Ch 15: Vestibular system Flashcards
what does the vestibular system consist of?
receptors located in the inner ear on both sides of the head
peripheral nerve fibers of the vestibular division of CN 8
central connections that analyze information about the position and movement of the head in space
what are the functions of the vestibular system?
maintain body balance
coordinate eye, head and body movements
permit the eyes to remain fixed on a point in space as the head moves
what structures mediate the functions of the vestibular system?
commissural connections between the vestibular nuclei
vestibulocerebellar tracts
vestibulospinal tracts
vestibulo-ocular tracts
thalamocortical tracts
the thalamocortical pathway to the parietal lobes serves as:
the conscious sense of head position and acceleration
the inner ear (labyrinth) consists of:
bony labyrinth:
-consists of a series of interconnected cavities in the petrous portion of the temporal bone
membranous labyrinth (lies inside the bony)
- a system of tubes and sacs of fine membranes
- contains 5 compartments: utricle, saccule and 3 semicircular canals
what is perilymph?
a fluid that fills the space between the bony and membranous labyrinth
the perilymphatic space of the vestibular part of the bony labyrinth is continuous with the perilymphatic spaces of the cochlea: the scala vestibule and scala tympani
what is endolymph?
fills the membranous labyrinth
communicates through a small channel with that in the cochlear duct
what are the peripheral receptors of the vestibular system? where do they reside? what do they do?
the vestibular hair cells
reside in specialized receptor areas within the membranous labyrinth
dynamic functions:
detect both linear (translational) and angular (rotational) motion of the head in space
static functions:
allow detection of the position (tilt) of the head
what requires all components of vestibular function?
the control of posture, locomotion and eye movements
what are the 2 swellings within the membranous labyrinth?
utricle
saccule
what does the floor of the cuticle house?
what is on the apical surface of (the above)?
a specialized receptor region= macula
the macula contains hair cells that synapse on the distal branches of vestibular ganglion cells
the apical surface of each hair cell bears numerous STEREOCILIA and on each side of the stereocilia, a single KINOCILIUM
macula hairs extend upward into an overlying gelatinous substance containing ____?
otoconia - consists of calcium carbonate crystals
how does the macula lie when the head is in an erect position? what happens to the gelatinous matrix?
the macula of the utricle lies in the horizontal plane
the gelatinous matrix with its otoconia rests directly on the hair cells
what happens to the otoconia if the head tilts or accelerates in the horizontal plane?
the inertia of the otoconia causes the gelatinous matrix to lag behind the movement of the skull and thereby bends the hairs of the receptor cells
how does head movement change the membrane potential of the receptor cells?
when the force on a hair cell bends the sterocilia toward the kinocilium, the cell becomes DEPOLARIZED
when the stereocilia en away from the kinocilium, the cell becomes HYPERPOLARIZED
this results in a corresponding increase or decrease of activity at the synapses between the hair cells and the peripheral processes of the vestibular ganglion cells
the macula of the utricle responds to:
changes in head position with respect to gravity or tilt and to each-horizontal linear acceleration
the arrangement of hair cells in the macula creates a response to acceleration in any diretion
what is the stroll?
a specialized strip through the middle
defines the site of change in orientation of the hair cells
each hair cell is oriented with the kinocilium toward the stroll. thus the hair cells on either side of the stroll are organized as mirror images
the saccule contains a macula of similar structure, but..
responds to?
it is oriented vertically
thus linear acceleration in the vertical direction stimulates the macula. this occurs in response to gravity or against gravity, as when we accelerate or decelerate in an elevator
how are the semicircular canals arranged?
anterior/ superior
lateral /horizontal
posterior
arranged in 3 planes that lie roughly at R angles to one another
what is the ampulla?
what resides in it?
the enlarged end of each semicircular canal
within the ampulla resides the ampullary crest (Crista ampullaris) - a ridge that bears hair cells like those of the maculae
what covers the ampullary crest?
what affect does it have?
a gelatinous capsule termed the CUPULA covers the ampullary crest and extends upward to the roof of the ampulla.
this gelatinous matrix has the same specific gravity as the endolymph and therefor can’t sense the effects of gravity, but when the head undergoes angular acceleration or rotation, the viscous endolymph in the semicircular ducts lags behind as a result of inertia and pushes on the cupola
distortion of the cupola during angular acceleration evokes a receptor potential in the hair cells on the ampullary crest, and this alters the level of activity in the vestibular fibers of VN 8 which the hair cells synapse.
the afferent nerve fibers from each crista amullaris respond with an increase in impulse frequency to rotation in one direction and with a decrease in impulse frequency to rotation in the opposite direction.
these increases and decreases modify a baseline tonic level of vestibular activity that balance input from the 2 ears.
what does damage to the inner ear or vestibular nerve on one side cause?
an imbalance that can produce nausea, vertigo, postural imbalance, and abnormal eye movements
the afferent fibers of the vestibular nerve have cell bodies in:
the vestibular ganglion
axons of bipolar cells of the vestibular ganglion pass:
through the internal auditory canal and reach the upper medulla in company with the cochlear nerve
the fibers of the vestibular nerve bifurcate into ascending and descending branches and terminate in the ? located?
vestibular nuclei which lie clustered in the lateral part of the floor of the 4th ventricle
how many vestibular nuclei are there? what are they called?
lateral vestibular nucleus
medial vestibular nucleus
superior vestibular nucleus
descending (spinal/inferior) vestibular nucleus
the cristae ampullares of the semicircular canals provide input over vestibular afferents primarily to:
the central region of the vestibular nuclear complex: at the boundaries between the superior, medial and descending vestibular nuclei
the maculae of the utricle and saccule project vestibular afferents predominantly to the:
lateral and descending vestibular nuclei
the vestibular nuclei project to:
the cerebellum and the RF of the BS
vestibulospinal pathways essential for postural reflexes and coordinated locomotion
to ascending pathways that regulate eye movements
to thalamocortical pathways that mediate the conscious sense of position and movement of the head
what are the 2 major projections into the SC that arise from the vestibular nuclei?
the uncrossed lateral vestibulospinal tract from the lateral vestibular nucleus
the medial vestibulospinal tract, which has both crossed and uncrossed axons, comes chiefly from the medial vestibular nucleus, with some fibers contributed by the descending (spinal) nucleus
the lateral vestibulospinal tract extends:
ipsilaterally from the cervical to lumbosacral levels
the medial vestibulospinal tract extends:
joins the descending MLF, extends bilaterally only through the cervical segments
both vestibulospinal tracts strongly facilitate:
motoneurons innervating antigravity muscles
theseeffects assist the local muscle stretch reflexes and reinforce the tons of the extensor muscles of the trunk and limbs to produce enough extra force to support the body against gravity and to maintain an upright posture
what is the vestibule-ocular reflex?
the vestibular system controls reflexive conjugate eye movements that compensate for brief head movements in any direction
stimulation of the vestibular receptors evokes eye movements of equal magnitude in the direction opposite to the head, to keep the retina focused on the same visual field
the vestibular nuclei (particularly the medial) and their connections with the cerebellum influence the pathways used for smooth pursuit eye movements
fibers from the medial vestibular nucleus synapse on the?
mediate the?
in response to?
abducens nucleus and mediate horizontal eye movements in response to stimulation of the horizontal semicircular canals
the ascending projections from the superior vestibular nucleus synapse on the?
mediate the?
in response to?
oculomotor nuclei
control vertical eye movements in response to stimulation of the anterior and posterior canals
what do the vestibule-ocular reflexes do?
enable the eyes to remain fixed on stationary objects while the head and body move.
what does turning the head slightly to the right do?
turning the head to the R causes a slight flow of endolymph in the horizontal semicircular canals – the flow moves to the LEFT because the inertia of the fluid causes it to lag behind the movement of the head.
this flow of endolymph increases neural activity in the hair cells of the ampulla in the R horizontal canal and decreases activity in the L.
this excites the vestibular nuclei on the R through the vestibular ganglion cells.
axons from the R medial vestibular nucleus cross the midline to excite the L abducens nuclear complex and induce contraction of the L lateral rectus muscle. Simultaneously, projections from the R medial vestibular nucleus inhibit the ipsilateral abducens nucleus.
From a separate group of cells in the L abducens complex, axons cross the midline and ascend through the MLF to the R oculomotor nucleus- activate oculomotor neurons to the R medial rectus muscle
as a result, the eyes turn the proper distance to the L to compensate for the head movement to the R and keep the field of vision unchanged
what is nystagmus?
persistent stimulation of hair cells in the ampulla of a semicircular canal causes the eyes to move slowly to one side until they reach the physical limit and then jerk quickly to the opposite side.
these movements occur repetitively in rapid succession and produce tremor like oscillations of the eyes
nystagmus consists of a normal reflex response to rotation or to unilateral (unbalanced) stimulation of one of the semicircular canals
what are 2 ways to clinically test for nystagmus?
rotation test
caloric or thermal tests
what is the rotation test? how is it performed?
performed by turning the patient in a revolving chair with the head tilted forward 30 degrees to bring the horizontal canals parallel with the floor
after 10-12 turns, movement stops abruptly
during rotation, endolymph moves in the direction opposite to the movement of the head, but with stopping, momentum causes the endolympth to reverse direction and to flow in the direction in which the head had been turning, even though the head has become stationary.
the induced nystagmus (Post rotary nystagmus) lasts about 30 seconds in neurologically normal persons
when rotation is to the L, on cessation of movement, endolymph flows to the L and the slow component of the nystagmus moves to the L. because the quick component moves to the R, it is called “nystagmus to the R”
what is the caloric test? how is it performed?
permits testing of each side of the vestibular system separately
patient is either prone with head tilted forward 30 degrees or seated with the head backward 60 degrees– brings the horizontal semicircular canal into the vertical plane
examiner irrigates the patent’s external auditory canal with either cold or warm water– lowers or raises the temp of the endolymph on the side of the semicircular canal closest to the middle ear and causes a convection current to develop in the endolymph-causes deception of the ampullary crest, which stimulates the hair cells and leads to nystagmus.
warm water in the R ear: convection current moves superiorly, causing endolymph to flow toward the ampulla– nystagmus includes a slow component to the L and a quick component to the R
cold water: causes current to be reversed, and the nystagmus consists of a slow component to the R and a quick component to the L
caloric tests can be recorded graphically by placing recording electrodes on the patient’s face near the eyes and using appropriate amplification and recording equipment
loss of the tonic labyrinthine discharge on one side unbalances the stream of impulses from the 2 sides and leads to ?
spontaneous nystagmus and vertigo
unbalanced discharge can occur with:
destruction of:
vestibular receptors
section of the vestibular nerve
damage to vestibular nuclei
what is vertical nystagmus usually associated with?
horizontal and rotatory nystagmus?
vertical= central lesions
horizontal & rotatory= peripheral lesions affecting the labyrinth or vestibular nerve
what is the vestibulothalamocortical pathway?
projections from the vestibular nuclei to the ventral lateral and ventral posterior nuclei of the thalamus
provide conscious awareness of the position and movements of the head
from these thalamic nuclei, vestibular information projects to several separate areas associated with the somatosensory cortex (including primary somatosensory areas which receives primarily joint sensations, and 3a which receives info from muscle spindles
stimulation of the vestibulothalmocortical pathway causes:
a sense of motion
what are sensory aspects of vestibular stimulation?
vertigo= consists of a sense of whirling
motion sickness= manifestation of prolonged and excessive stimulation of the vestibular apparatus
dizziness= (sensation of vertigo); describes feelings of giddiness, faintness, lightheadedness, visual disturbances
what is Meniere’s disease?
a condition characterized by sudden attacks of severe vertigo, usually associated with nausea, vomiting, and prostration and usually lasting a few days at a time.
fluctuating but progressive unilateral deafness and tinnitis frequently accompany the attacks of vertigo.
results from accumulation of endolymphatic fluid in the labyrinth
