Lec 20 Vestibular System

Question 1

What two things does the vestibular system sense?

Answer 1

head motion and head position in space by supplying info about

• angular acceleration via semicircular canals
• linear acceleration via otolith organs

Question 2

To what effector pathways does vestibular system distribute its sensory info?

Answer 2

to regions that

• control eye movements/maintain stable faze during head movement [vestibulo-ocular reflex + otolith-ocular reflex]
• postural equilibrium [vestibulo-spinal tracts]
• maintain head stability during body movement [vestibulo-colic reflex]
• autonomic function [vestibulo-autonomic reflex]
• conscious perception of balance [vestibulo-thalamo-cortical projections ]

Question 3

What is result of damage to vestibular system?

Answer 3

• poor balance, dizziness, vertigo, motion sickness, fainting

Question 4

What is structure of semicircular canals?

Answer 4

• bony canals oriented in orthoganol planes [horizontal, anterior, posterior]
• filld with perilymph + contain membranous chambers filled with endolymph

Question 5

What is the ampulla of semicircular canal?

Answer 5

• swelling at one end of each canal
• the structure from which sensory epithelium [crista ampullaris] with hair cells and support cells hangs [like a swing/hammock from 1 wall of ampulla to the other]

Question 6

What is the cupula?

Answer 6

dense gelatinous solution from which hair cell bundles protrude

Question 7

What happens in semicircular canal when there is angular head acceleration?

Answer 7

• endolymph fluid in the membranous chambers moves –> differentially displaces cupula to exert force on hair cell stereocilia
• in turn depolarizes or hyperpolarizes hair cell receptor potential [which activates or inhibits sensorineural transduction]

Question 8

What are the two otolith organs? purpose of each?

Answer 8

saccule = vertical acceleration
utricle = horizontal acceleration

Question 9

What is structure of otolith organs?

Answer 9

• filled with endolymph, surrounded by perilymph

- sensory epithelial with hair/support cells oriented horizontally in utricle or vertically in saccule

Question 10

Where is cochlea located with respect to semicircular canals?

Answer 10

cochlea is more anterior/medial than semicircular

Question 11

Which of the semicircular canals is most superior? most inferior?

Answer 11

superior = anterior
inferior = posterior

Question 12

What is the macula?

Answer 12

the sensory epithelial of the otolith organs

Question 13

what are otoconia?

Answer 13

calcium-carbonate crystals that cover the otolithic membrane

Question 14

What happens to otoconia in linear head accelerations?

Answer 14

they are displaced –> causes shear force on stereociliarly bundles that activates or inhibits sensorineural transduction via changes in potential

Question 15

What is equivalent to the crista ampullaris in the cochlea?

Answer 15

basilar membrane

both are sensory sheets which contain hair cells

Question 16

What can damage hair cells?

Answer 16

• trauma
• vascular disease
• aminoglycoside toxicity
• normal aging they diminish in number

Question 17

What is contained in a hair bundle? how does it work?

Answer 17

• contains many sterocilia [St] and a single kinocilium [Ki] that is bigger
• displacement of stereocilia toward kinocilium –> depolarization
• displacement of stereocilia away from kinocilium –> hyperpolarization

Question 18

What are the 3 planes of the semicircular canals?

Answer 18

horizontal = both L and R in same plane, horizontal but skewed 30 degrees above earth horizontal
RALP = right anterior, left posterior
LARP = left anterior, right posterior

Question 19

What does the saccule detect?

Answer 19

linear acceleration in vertical plane

Question 20

what does the utricle detect?

Answer 20

linear acceleration in horizontal plane

Question 21

What are the maculae? its equivalent in the other part of the vestibular system?

Answer 21

sensory sheets of the otolith organs

equivalent in the semicircular canals = crista ampullaris

Question 22

Where are the cell bodies located for both canal and otolith primary afferents?

Answer 22

scarpa’s ganglion in temporal petrous bone

Question 23

Where do the vestibular nerve fibers from canal/otolith terminate?

Answer 23

the vestibular nuclei in the medulla/caudal pons

4 –> superior, inferior, medial, lateral

Question 24

What is the path from inner ear to vestibular cortex [for conscious perception of balance]?

Answer 24

• canal/otolith bipolar afferent receive input from hair cells and form cochlear portion of CN VIII
• terminate in vestibular nuclei in medulla and caudal pons [plus a few in cerebellum]
• superior/lateral vestibular nuclei neurons cross midline –> VP of thalamus
• VP –> vestibular cortex in parietal lobe just posterior to S1 somatosensory cortex

Question 25

What is blood supply to vestibular nuclei?

Answer 25

PICA

Question 26

What structures make up the vestibular cortex?

Answer 26

2 places

• posterior parietal cortex
• cortex posterior to primary somatosensory cortical face area [just posterior to postcentral gyrus]

Question 27

What is the vestibulo-ocular reflex [VOR]?

Answer 27

• compensatory eye movement that stabilizes gaze during head rotation
• afferent = detection of head movement by vestibular CN 8
• efferent= oculomotor/abducens in direction opposite to that of head movement [coordinated lateral rectus + medial rectus firing]

Question 28

What is the path of afferent vestibulo-ocular reflex [VOR]?

Answer 28

• CN8 carrying fibers from horizontal semicircular canals synapse on medial vestibular nucleus
• medial vestibular nucleus axons

Question 29

what is the brainstem arc of the VOR?

Answer 29

• axons from medial vestibular nucleus project to contralateral CN VI nucleus –> excites both contralateral lateral rectus motor neuron + interneuron that synapse in ipsilateral CN III nucleus via MLF
• axons from medial vestibular also project to ipsilateral CN VI nucleus where inhibit ipsilateral lateral rectus efferents so no excitation of contralateral medial rectus

Question 30

What are the two descending motor tracts from the vestibular nuclei? function?

Answer 30

1. lateral vestibulospinal tract [LVST]
   - balance + postural control, influence motor neurons in spinal cord
2. medial vestibulospinal tract [MVST] [= vestibulo-colic path]
   - maintains stability of head on neck as body moves

Question 31

Where does the lateral vestibular nucleus project to? terminate? function?

Answer 31

• all levels of ispilateral spinal cord via lateral vestibulospinal tract
• terminates on medial motor neurons
• activates antigravity muscles
• cerebellum influences posture in part by modulating activity of vestibular nuclei

Question 32

Where does the MVST project to? function?

Answer 32

• sends bilateral projects to cervical and upper thoracic spinal cord via medial longitudinal fasciculus [MLF]
• in medulla MLF is just dorsal to tectospinal tract
• regulates head pos by activating neck muscles in response to canal stimulation [vestibulo-colic reflex] –> ex. lunge forward activates neck extension

Question 33

What is the vestibulo-sympathetic reflex?

Answer 33

VSR = vestibular otolith organs provide direct input to allow for adjustments in BP during locomotion and changes in body position

• vestibular nuclei receive primarily vestibular input and send projections to brainstem presympathetic control centers

from there –> path converges with baroreflex

Question 34

What can commonly happen in VSR dysfunction?

Answer 34

• orthostatic hypotension [common in elderly]

Question 35

Which of the functional paths of vestibular system are under cerebellar control?

Answer 35

all 5

Question 36

What is the crista ampullaris?

Answer 36

sensory sheet in ampulla containing hair cells

- the swing/hammock in the ampulla