Download 46 Vestibular analyzer

Vestibular system - organ of position and balance
sense - placed in the semicurcular canals in
petrous bone lying in three mutually perpendicular
planes. The canals start in utricle, which is
connected with sacculus. Both parts are placed in
vestibulum communicating with ductus cochlearis.
 One outlet of each canal is transformed in
ampulla, divided by the ampullary crist into two
parts. Macula utriculi is in the lower part of
utricle, the macula sacculi in sacculus. The crists
and ampullae are covered by sensory epithelium
composed of hair-cells. There are also gelatinous
cupulae on ampullary crists and the statoconia
membranes in maculae. Their function is to
stimulate stereocilia of sensory cells. The
statoconia are crystals of CaCO3 - it increases the
mass of gelatinous membranes.

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The semicircular canals allow analyse the
rotational motion of the head. Receptors of
ampullary crists react on angular acceleration.
The cupulas of crists work as valves, which are
deflected by streaming endolymph and stimulate
the hairs of sensory cells by bending –
depolarisation or hyperpolarisation takes place.
 The receptors of utricle and sacculus react on
linear acceleration and gravitation. When
changing the head position, the membrane with
statoconia shifts against hairs of sensory cells excitation arises. Important for keeping erect
position - static reflexes.

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
generates compensatory responses to head
motion



postural responses
ocular-motor responses
visceral responses
 To
achieve this the vestibular system
measures


Head rotation
Head acceleration

Einstein’s equivalency theorem states that an
accelerometer cannot distinguish between translational
accelerations and tilts
► Spatial
arrangement of the 6 SSC cause 3
coplanar pairings

R & L lateral, L anterior and R posterior; l posterior
& R anterior; R & L horizontal
► Allows
for a Push-Pull arrangement of the two
sides (e.g., as head turns right, right SSC will
increase firing rate & the left SSC will
decrease firing rate)
► Advantages



sensory redundancy
common mode rejection/noise
assist in compensation for sensor overload
► Depolarization
of the ipsilateral hair cells
occurs during angular head movements
► Hyperpolarization of contralateral hair
cells occurs at the same time
► Hair cells are only able to hyperpolarize to
what they were at rest = cut off of
inhibitory influences from the movement
going in the opposite direction even if the
ipsilateral hair cells continue to spike
higher firing rates
Purves 2001
► Utricle
and saccule
► Otolith sensory structures



Maculae
Otolithic membrane
Otoconia
► Movement
of gel membrane & otoconia cause
a shearing action to occur over the hair cells
→ sensitivity of otoliths
► Respond



to:
Linear head motion on acceleration
Static tilt
Two organs respond to respective accelerations
or tilts in their respective planes
►
►
Saccule has vertical orientation of maculae
Utricle has horizontal orientation of maculae
Bear 1996
►2
types: kinocilium & stereocilia
► Sensory structures for the peripheral end
organs (maculae and ampula)
► Hyperpolarized or depolarized depending
upon the direction of deflection of the
stereocilia (movement of stereocilia towards
the kinocilium causes depolarization of the
hair cell)
► Affect the firing rate of the primary
vestibular afferents to the brainstem
Bear 1996
► Striola
serves as a structural landmark
► Contains otoconia arranged in narrow trenches,
dividing each otolith
► Orientation of the hair cells change over the
course of the macula
► Allows otoliths to have multidirectional
sensitivity
► Tonic
firing rate
► Vestibular Ocular Reflex
► Push-pull mechanism
► Inhibitory cutoff
► Velocity storage system

Vestibular
apparatus and
cochlea form the
inner ear

Vestibular
apparatus –
provides sense of
equilibrium

consists of otolith
organs (utricle and
saccule) and
semicircular canals

Sensory structures located within membranous
labyrinth

filled with endolymph and located within bony labyrinth

Utricle and saccule provide info about linear acceleration

Semicircular canals, oriented in 3 planes, give sense of
angular acceleration

Hair cells – receptors for equilibrium

Each contains 20-50 stereocilia (hair-like extensions)
 1 of these is a kinocilium---a true cilium

Stereocilia bend toward kinocilium – hair cell
depolarizes


releases NT that stimulates CN VIII
When bent away from kinocilium – hair cell
hyperpolarizes

In this way, frequency of APs in hair cells carries information
about movement


When stereocilia are bend away from
kinocilium, hair cell is hyperpolarized, i.e.
inhibited. It occurs because acceleratory
force acts to flow of fluid in semicircular
canals during circular motion of the head or
whole the body.
Hair cells are located along crista ampularis
and protect their cilia in cupula. Hair cells are
secondary sensor cells, which synapse with
neurons. Axons of these nerve cells compose
vestibular nerve.
cellbio.utmb.edu/.../Ear/ organization_of_the_inner_ear.htm.
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
Have a macula that
contains hair cells

Hair cells embedded in
gelatinous otolithic
membrane
 contains calcium
carbonate crystals
(otoliths) that resist
change in movement

Utricle sensitive to
horizontal
acceleration


Hairs pushed backward
during forward
acceleration
Saccule sensitive to
vertical acceleration

Hairs pushed upward
when person descends

Provide information
about rotational
acceleration

Project in 3 different
planes

Each contains a
semicircular duct

Crista ampullaris –
where sensory hair
cells are located
10-42
Ampula is enlargement at
one end of semicircular
canal. It has a small crest
on top of which is a
gelatinous mass known as
cupula. Hair cells have
two kinds of cilia –
kinocilium and
stereocilia.
 Kinocilium is large cilium
located at one end of hair
cell. Stereocilia are
small. When stereocilia
are bent towards
kinocilium, hair cell is
depolarized, i.e.
stimulated.


Hair cell processes
embedded in cupula of
crista ampullaris

When endolymph
moves cupula moves

Sensory processes bend in
opposite direction of
angular acceleration
From Kandel
and Schwartz
► Vestibular
nerve
► Vestibular nuclei
► Cerebellum
► Oculomotor complex


CN 3, 4, and 6
Along with vestibulospinal reflexes coordinate head
and eye movements
► Thalamus

Connection with vestibular cortex and reticular
formation → arousal and conscious awareness of
body; discrimination between self movement vs. that
of the environment
► Vestibular


Cortex
Junction of parietal and insular lobe
Target for afferents along with the cerebellum
►
Both process vestibular information with somatosensory and
visual input
► Vestibular
nerve and vestibular nuclei have
a normal resting firing rate (70-100
cycles/sec)
► Baseline firing rate present without head
movement
► Tonic firing is equal in both sides; if not, a
sense of motion is felt e.g., vertigo, tilt,
impulsion, spinning
► Excitation and inhibition of the vestibular
system can then occur from stimulation of
the hair cells
► Spontaneous recovery with light
► Causes
eyes to move in the opposite direction to
head movement
► Speed of the eye movement equals that of the
head movement
► Allows objects to remain in focus during head
movements
► VOR
► Optokinetic
reflex
► Smooth pursuit reflex, saccades, vergence
► Neck reflexes

combine to stabilize object on the same area of the
retina=visual stability
► Keeps
eye still in space while head is moving
► Ratio of eye movement to head movement
(equals 1)
► Perseveration
of neural firing in the
vestibular nerve by the brainstem after
stimulation of SSC to increase time constant
(10sec.)

SSC respond by producing an exponentially decaying
change in neural firing to sustained head movement
► Otolith
& somatosensory input also drive
mechanism
► Direction
of gaze will shift with the head
movement
► Cause degradation of the visual image
► In severe cases, visual world will move with
each head movement
► Visual
illusion of oscillating movement of
stationary objects
► Can arise with lesions of peripheral or central
vestibular systems
► Indicative of diminished VOR gain


motion of images on fovea
diminished visual acuity
► Monitors
vestibular performance
► Readjusts central vestibular processing of static
& dynamic postural activity
► Modulates VOR
► Provides inhibitory drive of VOR (allows for
VORc)
► Provide
motor output from the vestibular
system to:


Extraocular muscles (part of VOR)
Spinal cord & skeletal muscles (generate antigravity
postural activity to cervical, trunk & lower
extremity muscles)
► Response
to changing head position with
respect to gravity (righting, equilibrium
responses)
► Generates
compensatory body movement to
maintain head and postural stability, thereby
preventing falls
 Vestibular
of eyes


occur when spinning person stops
Eyes continue to move in direction opposite to spin,
then jerk rapidly back to midline
 Vertigo


nystagmus – involuntary oscillations
– loss of equilibrium
Natural response of vestibular apparatus
Pathologically, may be caused by anything that alters
firing rate of CN VIII

Often caused by viral infection
Otolith Ambiguity
Translation
Tilt
Acceleration
Both Angles are equal.
•The otolith organs transduce acceleration
•Thus, translational accelerations and tilts of the head
cause similar activity on otolith afferents.
•Otolith information is therefore ambiguous
(Einstein’s equivalency principle) and must be
resolved in order to provide proper stabilizing and
orientation responses.



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Consists of three semicircular canals
Monitors the position of the
head in space
Controls balance
Shares fluid with the cochlea
Cochlea & Vestibular system
comprise the inner ear