipatory strategies pre-tune sensory and motor systems based on prior learning

-cognitive “override” –> you can cognitively override due to intent, motivation, and intention (sliding to hit a volleyball)

Question 6

OUTPUTS of the vestibular system

Answer 6

VOR - vestibular ocular reflex
VSR - vestibulo-spinal reflexes
VCR - vestibulo-collic reflex

Question 7

What are the peripheral vestibular structures?

Answer 7

vestibular end organ- 3 pairs of semicircular canals and otolith organs

cranial nerve VIII (vestibular portion)

Question 8

Blood supply of the peripheral vestibular structures?

Answer 8

basilar artery from vertebral artery
-AICA coming of of the basilar artery
–labyrinthine artery (branch of the AICA)
—anterio-vestibular artery
—common cochlear artery

Question 9

What are the components of the vestibular end organ?

Answer 9

-bony labyrinth - bony portion of the temporal bone; houses the vestibular apparatus and cochlea

-membranous labyrinth

-endolymph - within semicircular canals

perilymph - surrounds whole system

-otoliths- saccule and utricle help detect linear movement

-semicircular canals- detect angular movement

-hair cells that convert head motion into neural firing

Question 10

Otoliths- utricle and saccule characteristics

Answer 10

utricle
-oriented in a more horizontal plane
-detects lateral tilting of the head

saccule
-oriented in a more vertical plane
-detect forwards/backwards and up/down motion

the otolithic membrane has a gelatinous macula that otoconia sit on top of –> otoconia provide shear forces for hair cell deflection —> information transmitting to vestibulocochlear nerve CN VIII

-there is a tonic discharge from the hair cells
-push/pull relationship: the hair cells on the right will be excited while the hair cells on the left will be inhibitd

Question 11

Semicircular canals characteristics

Answer 11

6 total (3 on each side):
-anterior
-posterior
-horizontal

FUNCTION:
-detect angular motion
-provide afferent input to the central vestibular nuclei about head position due to endolymph movement

-perpendicular in nature
-push/pull relationship

Question 12

SCC excitation

Answer 12

horizontal: rotation to same side
posterior: posterior rot of the head (think about dix hallpike)
anterior: ant rotation of the head

MECHANISM:
endolymph flow deflects the cupula in the semicircular canal–> depolarization–> sends a signal to the brain that the head is moving
** endolymph moves in opposite direction to head motion

Question 13

What is the ampulla of the semicircular canals?

Answer 13

houses the cupula –> which carries many hair cells that sense directional changes

Question 14

Together the semicircular canals and otoliths:

Answer 14

detect motion in all planes of movement

Question 15

Where do the peripheral and central vestibular structures meet?

Answer 15

the vestibular portion of CN VIII is a bipolar neuron with cell bodies in Scarpa’s ganglion

CN VII and VIII–> travel together through the internal auditory meatus and enter the pontomedullary junction of the brain

cerebellopontine angle–> where CN VII and VIII enter the brain
** common site for acoustic neuroma (noncancerous tumor along the branches of the eighth cranial nerve)

Question 16

Where do vestibular afferents project in the central vestibular structures?

Answer 16

-vestibular nuclei
-cerebellum

Question 17

After info reaches the vestibular nucleus where does it go?

Answer 17

-extraocular motor neurons –> 3 neuron reflex arc for the VOR (a peripheral vestibular function)
-SC motor neurons
-cerebellum, thalamus, cerebral cortex

Question 18

VOR characteristics

Answer 18

FUNCTION: stabilizes gaze during head movement; 1:1 head to eye movement

3 NEURON REFLEX ARC:
Scarpa’s ganglion –> vestibular nuclei –>
oculomotor nuclei in brainstem (CN III, IV, VI)
-inhibit antagonistic muscles and produce compensatory eye movements in the plane of a stimulated canal
-excitation of certain muscles and inhibition of other muscles

Question 19

Origin of central vestibular system

Answer 19

CN VIII enters the skull through the internal auditory meatus and becomes the central vestibular system

Question 20

Vestibular nuclei characteristics:

Answer 20

subnuclei:
-superior
-inferior
-lateral
-medial

input from the peripheral vestibular system and vision/somatosensory input–>vestibular nuclei–> MLF –> oculomotor nuclei

FUNCTION:
-process and relay station for VOR, between cerebellum, brainstem and the cortex

Question 21

Function of the cerebellum in the vestibular system:

Answer 21

vestibulocerebellum- flocculonodular lobe

-plays a role/interacts with vestibular spinal reflexes

-afferent impulse from peripheral end organ –> cerebellum

-modulation of VOR/ VOR cancellation

-modulation of postural reflexes

Question 22

VSR - vestibulo-spinal reflexes and tracts involved:

Answer 22

Coordinate head and body movements to maintain head in upright position

LVST
-adjusts limb movement for balance
-control of head and body movement in space, staying upright

MVST
-terminates in the cervical cord
-responsible for controlling head movements and integrating head/eye movements
-make sure head and eye movements coordinate
-MLF descending fibers help to coordinate

Question 23

vestibular connections to thalamus

Answer 23

assists in discrimination between self movement and environmental movement
— ex: car vs self

-thalamus acts as a sensory relay system

-influence on arousal –> connection with vestibular cortex and RAS

Question 24

Reticular formation and autonomic connections

Answer 24

RST receives input from all vestibular nuclei

vest. projections to locus coeruleus (nuclei deep in brainstem): stress and panic centers

vest. projections to amygdala: emotional memory

*the autonomic connection is very strong

Question 25

Vestibular cortex characteristics

Answer 25

*Junction of parietal and insular lobe

• Multisensory integration of
  vestibular information with
  somatosensation and vision
• Sensory integrative capacity
  for postural control evolves
  between 7 and 15 years of age

Question 26

Blood supply to central vestibular structures:

Answer 26

vertebral arteries:
-PICA –> inferior vestibular nuclei, inferior portions of the cerebellum

basilar artery:
–> supply to pons and superior vestibular nuclei

Question 27

VCR function

Answer 27

cervical reflex pathway
-head righting
-stabilizes head and neck
-coordinated movements to track a moving target
-ex: chicken

Question 28

examples of cervical reflex pathways

Answer 28

VCR

tonic neck reflexes:
ATNR- Neck rotation excites ipsilateral extensors and contralateral flexors
STNR- Neck flexed excites bilateral UE flexors and LE extensors

cervico-ocular reflex (COR)
- eye movements are driven by neck proprioceptors
-supplements the VOR when needed
-facilitated when vestibular organ is injured
-afferents come from cervical proprioceptors to supplement VOR

CSR - supplement VSR by altering body tone, related to tonic neck reflexes

CCR- stabilizes head on body, may be facilitated after vestibular loss ; compensate for loss of stronger reflexes

Question 29

oculo-cephalic reflex

Answer 29

A positive doll’s eye reflex (eyes move in opposite direction of head movement) indicates an intact brainstem. A negative doll’s eye reflex (eyes remain midline or move in same direction of head movement) indicates severe brain stem dysfunction.

SEEN IN: those that are comatose (move in same direction of head movement)