Vestibular System Flashcards
vestibular system consists of…
1) membranous labyrinth and sensory receptor (endorgan)
- housed in in the bony labyrinth (series of cavities in the temporal bone)
2) cranial nerve VIII
3) vestibular nuclei
4) ascending and descending tracts
where are the vestibular nuclei located
lie on the floor of the 4th ventricle at the junction of the medulla and pons
vestibular system functions
sensory system
influences muscle tone in appendicular and axial muscles for control of postural support and balance (vestibulo-spinal and vestibulocollic reflexes)
provides conscious awareness of spatial orientation
vestiublar system: sensory system function
- vestibular signals come from…
- signals generated by…
- regulate…
vestibular signals from labyrinths
generated by head movement
regulate eye position in the orbit during head movements in order to assure a stable image on the retina (vestibulo-ocular reflex)
membranous labyrinth
-what is it
series of tubes made of fine membranes that contain the peripheral sensory receptors (hair cells) of the vestibular system
membranous labyrinth contains a fluid called…
endolymph
membranous labyrinth
-separated from _____ by _____
separated from the bone cavity within the petrous part of the temporal bone (bony labyrinth) by a fluid called perilymph
vestibular appartus receptors
-location
very specific sites within the membranous labyrinth
five sites on each side
-one in each of the three semicircular canals
-two in the vestibule
the vestibule is composed of…
otolith organs
- saccule
- utricle
semiciruclar canals
- how many
- arrangement
- names
3
arranged at right angles to each other
horizontal, anterior, posterior
horizontal canals
- orientation
- activated with…
in horizontal plane when the head is flexed forward 30 degrees
activated with rotation around the body axis
anterior and posterior canals
-activated with…
activated with rotation in an oblique orientation between the frontal and sagittal planes
work together on opposite sides of body
semicircular canals
-overall function
perform the dynamic function of detecting angular velocity of the head in all planes
especially important for stabilizing the eyes during head movement
semicurcular canals
-filled with
endolymph
semicurcular canals: ampullae
- location
- contain
located at base of canals
contain hair cells called crista
semicurcular canals: crista (hair cells)
- innervated by…
- protrude…
peripheral ends of the vestibular nerve fibers
protrude upwards into a gelatinous material called the cupula
semicurcular canals
- angular movement (rotation) causes the endolymph to…
- -as a result…
endolymph lags behind (inertia) in the opposite direction
causes displacement of the cupula, thereby moving the hair cells
semicurcular canals
-mechanical deflection of the hair cells produces a…
receptor potential that may either depolarize or hyperpolarize the vestibular nerve that is tonically active
semicurcular canals
-vestibular nerve resting discharge rate
100 to 300 spikes per second
semicurcular canals
-effect on the resting discharge rate depends on…
direction the hair cells bend
semicurcular canals
- each hair cell is made up of…
- depolarization occurs when…
- hyperpolarization occurs when…
two types of projections
-kinocilium (one)
-stereocilium (40-70)
depolarization occurs when the stereocilium bend towards the kinocilium
hyperpolarization occurs when the stereocilium bend away from the kinocilium
semicurcular canals
-example of how direction of rotation is determined by the vestibular nerve (L rotation)
with rotation around the body axis to the left, the hair cells in the L horizontal canal become more depolarized, resulting in an increase in frequency of action potentials along the L vestibular nerve
hair cells in the R horizontal canal become more hyperpolarized, resulting in a decrease in the frequency of action potentials along the R vestibular nerve
semicurcular canals
- signal from hair cells is _____ due to _____
- therefore, semicircular canals are _____ receptors, with a response lasting howlong?
signal from the hair cells is temporary because of a spring-like action of the cupula, which restores it to its resting position
the semicircular canals are rapidly adapting receptors, with a response lasting 20 seconds or less
semicurcular canals
- threshold level
- -as a result…
low threshold (highly sensitive receptors) respond to very small angular changes
semicurcular canals
- response from the rector is _____
- i.e.
response from receptor is graded
i.e. as velocity increases, more hair cells are activated, each to a greater degree
otolithic organs
- functions
- important role as a result of their functions
detect linear acceleration and deceleration of the head
encode the static position of the head with respect to gravity
important role
-maintain the head upright in space and modulate postural muscle tone relative to the position of the head
structurally, what are the utricle and saccule
two large chambers located between the semicircular canals and cochlear duct
both ends of the semicircular canals are continuous with the…
utricle
utricle and saccule are filled with…
endolymph
utricle and saccule
-overproduction of endolymph results in…
Meniere’s disease
- auditory disturbances
- severe vertigo
- nausea
- nystagmus
name of the sensory organ on the wall of the utricle and saccule
macula
macula
- covered with…
- hair cells project into…
covered with a gelatinous layer with embedded small crystals of calcium carbonate called otoconia
hair cells project into the gelatinous layer
macula hair cells
-synapse with…
synapse at their base with the sensory axons of the vestibular nerve (same as with the hair cells of the ampulla)
when the head is upright, what is the positioning of the…
- macula of the utricle
- macula of the saccule
utricle
-macula in horizontal plane
saccule
-macula vertical
why is the complex arrangement of macula in the utricle and saccule necessary
-these receptors are _____
allows for the utricle and saccule to be sensitive to static head tilt in multiple planes
receptors are non-adapting and continue to respond at all times
the anatomical relationship between the utricle, macula, and semicircular canals is clinically significant in which condition
benign paroxysmal positional vertigo (BPPV)
BPPV
- pathophysiology
- symptoms
otoconia from the macular organs break loose and float into the semicircular canals, most commonly the posterior canal
with specific head motions, this “debris” stimulates (increases the inertia of the cupula) the hair cells in the ampula and produces symptoms of vertigo, characteristic nystagmus, and sometimes nausea
vestibular portion of 8th cranial nerve
- type of nerve
- cell bodies form _____ near _____
bipolar nerve
cell bodies form Scarpa’s ganglion near the internal auditory meatus
vestibular portion of 8th cranial nerve
-after leaving Scarpa’s ganglion, the central processes….
join the cochlear fibers
travel in the internal auditory canal, along with the facial nerve, pass through temporal bone into the posterior fossa, and enter the brainstem at the pontomedullary junction
vestibular nuclei
-primary vestibular afferents synapse on…
2nd order neurons in the vestibular nuclei of the brainstem and in the cerebellum
vestibular nuclei
-form a _____
diamond-shaped complex located in each side of the brainstem in the floor the 4th ventricle
vestibular nuclei
-surrounded by….
inferior cerebellar peduncle laterally
pontine reticular formation medially
middle cerebellar peduncle rostrally
nucleus and spinal tract of the trigeminal nerve ventrally
vestibular nuclei
-main nuclei
superior
lateral
medial
descending (inferior and spinal)
vestibular nuclei
-nucleus Y importance
integrative
vestibular nuclei
-different methods of organization
in some areas, there is convergence of sensory input from multiple vestibular receptors (e.g. canals and otolith) onto 2nd order neurons
in other locations, there is clear topographic organization of input from specific endorgans to specific nuclear areas
vestibular nuclei
-_____ areas support the vestibular nuclei in the task of _____
surrounding brainstem areas support the vestibular nuclei in the task of controlling eye movements in the horizontal and vertical planes
vestibular nuclei
- in general, each nucleus receives input from…
- what is the exception
both the semicircular canals and otoliths
exception
-lateral nucleus receives majority of its vestibular input from the utricle
fibers from the cerebellar flocculonodular lobe and fastigial nucleus terminate in _____
all four vestibular nuclei
afferents to the vestibular nuclei from _____ are relayed through _____
-what is their function
from the eyes as well as from the neck and limbs relayed through the cerebellum help modulate -vestibulo-ocular -vestibulocollic -vestibulospinal reflexes
how does the cerebellum exert influence on limb extensor tone
indirectly through tonic inhibition of the lateral vestibular nucleus
vestibular nuclei: ascending projections
- to the…
- travel via…
to the motor nuclei of the extraocular eye muscles in the pons and midbrain
tarvel via the MLF that runs in the floor of the fourth ventricle
vestibular nuclei: descending projections
- travel via…
- travel to…
via
- MLF
- lateral vestibulospinal tract ipsilaterally
- medial vestibulospinal tract to both sides of the spinal cord
the _____ and _____ of the cerebellum receive extensive input from the vestibular nuclei
fastigial nuclei
flocculonodular lobe
vermis
- what is it
- receives input from…
- function
principal region of the spinocerebellum
receives input from the vestibular nuclei
info is integrated with somatosensory mapping from the limbs and trunk
what serves the third function of the vestibular system involving perception of self in space
ascending vestribulo-thalamo-cortical projections
ascending vestibulo-thalamo-cortical projections
- travel in _____ adjacent to _____
- what fibers carry this information
- terminate in…
travel in reticular formation adjacent to the MLF and lateral lemniscus (auditory pathway)
carried by secondary fibers from the lateral and superior vestibular nuclei
terminate in thalamus
ascending vestibulo-thalamo-cortical projections: thalamocortical projections
- project to
- function
project to regions of the parietal lobe (association areas)
produce a subjective sensation of rotation and body displacement
ascending vestibulo-thalamo-cortical projections: thalamocortical projections
- does not project to…
- why is this impoortatn?
does not project to primary somatosensory cortex
therefore there is no perception of specific location in space (proprioception)
peripheral vestibular endorgan blood supply
labyrinthine artery
- most often a branch of the AICA
- occasionally branches directly off of the basilar artery
labyrinthine artery ischemia
- how possible is it?
- results in…
highly susceptible to ischemia
results in hearing loss and sudden onset of vertigo
area of brainstem containing the vestibular nuclei blood supply
PICA (caudal aspects) AICA (rostral aspects)
area of brainstem containing the vestibular nuclei
-specific clinical syndromes with symptoms of vestibular system involvement occur with occlusitons of the…
basilar artery
labyrinthine artery
AICA
PICA
vestibulo-ocular reflex (VOR)
-function
maintain stable vision during head motion
VOR
-describe the reflex
when head is turned to the left, the L vestibular nerve increases firing and right decreases firing
these changes in firing rates influence discharge from the medial vestibular nuclei, which in turn sends signals via the MLF to activate R abducens nucleus (R lateral rectus) and L oculomotor nucleus (L medial rectus)
inhibitory impulses are sent to nuclei that control the antagonistic muscles that result in a lateral compensatory eye movement towards the right called nystagmus
nystagmus
- characterized by…
- nystagmus named by direction of…
slow phase away from midline (caused by vestibular input) and a fast phase as eyes rebound towards midline
named by direction of fast phase because it is more readily identified
in the VOR example, what would we call that nystagmus?
left beating
eye movements are parallel to the plane of ____
the canal that has been stimulated
is nystagmus normal?
yes, it is a normal response to rotatory head velocity
a pathologic nystagmus is doe to lesions either in the _____ or _____
peripheral vestibular organ
central vestibular nuclei or pathways
vestibulo-spinal reflex
-influences…
ipsilateral extensors
-primarily of the axial musculature and the LE (anti-gravity) muscles
output of the lateral vestibular nucleus gives rise to…
lateral vestibulospinal pathway
lateral vestibulospinal pathway
-axons are seen in cross section where?
as they pass through the inferior vestibular nucleus
vestibulo-collic reflex
-how does it get its name
from the musculature of the neck
vestibulo-collic reflex
- uni- or bilateral?
- most active through….
bilateral
most active through the cervical and upper thoracic regions of the spine
vestibulo-collic reflex
-primary source of these fibers is from…
medial vestibular nuclei of both sides
vestibulo-collic reflex
- while one side of the system is sending _____ signals to _____, the other side is carrying signals that _____
- why is this concept important?
while one side is sending excitatory signals to the extensors of the UE and axial musculature on both sides, the other side is carrying signals that are decreasing, but are also going to both sides
important concept since the control of axial musculature is dependent on this balance of increasing and decreasing signals
superior vestibular nucleus
-major input
semicircular canals
cerebellum
contralateral vestibular nuclei
superior vestibular nucleus
-major output
MLF to oculomotor nuclei cerebellum dosal pontine RF adjacent RF thalamus
superior vestibular nucleus
-function
maintain eye position during head movement
arousal
conscious awareness of spatial orientation
lateral vestibular nucleus (Deiter’s)
-major input
cerebellum
utricle and saccule
semicircular canals (minimal)
lateral vestibular nucleus
-major output
ipsilateral lateral vestibulo-spinal tract
thalamus
lateral vestibular nucleus
-function
facilitates antigravity limb tone through vestibulospinal reflexes
spatial orientation
medial vestibular nucleus
-major input
semicircular canals - primary horizontal utricle and saccule cerebellum contralateral vestibular nuclei reticular formation
medial vestibular nucleus
-major output
bilateral medial vestibulo-spinal tract
MLF to oculomotor nuclei
cerebellum
adjacent RF
medial vestibular nucleus
-function
coordination of eye, head and neck movements
gaze holding in the horizontal plane
head control and stability - head turning and righting
inferior (spinal or descending) vestibular nucleus
-major input
utricle and saccule
cerebellum
inferior (spinal or descending) vestibular nucleus
-major output
cerebellum
RF
contralateral vestibular nuclei
inferior (spinal or descending) vestibular nucleus
-function
integrates vestibular signals from two sides with signals from cerebellum and RF
nucleus Y
-major input
utricle and saccule
nucleus Y
-major output
RF
contralateral vestibular nuclei
nucleus Y
-function
integrates vestibular signals from two side with signals from cerebellum and RF
