Vestibular System Flashcards
We have evolved a vestibular sense based on
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)
three functions of the vestibular system
• 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
The vestibular system resides in the inner ear in a series of Labyrinths
The Inner Ear Contains a Bony and Membranous Labyrinth
Bony labyrinth
• 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.
Inside the bony labyrinth is the membranous labyrinth, containing sensory organs
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
2 types of labyrinths
Kinetic and static labyrinths
Kinetic labyrinth responds to
head movement
- -Angular velocity
- -Shaking yes or no
- -Ampullae/Cristae
** Semicircular canals
Static labyrinth responds to
changes in head position
Linear velocity
Walking forward and backward, riding an elevator
–Utricle and Saccule/Maculae
** Vestibule
one side’s superior goes with
the other side’s posterior
superior canals- flex the neck
posterior canals- extend the neck
laterals partner with
contralaterals (turning head right to left)
Kinetic Labyrinth:Orientation of semicircular ducts
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
Kinetic Labyrinth : Designed to detect
Angular Acceleration
The kinetic labryrinth is made of 3 semicircular canals/ducts. Sensory transduction takes place in canal swellings called Ampullae .
Cristae contain
sensory hair cells and supporting cells
All vesitubular hair cells contain
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
Deflection of the cilia in a particular direction cause the hair cell to
depolarize. Deflection in the opposite direction causes the hair cell to hyperpolarize
Cupula is
a gelatinous mass holding the cilia of the hair cells
Angular movements of the head do what?
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
semicircular system is best for noticing changes in…
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.
Transduction in inner ear hair cells
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
The movement of the cilia (for different ampullae) will be
in a different orientation, as they sit orthogonally to each other
4 vestibular nuclei
There are 4 vestibular nuclei that control vestibulospinal, vestibulocerebellar, vestibulo-ocular, and vestibulo-autonomic pathways
Superior
Medial
Lateral
Inferior (Spinal)
At rest, a balance between the input from each semicircular canal
keeps the eyes and the body centered around midline
If there is ever an imbalance, can see in
excessive eye and gait abnormalities:
Nystagmus
Gait instability
Spatial disorientation
Nasuea and vomiting
Vestibulo ocular reflex (VOR):
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)
Caloric reflex test activates the same pathways
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
the cold and warm water mnemonic
direction of the ping phase names the nystagmus
COWS
Cold Opposite Warm Same
The anatomy of the static labyrinth differs from the kinetic labyrinth: designed to detect
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
Otoconia/otolith displace hair cells during
vertical or horizontal acceleration
Direction is conferred by the
specific hair cells depolarized in the macula
Striola are mirror image point. It’s a groove within the macula.
What happens when you trip
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
Vestibulocerebellar tract takes
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.
Conscious awareness of movement is processed through
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)
Vertigo clinical manifestations: alcohol, aging/ trauma
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
lateral medullary syndrome
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
A final vestibular note…
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
