Vestibular Anatomy and Function Flashcards
Functions of the vestibular system
-postural control
-gaze stability
-provide sensory information for spatial orientation
- postural control, postural stability, postural orientation
postural stability
maintenance of the COM within the BOS
postural orientation
ability to maintain a relationship between body segments and between body and environment for a task
postural control inputs and outputs
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
What is central processing in the vestibular system?
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)
OUTPUTS of the vestibular system
VOR - vestibular ocular reflex
VSR - vestibulo-spinal reflexes
VCR - vestibulo-collic reflex
What are the peripheral vestibular structures?
vestibular end organ- 3 pairs of semicircular canals and otolith organs
cranial nerve VIII (vestibular portion)
Blood supply of the peripheral vestibular structures?
basilar artery from vertebral artery
-AICA coming of of the basilar artery
–labyrinthine artery (branch of the AICA)
—anterio-vestibular artery
—common cochlear artery
What are the components of the vestibular end organ?
-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
Otoliths- utricle and saccule characteristics
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
Semicircular canals characteristics
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
SCC excitation
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
What is the ampulla of the semicircular canals?
houses the cupula –> which carries many hair cells that sense directional changes
Together the semicircular canals and otoliths:
detect motion in all planes of movement
Where do the peripheral and central vestibular structures meet?
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)
Where do vestibular afferents project in the central vestibular structures?
-vestibular nuclei
-cerebellum
After info reaches the vestibular nucleus where does it go?
-extraocular motor neurons –> 3 neuron reflex arc for the VOR (a peripheral vestibular function)
-SC motor neurons
-cerebellum, thalamus, cerebral cortex
VOR characteristics
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
Origin of central vestibular system
CN VIII enters the skull through the internal auditory meatus and becomes the central vestibular system
Vestibular nuclei characteristics:
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
Function of the cerebellum in the vestibular system:
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
VSR - vestibulo-spinal reflexes and tracts involved:
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
vestibular connections to thalamus
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
Reticular formation and autonomic connections
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
Vestibular cortex characteristics
*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
Blood supply to central vestibular structures:
vertebral arteries:
-PICA –> inferior vestibular nuclei, inferior portions of the cerebellum
basilar artery:
–> supply to pons and superior vestibular nuclei
VCR function
cervical reflex pathway
-head righting
-stabilizes head and neck
-coordinated movements to track a moving target
-ex: chicken
examples of cervical reflex pathways
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
oculo-cephalic reflex
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)
