Lecture - 06 - Vestibular Systems Flashcards
Vestibular system
sense head motion
- output to cerebral cortex, 3 reflex paths, cerebellum
- visual system helps
- visuo-vestibular conflict causes dizziness/nausea
- peripheral and brainstem vestibular dysfunction causes pathological sense of self-motion due to visuo-vestibular conflict
semicircular canals
sense head angular acceleration (rotation)
- ampulla inside has tissue ridge (crista) with hair cells embedded in gelatinous mass called cupula
- hair cells bent by fluid moving cupula
- vestibular nerves have constant spontaneous spike rate, head moving to left increases firing and moving to right decreases firing (towards kinocilia = excite)
- compares with inputs from other ear
- horizontal canals on both sides work as team (one side excites, one side inhibits)
- send input to brain via MEDIAL vestibulospinal tract
- vestibulo-cervical reflex
otoliths
sense linear acceleration and gravity
- hair cells sit in utricle and saccule in macula and are embedded in gelatinous otolithic membrane
- topped with Ca carbonate crystals (otoliths)
- tilt head backward/forward, gravity causes heavy otoliths to bend hair cilia to either excite or inhibit
- same with acceleration
- utricle macula: horizontal plane (forward/backward, side to side acceleration, head tilts)
- saccule macula: vertically oriented (up/down)
- movements in all directions are covered
- send input to brain via LATERAL vestobulospinal tract
- vestibulo-spinal reflex
Vestibulo-ocular reflex
oculomotor system for gaze stabilization
3 sets of semicircular canals lie in 3 orthogonal planes and sense rotation by working in pairs
-each pair of canals use extraocular muscles that move the eyes in the same plane
-horizontal canals = medial rectus and lateral rectus
-eye movement: synapse on abducens to stim MR and LR, inhibit medial longitudinal fasciculus to inhibit CN3 movement of other MR and LR pair
Vestibulo-cervical (collic) reflex
vestibulospinal system to control head stabilization during movement and head “posture”
Vestibular-spinal reflex
to vestibulospinal systems to control body stabilization during movement and limb movement
Vestibular organ
vestibular part of inner ear
- hair cells are sensory receptors
- sit in membranous labyrinth at 5 locations
- 3 ampulla at ends of semicircular canals, 2 otoliths (utricle and saccule)
- membranous labyrinth pasted up against bony walls of bony labyrinth so sound vibrations don’t cause hair cell vibration
Blood supply
basilar –> AICA –> int. auditory artery
Vestibular hair cell function
activated/excited by bending of cilia
- K rushes in to depolarize, causing Ca channels to open and come in to release transmitter and activate vestibular nerve
- at “rest” many vestibular neurons are spontaneously active, so bending hair cell cilia signals CNS to increase or decrease firing
- signals from vestibular apparati bilaterally compared by brain
Input to brain
several subdivision of vestibular nuclei
- pons/medulla border
- vestibular hair cells –> vestibular nuclei via CN8
Vestibulocortical
consciously sensing motion
- bilaterally (but more IPSI) from nuclei to posterolateral thalamus
- insula and somatosensory cortex light up
- parietal operculum
Vestibulocerebellar
cerebellum gets info about motor plan, compare with muscles present situation, then send corrective info back to motor cortex
- cerebellum does this for reflex pathways of vestibular system
- cerebellum gets info from hair cells
- reflexes work without cerebellum but not as well
- vestibular input to cerebellum goes to flocculonodular lobe
Optokinetic Nystagmus
if head is fixed in position, eyes will follow an object until they can’t go any further then they rapidly snap back and look for something else to focus on
-snapback direction = nystagmus direction
Caloric testing
tests integrity of vestibular apparatus of each ear independently
-perfuse warm/cold water into ear
Warm Same, Cold Opposite
Also tests brainstem/cortex integrity
- fast eye movements occur due to cerebral cortex whereas slow movements do not
- no beating = cortical/corticobulbar issue
- slow phase changes - brainstem problem
Benign paroxysmal vertigo (BPPV)
normally hair cells in canals only sense head turning
- sometimes otoconia fall off utricle’s otolithic membrane and get into canal –> cause hair cells in cupola to sense gravity
- misplaced otoconia activate canal hair cells –> vertigo
