Vestibular Flashcards
Hair cell function
Stereocilia on apical membrane
- deflection -> opening of K+ channels -> K+ influx from endolymph -> depolarization
Basal membrane voltage-dependent Ca channels -> Ca influx from perilymph -> transmitter release -> K+ efflux to perilymph to re-establish potential
Relies on K+ difference between endo and perilymph (+80 mV)
- hair cell -125 mV vs endo, -45 vs peri
Deflection towards kinocilium -> more transmitter release, away -> less vs baseline
Overview of vestibular anatomy
Bony labyrinth = vestibule, semicircular canals, aqueduct
- perilymph - almost identical, continuous with CSF
Membranous labyrinth =
- utricle, saccule - contain maculae -> linear acceleration
- semicircular canals - right angles to each other (not to head)
- ampulla -> cristae
- endolymph: stria vascularis -> aqueduct -> resorbed in sac
Adaptation of hair cells
Adjacent stereocilia linked via tip filaments
- K+ channel -> voltage-sensitive motor protein
- low stimulation -> low Ca -> motor moves towards tip -> pulls K+ channel open -> higher Ca -> higher transmitter release
- > consistent baseline level of transmitter release
- > fast adaptation to slowly changing position (ie airplane pilot)
Macula structure
Within utricle and saccule
Small patch of hair cells
Polarity - all different directions
- striola = center line -> divides opposite polarity
Hair cells embedded in gel
- otoliths/otoconia - Ca carbonate crystals embedded in gel (greater density -> detect gravity)
Macula function
Linear acceleration in all directions
- does NOT respond to velocity, just acceleration (ex driving at constant speed)
Gravity (head position - equivalent to acceleration)
- adapts slowly to constant stimulus (ie head tilt)
- direction determined by combination of + and (-) orientation of hair cells
Cristae structure
In ampulla of semicircular canal
All oriented in a single direction (across opening of canal)
Cupola = gel
- no otoliths (no effect of gravity)
Cristae function
Angular acceleration (rotation)
Detect current in semi-circular canals
Rotation from all three canals is integrated
- ex rotation of head to the right -> fluid pushes on R christa -> increased firing in R, decreased firing in L
Each ear has a full complement of directions
- can compensate fully after loss of one inner ear
Vestibular innervation
Cell bodies in Scarpa’s/vestibular ganglion ->
Vestibulo-cochlear nerve -> medial to inferior peduncle ->
Vestibular nuclei (dorsolateral medulla and pons)
- corresponds to vestibular trigone
- inferior, medial, lateral
Outputs from vestibular nuclei
Flocullonodular cerebellum = vestibulocerebellum
- inputs direct from CN VIII
- inputs from vestibular nuclei
Medial longitudinal fasciculus -> nuclei for CN 3, 4, 6
- vestibulo-ocular reflex
- bilateral input (compensates for loss of one side)
Vestibulo-spinal tracts (medial and lateral) -> muscle tone
- vestibular righting reflex
Reticular formation -> autonomic (nausea/vomiting, sweating, etc)
Ventral posterior nucleus of thalamus -> parietal cortex
- conscious sensation
Interconnection between vestibular systems -> compensation
Inputs to vestibular nuclei
Mainly from ipsilateral CN VIII
- interconnection -> compensation for loss
Vestibulo-cerebellum - flocculus, fastigial nucleus
- integrates visual information
-> crucial for adaptation of vestibulo-ocular reflex (ex inner ear damage)
Spinal - neck proprioception
Nystagmus
Normal eye movement if head is spinning
- “jerk” phase is in direction of head spin
Slow phase = vestibulo-ocular reflex (fixates on point)
Rapid phase = visual cortex correction
Pathology:
Unilateral loss -> sensation of spin to opposite side, nystagmus -> compensation
Cold water -> endolymph sinks - simulates movement away from that side
- cold opposite, warm same (rapid phase)
Coma - slow phase but missing rapid phase
Vestibulo-ocular reflex is from semi-circular canals!
