Vestibular System (Week 3--Bisley)

Question 1

Vestibular organs

Answer 1

Otolith organs: utricle and saccule

Semicircular canals/ducts: anterior, posterior, horizontal

Question 2

Semicircular canal

Answer 2

Part of bony labyrinth

Perilymph is fluid between bone and duct, and has low K+ and high Na+ (similar to CSF)

Question 3

Semicurcular duct

Answer 3

Membranous tube within semicircular canal

Endolymph is fluid within the ducts and has high K+ and low Na+; made in dark cells in vestibular organs and by marginal cells in the cochlea

Question 4

What does each hair cell contain?

Answer 4

Many stereocilia

One kinocilium

Kinocilium lies on one side of stereocilia, which line up in height

Stereocilia and kinocilium are linked by “tip links” which are linked to K+ channels

Note: stereocilia and kinocilium are poking into endolymph (within duct) and rest of the cell is in perilymph (between bone/canal and duct)

Question 5

How do you depolarize a hair cell?

Answer 5

1) Hairs all move toward kinocilium
2) Tip links are stretched, which causes K+ channels to open and let K+ in (endolymph has a high concentration of K+)
3) Voltage-gated Ca2+ channels activated and let Ca2+ in
4) Cell is depolarized
5) NT (glutamate and aspartate) released onto afferent nerve ending (to CN VIII)

Question 6

How do you hyperpolarize a hair cell?

Answer 6

1) Hairs move away from kinocilium
2) Channels close
3) Cell hyperpolarizes

Question 7

What does movement orthogonal to the stereocilia-kinocilium axis do?

Answer 7

Nothing!

Movement orthogonal to stereocilia-kinocilium axis does not change the membrane potential

Because tip links are not stretched so no K+ channels open

Question 8

How are the semicircular canals arranged?

Answer 8

3 semicircular canals in each inner ear

Orthogonal to each other

Each have a swelling (ampulla) close to where they intersect with the utricle, and the hair cells sit in the ampulla

Question 9

Internal organization of the semicircular canals

Answer 9

Hair cells within ampulla sit among supporting cells in the crista

Kinocilia and stereocilia protrude into a gelatinous mass (cupula)

Hair cells all lined up with same polarity: kinocilium closest to utricle in horizontal canals and farthest from utricle in anterior and posterior canals

So if cupula pushed towards or away from the utricle, all hair cells in one ampulla will respond the same way

Question 10

What induces cupula movement?

Answer 10

Angular acceleration of the head in a direction parallel to the axis of the semicircular canal

When head starts to move, endolymph initially stays in place due to inertia, so cupula pushed by it and will either depolarize or hyperpolarize hair cells

When head rotating constantly, endolymph moves at same speed as head so no pressure on cupula and hair cells have same response as if head was stationary

When head stops rotating, endolymph briefly continues to rotate due to inertia, so cupula pushed in opposite direction

Question 11

Push-pull response

Answer 11

Because bony labyrinths are mirror images of each other, each canal has reverse polarity from partner

Angular rotation that increases activity from left canal will reduce activity from paired right canal

Advantage of push-pull response is that small changes in firing rate are more easily discernable because CNS takes difference in activity!

Question 12

Why does the room spin when you’ve drank too much alcohol?

Answer 12

Alcohol changes the relative density of the cupula (alcohol makes the very vascular cupula less dense) so that it floats in the endolymph, pulling the hair of the hair cells with it no matter what position you’re in

Hair cells are telling you that there’s constant angular acceleration!

Question 13

How are the utricle and saccule aligned?

Answer 13

Utricle and saccule are orthogonal to one another

Utricle points toward eyes, but 30 degrees up (because natural to walk looking down to avoid tripping)

Saccule points down toward chin

Question 14

Macula

Answer 14

Layer of sensory epithelium within small section of both the utricle and saccule

General area of lots of receptors

Question 15

Hair cells of the utricle and saccule

Answer 15

Hair cells sit among supporting cells in macula and hairs poke into gelatinous membrane (otolithic membrane) which has otoconia on top

Hair cells do not line up same way and instead there is an organized arrangement in which hair cells on either side of a groove (striola) in otolithic membrane face in opposite directions (different from semicircular canals!)

Because utricular macula and striola macula are orthogonal to each other and because striola are not straight, all potential directions of linear acceleration are represented

Question 16

How does linear acceleration work in the utricle and saccule?

Answer 16

Otoconia are calcium carbonate crystals embedded in otolithic membrane that are more dense (heavier) than endolymph

Movement causes heavy otoconia to initially stay behind because of inertia and this moves the otolithic membrane –> membrane movement changes K+ influx in hair cells (toward kinocilium means depolarization, etc like in semicircular canals)

When body stops moving, heavy otoconia take a little longer to stop due to inertia so again changes in K+ influx occur in the hair cells

Question 17

Does gravity affect the otoconia?

Answer 17

Yes, gravity pulls the otoconia down because remember utricle angled up 30 degrees

Question 18

Why don’t we confuse tilt with linear acceleration?

Answer 18

Visual input has strong influence on perception

Semicircular canals provide confirmation

Efference copy provides additional confirmation

Question 19

Efference copy (corollary discharge)

Answer 19

Copy of a motor command that is sent to other regions of the brain so that the CNS has an internal representation of what movements it makes

“I pulled control stick up so should be accelerating now…”

Question 20

Where to afferent nerves have their cell bodies?

Answer 20

Vestibular (Scarpa’s) ganglion

Central processes of these bipolar neurons enter the brainstem via CN VIII at pontomedullary junction

Question 21

Where do central processes of bipolar neurons (afferents) project to?

Answer 21

Vestibular nuclei: inferior, superior, medial, lateral

Cerebellum (via juxtarestiform body in inferior cerebellar peduncle)

Question 22

Where do second order neurons (cell bodies in vestibular nuclei) go?

Answer 22

Lateral vestibular nuclei

Medial vestibular nuclei

Cerebellum

Reticular formation

Thalamus and cortex

Eye movement control nuclei

Contralateral vestibular nuclei

Question 23

Lateral vestibulospinal tract (LVST)

Answer 23

Projections from lateral vestibular nuclei (second order neurons now) descend ipsilaterally in lateral vestibulospinal tract (LVST)

Project to ventral horn of spinal cord and synapse on motor neurons controlling limbs and trunk and are used to mediate balance and posture

Question 24

Medial vestibulospinal tract (MVST)

Answer 24

Projections from medial vestibular nuclei (second order neurons now) descend bilaterally in medial longitudinal fasciculus (MLF) as the medial vestibulospinal tract (MVST)

Project to ventral horn of upper cervical spine and used to mediate head position

Question 25

What are projections to the reticular formation thought to mediate?

Answer 25

Feeling of nausea that is linked to vestibular system

Question 26

Second order then third order neurons to thalamus and cortex

Answer 26

Vestibular nuclei (second order neurons) project bilaterally to ventral posterolateral nucleus (VPL) and the posterior nuclear group within the thalamus

Third order neurons in thalamus project to parietal cortex (areas 3a (primary somatosensory), 2v, posterior parietal areas 5, 7)

Question 27

Projections from vestibular nuclei to oculomotor nuclei are responsible for what?

Answer 27

Vestibulo-ocular reflex

Projections are bi-lateral but sign of activity is different (excitatory vs. inhibitory!)

Important for maintenance of gaze

Question 28

Do neurons in vestibular nuclei project to their reciprocal vestibular nuclei?

Answer 28

Yes, neurons in vestibular nuclei project contralaterally to reciprocal vestibular nuclei to enable push-pull analysis of signals from functional pairs of canals on either side of the head

Question 29

What organs convey information about linear vs. rotational acceleration?

Answer 29

Otolithic organs (utricle and saccule) convey information about linear acceleration

Semicircular canals convey information about rotational acceleration

Question 30

How are vestibular signals brought into the CNS and what do they do from there?

Answer 30

1) Vestibular signals brought into CNS via CN VIII and terminate in cerebellum and 4 vestibular nuclei
2) 4 vestibular nuclei are connected to 2 descending tracts, cerebellum, reticular formation, thalamus, and ocular motor nuclei
3) Info from vestibular nuclei is used to maintain gaze, balance and posture, and is used in cortex to track position in space

Question 31

Orientation of kinocilium in utricle vs. saccule

Answer 31

In utricle, the kinocilium is on the side of the striola

In saccule, the kinocilium is on the side away from the striola