Vestibular System Flashcards
What are the five components of the vestibular system?
- peripheral receptor apparatus (inner ear, transduces head motion and position)
- central vestibular nuclei (brainstem, integrates and distributes info that controls motor activities and spatial orientation)
- vestibuloocular network (vestibular nuclei, control of eye movements)
- vestibulospinal network (head movements, axial musculature, postural reflexes)
- vestibulothalamocortical network (conscious perception of movement/spatial orientation)
What receptor structures make up the vestibular labyrinth?
Three semicircular canals
Two otolith organs
Describe the bony labyrinth
the protective shell that houses the fluid-filled system (the membranous labyrinth)
Describe the membranous labyrinth
Connecting tubes and prominences that house vestibular receptors
Describe perilymph and where does it exist?
- between membranous and bony labyrinths
- similar to CSF
- bathes vestibular portion of CN VIII
Describe endolymph and where does it exist?
- within the membranous labyrinth
- covers sensory receptors of the vestibular and auditory systems
Are ionic concentrations of perilymph and endolymph the same? What is the purpose of this?
- ionic concentrations are different
- if distribution or ionic content of endolymph is disturbed, often results in vestibular disease
- balance is maintained by cells in the membranous labyrinth and endolymphatic sac
What kind of motion is received by semicircular canals?
Rotational head movements (angular accelerations)
What kind of motion is received by otolith organs?
Translational head movements (linear accelerations)
What innervates the receptor cells in vestibular organs?
Primary afferent fibers of vestibular (Scarpa) ganglion
Describe the location of the primary afferent fibers that innervate the receptor cells of vestibular organs
- bipolar cells
- central processes enter the brainstem
- terminate in ipsilateral vestibular nuclei and cerebellum
- cell bodies are in the ganglia
Describe the organization of planes of the vestibular organs
- Horizontal SC canal/utricle approx. 30 degrees anterodorsal to the nose (head is pitched down when walking/running, so the plane of these organs becomes parallel with the earth/perpinducular to the gravity)
- Anterior/Posterior SC canals/saccules are vertical and at right angles to the horizontal SCC/utricle
- Vertical canals are at right angles to each other, plane of one anterior canal is coplanar with contralateral posterior canal
Describe the blood supply to the labyrinth
The labyrinthine artery (branch of AICA)
-enters temporal bone via internal auditory meatus
Sylomastoid artery also provides some branches to the labyrinth (semicircular canals)
Interruption of blood supply compromises vestibular and cochlear function (vertigo, nystagmus, unstable gait)
Describe the anatomical organization of the membranous labyrinth
- supported in the body labyrinth by connective tissue
- semicircular canal ducts connect to the urticle
- each duct ends with an ampulla (sensory receptors are in the base of each ampulla)
- receptors in utricle are longitudinal along the base
- receptors in the saccule are vertical along medial wall
- endolymph drains into endolymphatic sinus via endolymphatic duct
- saccule connected to cochlea by ductus reuniens
Meniere’s Disease
Increased endolymph volume, causing distention of membranous labyrinth
- fluctual hearing loss, vertigo, positional nystagmus, and nausea
- TX: diuretic and salt-restricted diet, implantation of shunt into swollen endolymphatic sac
Describe a Type I vestibular sensory receptor
- chalice shaped
- surrounded by afferent terminal (nerve calyx)
Describe a Type II vestibular sensory receptor
- cylindrical
- innervated by synaptic boutons
Commonalities of TI and TII vestibular sensory receptors
- hair cells with stereocilia that project from apical surface
- a single longer kinocilium
- both receive synapses from vestibular efferents that control receptor sensitivity (activated by behaviorally arousing stimuli or CN V stimulation)
Describe vestibular neuritis
- severe vertigo, nausea, vomiting
- no hearing loss or other CNS deficits
- edema of vestibular nerve or ganglion (acute viral infections such as herpes simplex)
- some have recent history of URI/cold/influenza
- treat with antiemetics, vestibular suppressants, corticosteroids, and anti-virals
Describe vestibular schwannoma
- benign tumor of Schwann cells of vestibular root (rarely from cochlear root)
- 5-10% of all intracranial tumors
- usually in the cerebellopontine angle; inpinges on structures in the internal auditory meatus (CN VII/VIII/labyrinthine artery)
- slow growing, hearing loss (all cases), gait issues (most), tinnitus (most)
Describe benign paroxysmal positional vertigo
- one of most common vestibular disorders
- brief episodes of vertigo w/ certain changes in body position
- turning over in bed, getting up in the morning, bending over, rising from a bent position
- otoconial crystals from utricle separate from otolith membrane; become lodged in cupula of semicircular canal (abnormal cupula deflections occur)
Describe subjective vertigo
the patient feels that they are spinning
Describe objective vertigo
the patient feels like the room is spinning
nystagmus beats in the direction opposite to the original direction of rotation
Describe dizziness
nonspecific, spatial disorientation that may or may not involve feelings of movement
- may be accompanied by nausea or postural instability
- not exclusively a vestibular phenomenon
Describe the ampulla
- hair/supporting cells lie embedded in the crista, goes across the base of the ampulla
- -TI in the central regions, TII in the peripheral regions
- enveloped in the cupula (gelatinous structure)
- cupula allows movement of the stereocilia towards or away from the kinocilium
- cupula has same specific density as endolymph
Describe how the ampulla helps detect motion
- rotational head movements produce angular acceleration
- endolymph is displaced
- cupula is pushed to one side
- stereocilia is displaced in the same direction
(if you move your head to the right, the R-sided cells depolarize and activate, while the L-sided cells hyperpolarize and are inhibited)
Describe to macula
- Stereocilia extend into the otolith membrane (gelatinous coating)
- The otolith membrane is covered by otoconia (calcium carbonate crytals)
- more dense than surrounding endolymph
- not displaced by normal endolymph movements
Describe how the macula helps detect motion
changes in head position relative to gravity/linear accelerations (forward/backward or upward/downward) displace otoconia, which bends the underlying hair cell stereocilia
What is the function of vestibular nuclei?
process positional and movement info for control of visual and postural reflexes
-each nucleus differs in their afferent/efferent connections
Describe the location of the superior vestibular nucleus
superolateral in central pons
bordered by restiform body and 4th ventricle
Describe the location of the medial vestibular nucleus
lateral floor of 4th ventricle
rostrocaudal extent
Describe the location of the lateral vestibular nucleus
lateral to the medial vestibular nucleus
contains large neurons (Deiters)
Describe the location of the inferior vestibular nucleus
lateral to medial vestibular nucleus
extends through the medulla
Describe areas 2v and 3a of the vestibular cortex
Primary Somatosensory Cortex
- stimulation of 2v: sensations of whole body motion
- 3a integrates motor control of the head and the body
Describe area 7 of the vestibular cortex
Parietal Cortex
- spatial coding, visual (optic flow), and vestibular motion signals
- integrates cues of body motion in space
- *lesions=confusion in spatial awareness**
Describe the insular areas of the lateral sulcus and the parietoinsular vestibular cortex
cells respond to body motion, somatosensory, proprioceptive, and visual motion stimuli
lesions cause vertigo, unsteadiness, loss of perception for visual vertical
Describe the prefrontal cortex and superior frontal gyrus
receives vestibular signals, related to the frontal eye fields
control of saccades and smooth pursuit eye movements
Vestibulovestibular Fibers
- arise from all vestibular nuclei
- most prominent in superior and medial nuclei
- form reciprocal connections with analogous contralateral nucleus
- coordinate verious motions of the eyes
Spinovestibular Fibers
- arise from all levels of the spinal cord
- proprioceptive input to medial and lateral vestibular nuclei
- positional sense related to balance
Other input to vestibular nuclei
- reticular formation
- raphe nuclei
- thalamus
- several other cortical regions
maintain balance
Primary (Vestibular) Afferent Fibers
- enter at pontomedullary junction
- traverse restiform body and branch into ascending and descending fibers
Afferent Fibers from the Semicircular Canals (ampulla)
- project to superior and medial vestibular nuclei
- minor input to lateral and inferior vestibular nuclei
Afferent Fibers from the Otolith Organs (maculae)
project to lateral, medial, and inferior vestibular nuclei
Saccular Afferent Fibers
project to contralateral oculomotor nucleus
influence vertical eye movements
What is the only sensory organ in the body that sends direct primary afferent projections to the cerebellar cortex and nuclei?
Cerebellum
Describe the course of vestibulocerebellar fibers
Primary vestibulocerebellar fibers travel from the fastigial nucleus through the juxtarestiform body in the inferior cerebellar peduncle
Collaterals are sent to the dentate nucleus and terminate as mossy fibers
Vestibular nuclei send fibers to fastigial and dentate nuclei and secondary vestibulocerebellar projections
What is the purpose of the vestibuloocular network?
Keeping a fixed gaze on an object while the head is moving
How does the vestibuloocular network accomplish its purpose?
Vestibuloocular reflex (stabilizing eye movements)
- compensatory: equal in magnitude and opposite in direction to the head motion
- occurs for any direction and speed of head motion
- can be willingly suppressed to focus on a moving target (such as trying to run and catch a ball)
Which vestibular receptive organs are associated with vertical movements?
vertical SC canals
saccule
Which vestibular receptive organs are associated with horizontal movements?
horizontal canals
utricle
Which vestibular receptive organs are associated with torsional eye movements?
vertical SC canals
utricle
Describe the pathway of afferent nerves necessary to produce eye movement during head rotation
- Send axons through the medial longitudinal fasiculus to the contralateral abducens nucleus
- Abducens motor neurons excite the ipsilateral lateral rectus muscle via cranial nerve VI
- Abducens interneurons send excitatory impulses to contralateral oculomotor nucleus (innervates the medial rectus muscle)
- a second set of vestibular neurons send excitatory signals to ipsilateral oculomotor nucleus (medial rectus muscle)
- 3rd set of vestibular neurons carry inhibitory signals to the ipsilateral abducens nucleus
* *inhibit the abducens muscle on the ipsilateral side of head rotation, activate the abducens on the side contralateral of the head rotation)
Describe the pathway of afferent nerves necessary to produce eye movement during a righward head turn
- The right horizontal semicircular canal activates neurons in the right vestibular nuclei
- inhibitory signals from the left vestibular nuclei are decreased via the commissural neurons
- Neurons in the right vestibular nuclei excite the contralateral abducens motor neurons
- Produces contraction in the left lateral rectus and the right medial rectus muscles
- Compensatory leftward eye movement keeps the object of interest in the fovea
matching bilateral commections keep the right lateral rectus and left medial rectus inhibited
What is the purpose of the linear vestibuloocular reflex?
-way to stay balanced during non-rotational movement
- side-to-side head motion cause horizontal eye motion in the direction opposite of head movement
- vertical displacement of the body cause oppositely directed vertical eye movement to stabilize gaze
- during roll tilts of the head, eyes “counter roll” (torsional eye movement)
Describe the phases of nystagmus
- Slow phase: vestibuloocular reflex moves eyes slowly in opposite direction of head movement
- Eye reaches limit of how far it can turn
- Fast phase: eyes rapidly move back to the central position (eye moves in same direction as head)
- Slow phase begins again
Named for the direction of the fast phase
Describe the caloric test
Used to test the vestibular layrinth
-warm water induces nystagmus that beats toward the ear into which the water was placed
-cold water induces nystagmus that beats away from the ear into which the water was placed
(COWS)
both ears should give equal responses: unilateral lesion of the vestibular pathway will cause reduced/absent nystagmus ipsilaterally
Describe the lateral vestibulospinal tract
- muscle tone and reflexive postural adjustments
- neurons located in the lateral and inferior vestibular nuclei
- projects to all levels of ipsilateral spinal cord (anterorostal to cervical, posterocaudal to lumbosacral)
- fibers travel through the lateral medulla and the anterior funiculus of the SC
- end on alpha and gamma motor neurons and interneurons to laminae VII-IX
- collaterals branch to different segments, ensuring coordination of postural control
Describe the medial vestibulospinal tract
- muscle tone and reflexive postural adjustments
- bestibular stimulation on neck nuscles
- medial bestibular nucleus, with some from interior and lateral vestibular nuclei
- input from vestibular receptorsm cerebellum, and PCMLS info from the spinal cord
- fibers bilaterally descend through the MLF and end in laminae VII-IX of the cervical spinal cord
- neck flecor and extensor motor neurons
- effects seen in the vestibulocolic reflex (stabilize neck)
Neurons from which vestibular nuclei project to the thalamus?
All four nuclei project neurons to the thalamus
most terminate in contralateral thalamic nuclei
Name and briefly describe the two main nuclei of the thalamus
Ventral posterolateral nucleus
Ventral posteroinderior nucleus
respond to bestibular and somatosensory stimulation
Describe the thalamic regional representation of vestibular information
Thalamic regions represent separate but parallel pathways of vestibular info necessary for processing of motion and body orientation information
