Vestibular Flashcards
The vestibular system consists of what 4 things?
1) the memranbous labyrinth and sensory receptor which is housed in the bony labyrinth
2) cranial nerve VIII and its cell bodies
3) vestibular nuclei which lie on the floor of the fourth ventricle at the junction of the medulla and pons
4) ascending and descending tracts
Explain the vestibulo-ocular reflex?
vestibular signals from the labyrinths, generated by head movement, regulate eye position in the orbit during head movements in order to assure a stable image on the retina
Explain the vestibulo-spinal and vestibulocollic reflexes?
the vestibular system influences muscle tone in appendicular and axial muscles for control of postural support and balance
Third role of the vestibular system?
to provide conscious awareness of spatial orientation
What is known as a series of tubes made of fine membranes that contain the peripheral sensory receptors (hair cells) of the vestibular system?
Membranous labyrinth
The membranous labyrinth contains a fluid called?
endolymph
The membranous labyrinth is separated from the bone cavity within the petrous part of the temporal bone (Bony labyrinth) by a fluid called?
perilymph
Function of the semicircular canals?
detecting angular velocity of the head in all planes, especially important for stabilizing the eyes during head movement
Semicircular canals are filled with what?
endolymph
Ampullae located at the base of the canals contain what?
hair cells called crista
The hair cells (crista) are innervated by what?
innervated by the peripheral ends of the vestibular nerve fibers
Hair cells protrude upwards into a gelatinous material called what?
cupula
Angular movement (rotation) causes the ednolymph to do what?
to lag behind (inertia) in the opposite direction, causing displacement of the cupula, thereby moving the hair cells
Mechanical deflection of the hair cells produces what?
a receptor potential that may either depolarize or hyperpolarize the vestibular nerve that is tonically active, with a resting discharge rate of 100 to 300 spikes per second
The effect on the resting discharge rate depends on what?
the direction the hair cells bend
What are the two types of projections that make up each hair cell?
- kinocilium (one)
- stereocilium (40-70)
When does depolarization occur?
when the stereocilium bends toward the kinocilium
When does hyperpolarization occur?
when the stereocilium bends away from the kinocilium
What signals the direction of rotation?
either an increase or decrease in the firing of the already tonically active nerve
Are the semicircular canals rapidly adapting or slowly adapting?
rapidly adapting receptors
Are the semicircular canals low or high threshold?
low threshold (highly sensitive receptors), responding to very small angular changes
Is the response from the receptor graded?
YES; as velocity increases, more hair cells are activated each to a greater change
What two structures make up the otolithic organs?
- utricle
- saccule
Function of the otolithic organs?
to detect linear acceleration and deceleration of the head as well as encoding the static position of the head with respect to gravity
What is the important role of the utricle and saccule?
to maintain the head upright in space and to modulate postural muscle tone relative to the position of the head
The utricle and saccule are two large chambers located between what?
between the semicircular canals and the cochlear duct
Both ends of the semicircular canals are continuous with what?
the utricle
The utricle and saccule are filled with what?
endolymph
Overproduction of endolymph results in what?
in auditory disturbances, severe vertigo, nausea and nystagmus (a condition known as Meniere’s disease)
The sensory organ on the wall of the utricle and saccule is called?
macula
The macula is covered with a gelatinous layer with embedded small crystals of calcium carbonate called?
otoconia
Hair cells project where?
into the gelatinous layer
Where do the hair cells of the macula synapse?
at their base with the sensory axons of the vestibular nerve
The macula of the utricle is positioned in what plane when the head is upright? The macula of the saccule is positioned in what plane when the head is upright?
utricle - horizontal plane
saccule - vertical plane
Is the macula non-adapting, slowly adapting, or rapidly adapting?
non-adapting and continue to respond at all times
The anatomical relationship between the utricle, the macula, and the semicircular canals is clinically significant in a condition called what?
benign paroxysmal positional vertigo (BPPV)
During BPPV, otoconia from the macular organs break loose and float into what part of the semicircular canals?
the posterior canal
Symptoms of BPPV?
- vertigo
- nystagmus
- nausea
The vestibular portion of CN VIII is a bipolar nerve with the cell bodies forming what?
Scarpa’s ganglion near the internal auditory meatus
After leaving the ganglion, the central processes of the vestibular nerve fibers join with what? then travel in what? along with what other nerve? passing through what? and entering the brainstem at what point?
cochlear fibers and travel in the internal auditory canal, along with the facial nerve, passing through the temporal bone into the posterior fossa, entering the brainstem at the pontomedullary junction
Primary vestibular afferents synapse on second order neurons in what>
in the vestibular nuclei of the brainstem and in the cerebellum
The vestibular nuclei form a diamond shaped complex located in each side of the brainstem in the fourth ventricle. They are surrounded by what?
- inferior cerebellar peduncle laterally
- pontine reticular formation medially
- the middle cerebellar peduncle rostrally
- the nucleus and spinal tract of the trigeminal nerve ventrally
What are the four main nuclei of the vestibular system?
- superior
- lateral
- medial
- descending (inferior or spinal)
Projections to the vestibular nuclei are primarily from where?
semicircular canals and the otoliths
Surrounding brainstem areas support the vestibular nuclei in the task of what?
controlling eye movements in the horizontal and vertical planes
Although, most vestibular nuclei primarily receive input from semicircular canals and the otoliths, which nucleus is the exception?
lateral nucleus
The lateral nucleus receives the majority of its vestibular input from where?
the utricle
Fibers from the cerebellar floculonodular lobe and fastigial nucleus terminate where?
in all four vestibular nuclei
Afferents to the vestibular nuclei from the eye as well as from the neck and limbs are relayed through what? to modulate what?
relayed through the cerebellum and help modulate vestibulo-ocular, vestibulocollic and vestibulospinal reflexes
Ascending projections to the motor nuclei of the extraocular eye muscles in the pons and midbrain travel via what?
the medial longitudinal fasciculus (MLF) that runs in the floor of the fourth ventricle
Projections out of the motor nuclei of the extraocular eye muscles descend in what three ways?
- in the MLF
- in the lateral vestibulospinal tract ipsilaterally
- in the medial vestibulospinal tract to both sides of the spinal cord
The fastigial nuclei and flocculonoduluar lobe of the cerebellum receive extensive input from where?
vestibular nuclei
The entire vermis, a principal region of the spinocerebellum, receives input from where?
the vestibular nuclei where it is integrated with somatosensory mapping from the limbs and trunk
Function of ascending vestibulo-thalamo-cortical projections?
perception of self in space
Traveling in the reticular formation adjacent to the MLF and the lateral leminiscus (auditory pathway), secondary fibers from the lateral and superior vestibular nuclei terminate where?
the thalamus
Thalamocortical projections to regions of the parietal lobe (association areas) produce a subjective sensation of what?
rotation and body displacement
Do thalamocortical projections project to the primary somatosensory cortex?
NO; therefore, there is no perception of specific location in space (perception)
The peripheral vestibular endorgan is supplied by what artery?
the labyrinthine artery, which is most often a branch of the anterior inferior cerebellar artery (AICA) but occasionally branches directly off of the basilar artery
The labyrinthine artery has no anastomotic network and is highly susceptible to ischemia, resulting in what?
hearing loss and sudden onset of vertigo
The area of the brainstem containing the vestibular nuclei is supplied by what two arteries?
the posterior inferior cerebellar artery (PICA) [caudal aspects] and the anterior inferior cerebellar artery (AICA) [rostral aspects]
Specific clinical syndromes with symptoms of vestibular system involvement occur with occlusions of what arteries?
occlusions of the basilar artery, the labyrinthine artery, AICA, and PICA
What is the purpose of the Vestibulo-ocular reflex (VOR)?
to maintain stable vision during head motion (when the head is turned to the left, the left vestibular nerve increases firing and the right decreases firing)
Changes in firing rates, when the head turns, influences discharge from where? which in turn sends signals via what? to activate what two nuclei?
Changes in firing rates influence discharge from the medial vestibular nuclei, which in turn sends signals via the MLF to activate right abducens nucleus (right lateral rectus) and left oculomotor nucleus (left medial rectus)
Inhibitory impulses are sent to nuclei that control what? results in what?
that control the antagonistic muscles that result in a lateral compensatory eye movement towards the right called nystagmus
Nystagmus is characterized by what?
by a slow phase away from midline (caused by vestibular input) and a fast phase (as the eyes rebound towards midline)
The direction of nystagmus is named by what?
the direction of the fast phase because it is more readily identified
Eye movements are parallel to what?
to the plane of the canal that has been stimulated
Nystagmus is a normal response to what?
to rotatory head velocity
Vestibulo-spinal reflex (VSR) influences what?
ipsilateral extensors, primarily of the axial musculature and the lower extremities
Output of the lateral vestibular nucleus gives rise to what?
the lateral vestibulospinal pathway
Axons of the lateral vestibulospinal pathway are seen in cross section as they pass through what?
the inferior vestibular nucleus
The vestibulo-collic reflex (VCR) is more active through what regions of the spine?
the cervical and thoracic regions
What is the primary source of the VCR fibers?
the medial vestibular nuclei of both sides
Explain the bilateral nature of this pathway and the balance it creates (the control of the axial musculature is dependent on this balance)?
while one side of the system is sending excitatory signals to the extensors of the upper extremity and axial musculature on both sides, the other side is carrying signals that are decreasing, but are also going to both sides
The superior vestibular nuclei receives input from where?
- semicircular canals
- cerebellum
- contralateral vestibular nuclei
The superior vestibular nuclei projects where?
- MLF to oculomotor nuclei
- cerebellum
- dorsal pontine RF
- adjacent RF
- thalamus
Function of the superior vestibular nuclei?
- maintain eye position during head movement
- arousal
- conscious awareness of spatial orientation
The lateral vestibular nuclei receives input from where?
- cerebellum
- utricle and saccule
- semicircular canals (minimal)
The lateral vestibular nuclei projects where?
- ipsilateral lateral vestibulo-spinal tract
- thalamus
Function of the lateral vestibular nuclei?
- facilitates antigravity limb tone through vestibulospinal reflexes
- spatial orientation
The medial vestibular nuclei receives input from where?
- semicircular canals (primary horizontal)
- utricle and saccule
- cerebellum
- contralateral vestibular nuclei
- reticular formation
The medial vestibular nuclei projects where?
- bilateral medial vestibulo-spinal tract
- MLF to oculomotor nuclei
- cerebellum
- adjacent RF
Function of the medial vestibular nuclei?
- coordination of eye, head and neck movements
- gaze holding in the horizontal plane
- head control & stability - head turning and righting
The inferior vestibular nuclei receives input from where?
- utricle and saccule
- cerebellum
The inferior vestibular nuclei projects where?
- cerebellum
- RF
- contralateral vestibular nuclei
Function of the inferior vestibular nuclei?
- integrates vestibular signals from two sides with signals from cerebellum and RF
The Nucleus Y receives input from where?
utricle and saccule
The Nucleus Y projects where?
- RF
- contralateral vestibular nuclei
Function of Nucleus Y?
- integrates vestibular signals from two sides with signals from cerebellum and RF
Where does the VCR get its name?
from the musculature of the neck (longus colli muscles on the anterior surface of the cervical vertebrae)
