Vestibular System

Question 1

We have evolved a vestibular sense based on

Answer 1

mechanoreceptors to determine our relative movement and position in the world
• If you are a fish, you care about water and swimming in it –> need to sense current and where you are in it (vestibular system), kind of like proprioception but more equilibrium (how am I sitting in this open world?)
• Fish also worries about what is coming toward it –> changes frequency of movement of waves that brush against fish (same thing as when you are listening) à rudamentory auditory system
• Fish have a lateral line –> mechanoreceptors, hairs that get pushed back when there is a wave
• For humans, we use the inner ear for that (vestibular system), inner ear also contains cochlea (person talking to you deflecting hair cells)

Question 2

three functions of the vestibular system

Answer 2

• Mediate awareness of movement
o Afferents from CN VIII
o Efferents to the thalamus, to cortex

• Adjust posture relative to position of head
o Afferents from CN VIII
o Efferents to the cerebellum

• ***Generate eye movements that compensate for head movements
o Afferents from CN VIII
o Efferents to reticular formation, cranial nerve nuclei controlling extraocular muscles

Question 3

The vestibular system resides in the inner ear in a series of Labyrinths
The Inner Ear Contains a Bony and Membranous Labyrinth

Bony labyrinth

Answer 3

• Bony or osseous labyrinth in the temporal bone
o Perilymph = fluid inside it, has same ionic concentrations as ECF and CSF (high Na and low K)
o Bony labyrinth is a cavity in the temporal bone. Perilymps is like CSF: high na+ and low k+. Opposite for endolymph, which is like intracellular comp. Tight junctions in membranous walls prevent mixing.

Question 4

Inside the bony labyrinth is the membranous labyrinth, containing sensory organs

Answer 4

o Endolymph
o Meniere’s disease: swelling of membranous labyrinth
o Vertigo, Hearing loss, Tinnitis
o Caused by defective circulation or absorption of endolymph
o Membranous traces bony labarynth carefully –> you have endolymph inside there – same makeup as intracellular environment (high K and low Na) –> needs to drain or else it stretches membranous labrynth –> Menier’s disease
o Usually recycled through endolymph ducts –> dura around temporal bone

Question 5

2 types of labyrinths

Answer 5

Kinetic and static labyrinths

Question 6

Kinetic labyrinth responds to

Answer 6

head movement

• -Angular velocity
• -Shaking yes or no
• -Ampullae/Cristae

** Semicircular canals

Question 7

Static labyrinth responds to

Answer 7

changes in head position
Linear velocity
Walking forward and backward, riding an elevator

–Utricle and Saccule/Maculae

** Vestibule

Question 8

one side’s superior goes with

Answer 8

the other side’s posterior

superior canals- flex the neck
posterior canals- extend the neck

Question 9

laterals partner with

Answer 9

contralaterals (turning head right to left)

Question 10

Kinetic Labyrinth:Orientation of semicircular ducts

Answer 10

There are 3 orthogonal semicircular canals bilaterally filled with semicircular ducts

Each lateral (or horizontal) duct works in concert with the corresponding contralateral duct

The superior duct of one side of the body functions in concert with the posterior duct on the contralateral side of the body

lateral ducts are activated by head turns
superior (anterior) ducts are activated by neck flexion

posterior ducts are activated by neck extension

Question 11

Kinetic Labyrinth : Designed to detect

Answer 11

Angular Acceleration

The kinetic labryrinth is made of 3 semicircular canals/ducts. Sensory transduction takes place in canal swellings called Ampullae .

Question 12

Cristae contain

Answer 12

sensory hair cells and supporting cells

Question 13

All vesitubular hair cells contain

Answer 13

2 kinds of cilia
Many stereocilia
Graduated array
One kinocilium
All hair cells of a given crista are aligned with their kinocilia facing in the same direction

Question 14

Deflection of the cilia in a particular direction cause the hair cell to

Answer 14

depolarize. Deflection in the opposite direction causes the hair cell to hyperpolarize

Question 15

Cupula is

Answer 15

a gelatinous mass holding the cilia of the hair cells

Question 16

Angular movements of the head do what?

Answer 16

shift the endolymph in the semicircular canals

The pressure from the endolymph moves the cupula as they are the same density

This results in the movement of stereocilia of the hair cells, thus stimulating them

Question 17

semicircular system is best for noticing changes in…

Answer 17

Because this is all about inertia, semicircular system is best for noticing changes in speed of rotation across a particular plane. Not so good at maintained rotations—only happens at amusement parks.

Question 18

Transduction in inner ear hair cells

Answer 18

Cupula deflection causes stereocilia and kinocilia to all tilt in the same direction, pulling open ion channels, and causing depolarization

The high K+ in the endolymph causes an influx into the cell (not like an AP, which uses Na+)

Ca2+ influx then causes release of neurotransmitter

Glutamate is usually released from the hair cell onto the vestibular nerve.

Use Ribbon synapses for massive release

Question 19

The movement of the cilia (for different ampullae) will be

Answer 19

in a different orientation, as they sit orthogonally to each other

Question 20

4 vestibular nuclei

Answer 20

There are 4 vestibular nuclei that control vestibulospinal, vestibulocerebellar, vestibulo-ocular, and vestibulo-autonomic pathways

Superior
Medial
Lateral
Inferior (Spinal)

Question 21

At rest, a balance between the input from each semicircular canal

Answer 21

keeps the eyes and the body centered around midline

Question 22

If there is ever an imbalance, can see in

Answer 22

excessive eye and gait abnormalities:

Nystagmus

Gait instability

Spatial disorientation

Nasuea and vomiting

Question 23

Vestibulo ocular reflex (VOR):

Answer 23

Moving the head through the kinetic labyrinth activates the Vestibular Ocular Reflex

Horizontal movement of head activates the medial longitudinal fasciculus (MLF)

Rule of thumb:
Head moves to the right, eyes move to the left

(The VOR also affects gaze in the upward/downward direction, uses the MLF across superior rectus, superior oblique and inferior oblique)

Question 24

Caloric reflex test activates the same pathways

Answer 24

Convection currents from heat cause movement of endolymph in canals, creating eye movements: caloric nystagmus

Nystagmus usually involves a fast phase as well as slow.
The fast phase gives the nystagmus its name
By moving head, get “dolls head” nystagmus followed by fast, jerky movement in the opposite direction
MLF controls dolls head nystagmus, superior colliculus causes fast-phase contralateral movement
Can use to determine brain damage site

Question 25

the cold and warm water mnemonic

Answer 25

direction of the ping phase names the nystagmus

COWS

Cold Opposite Warm Same

Question 26

The anatomy of the static labyrinth differs from the kinetic labyrinth: designed to detect

Answer 26

linear acceleration

There are no cristae in the saccule or utricle

There is no cupula in the saccule or utricle

There are patches in saccular and utricular walls containing support cells and hair cells–macula
Saccular macula is vertical
Utricular macula is horizontal

Instead of cupula, have otoconia
Calcium carbonate

Question 27

Otoconia/otolith displace hair cells during

Answer 27

vertical or horizontal acceleration

Question 28

Direction is conferred by the

Answer 28

specific hair cells depolarized in the macula

Striola are mirror image point. It’s a groove within the macula.

Question 29

What happens when you trip

Answer 29

The vestibular system may be thought of as a partner to the cerebellum.

 - The vestibular system has its own sensory pathway 
 - The vestibular system has its own motor pathways

The cerebellum receives vestibular afferents to integrate this information with proprioception from the rest of the body

Question 30

Vestibulocerebellar tract takes

Answer 30

vestibular input into cerebellum for processing

2 routes in—direct from sharpa’s ganglion or through the vestibular nuclei: GOES TO THE FOLOCCULUS of the cereberellum. This is the oldest region! In addition to muscle spindles telling proprioception, the vestibular system is communicating acceleration of the head.

Question 31

Conscious awareness of movement is processed through

Answer 31

relays in the VPM of the thalamus

There are also “primary” vestibular areas in the superior temporal gyrus and posterior insula

All 3 primary vestibular areas are responsive to visual stimuli too (more like association areas)

Question 32

Vertigo clinical manifestations: alcohol, aging/ trauma

Answer 32

Vertigo- illusions of movement

Alcohol
Makes the cupula change density. Movement of cupula without endolymph movement
Aging/trauma
Detachment of otoconia to semicircular duct, pushes cupula

Question 33

lateral medullary syndrome

Answer 33

Thrombosis of vertebral or posterior inferior cerebellar artery

Damage to vestibular nuclei = vertigo

• Also inaccurate reaching
• Eyeballs 10 deg off to one side
• Head tilt to same side
• Fall towards same side

Spinocerebellar fibers disrupted = ipsilateral ataxia
Sensory trigeminal damage = ipsilateral loss of nociceptivity and thermosensitivity
Sympathetic pathway = horner’s syndrome
Nucleus ambiguus = hoarseness and difficulty swallowing
Spinothalamic tract damage = contralateral loss of pain and temp in trunk and limbs

Question 34

A final vestibular note…

Answer 34

Vestibular, visual and somatosensory (cerebellar) input normally combined to produce sense of orientation
If one of these three is defective, can still function without a lot of trouble
But when two of three systems out, then disabling
Romberg sign
With labyrinthine stimulation, patient will fall in same direction as endolymph flow