Vestibular Pathway Flashcards
What structures make up the outer ear/ external ear?
- Auricle/Pinna (external ear)
- Auditory canal – ends at tympanic membrane
- Tympanic Membrane (Boundary between outer & middle ear)
What makes up the middle ear?
Ossicles
- Malleus
- Incus
- Stapes
What makes up the inner ear?
- Cochlea
- Labyrinth
- Vestibule – utricle & saccule
What are the 3 functions of the peripheral vestibular system?
- Maintain postural stability
- Stabilizing visual images during head movement to allow clear vision
- Provide information used for spatial orientation (head position relative to gravity)
What sensory input does the peripheral vestibular system receive?
- Vestibular
- Somatosensation
- Vision
What is the central processor of the peripheral vestibular system?
- Vestibular Nuclei
- Cerebellum (calibrates)
What is the motor output of the peripheral vestibular system?
- Postural reflexes
- Eye movement
What is the Bony Labyrinth filled with?
Compact bone filled with perilymph (fluid similar to CSF)
What is inside a membranous labyrinth?
- Suspended within bony labyrinth
- Filled with endolymph (fluid similar to intracellular fluid)
What are the 5 neural structures of the membranous labyrinth?
- 3 semi circular canals
1. Horizontal semi-circular canal
2. Posterior Semi Circular Canal
3. Anterior Semi-Circular Canal - 2 otolith organs
1. Saccule
2. Utricle
What does the semi circular canals detect?
- Detect angular acceleration around 3 axes
- Each canal paired with canal on opposite side (Anterior canal to opposite posterior canal)
What is the posterior semi circular canal innervated by?
inferior vestibular nerve
What is the anterior semi circular canal innervated by?
Superior vestibular nerve
What is the horizontal semi circular canal innervated by?
Superior vestibular nerve
How does the endolymph in the semi circular canals move and in response to what?
Freely in response to head angular movement (opposite the perilymph)
The semi circular canals are enlarged at one end to form what?
Ampulla
What does the Ampulla contain?
Gelatinous cupula (flexible membrane)
What does the cupula contain and where do they sit?
- Sensory hair cells (stereo cilia & kinocilia)
- Hair cells sit in the Crista ampullaris
When you turn your head the endolymph in the semi circular canals move (BLANK) direction of perilymph. This cause the (BLANK) to move and thus (BLANK) move.
- opposite
- cupula
- hair cells
When stereo cilia cells are deflected towards kinocilia does that cause excitation or inhibition?
excitation
When stereocilia cells are deflected away from kinocilia does this cause excitation or inhibition?
Inhibition
When you turn your head to the right the right horizontal canal is (BLANK) and the left horizontal canal is (BLANK)
- Right is excited (increase firing rate)
- Left is inhibited (decrease firing rate)
At rest how do we know our head is still and balanced?
- Tonic Firing Rate: Head is stationary
- The labyrinth is always firing action potentials at a constant rate down the vestibular nuclei
Healthy vestibular system have a resting firing rate at what?
70-100 spikes/sec
What does the otolith organs detect? and which specific axis does the utricle and saccule detect this on?
- Detect Linear Acceleration
1. Utricle: Horizontal Axis (Also detects head tilt)
2. Saccule: Vertical Axis
What does the otolith organs contain?
Maculae (resemble cristae ampullaris)
What does the maculae contain and what is it?
- Contain Otoconia
- Calcium carbonate crystals within otolithic membrane (Sit on gelatinous layer)
Hair cells are embedded within gelatinous layer in the Maculae.
- What are the hair cells?
- The hair cells that sit on medial wall of saccule detect what?
- The hair cells that sit on the floor of utricle detect what?
- Sit on Medial wall of saccule: Vertical acceleration
- Sit on Floor of utricle: Horizontal Acceleration
- Hair Cells= stereocilia and kinocilia
In the otoliths, with head tilt or acceleration, gravity/other acceleration pulling on crystals. What does this activate?
Pull of crystals activate hair cells – some maximally excited and some maximally inhibited
Why is there an increased sensitivity to gravity & linear acceleration?
Otolith membrane has more mass than cupula
Describe utricle response to acceleration if the head moves forward
- Calcium Carbonate crystals(otoconia) pull gelatinous membrane forward
- Causes stereocilia to move towards Kinocilia
- excitation
Describe saccule response to feeling your head moving up
Calcium carbonate crystals (otoconia) move up
- Cause hair cells to move up
- stereocilia move towards kinocilia
- excitation!
Describe the Vestibular Ocular Reflex
- Maintains Stability of an image on the fovea of the retina during rapid head movements
- Eyes move opposite the head!
- Peripheral Vestibular system connects to central vestibular pathways
In regards to SCC commincation with eye muscles:
For the Horizontal (left) what is the
Secondary nucleus?
Extraocular Motor Neuron?
Muscle?
- Secondary Nucleus: Medial vestibular nucleus
- Extraocular Motor Neuon: Left oculomotor nucleus & right abducens nucleus
- Muscle: Left medial rectus & right lateral rectus
For Posterior (left) what is the:
Secondary nucleus?
Extraocular Motor Neuron?
Muscle?
- Secondary nucleus: Medial vestibular nucleus
- Extraocular Motor Neuron: Right trochlear nucleus & right oculomotor nucleus
- Muscle: left superior oblique & right inferior rectus
For anterior/superior (left) what is the:
Secondary nucleus?
Extraocular Motor Neuron?
Muscle?
- Secondary nucleus: Lateral vestibular nucleus
- Extraocular Motor Neuron: right oculomotor nucleus
Muscle: Left superior rectus & right inferior oblique
What extra- ocular eye muscles does CN III innervate and what are their actions?
- Medial rectus: Rotates Eye Horizontally In
- Superior and Inferior Rectus mm: Rotates eye Vertically
- Inferior Oblique: rotates the eye upward and away from the nose
What extra-ocular eye muscle is innervated by CN IV and what is the action?
Superior Oblique: rotates the eye downward and toward the nose
What extra-ocular eye muscle is innervated by CN VI and what is the action?
lateral rectus muscles: rotate the eyes horizontally out
The relationship of eye velocity to head velocity is known as?
VOR Gain
When the head is moving below 60 degrees/sec what does this do the gaze?
- Gaze is maintained very well
- Gaze stability with smooth pursuits using eye muscle movements
- the ability to move the eyes with smooth, continuous motions in order to follow the movement of a target of interest and maintain the moving image on the fovea
What is solely responsible when the head is moved very fast (>60 deg/sec)?
VOR is solely responsible
What is inhibitory cut off?
- Excitation can be recorded as up to 400 spikes/sec
- Inhibition can only be recorded to a firing rate of 0 = inhibitory cut-off
- When there is a lesion in one side, the imbalance may be seen
What is VOR phase?
Relationship of amplitude between the eyes and the head
If the head moves 10 degrees to the right and eyes move 10 degrees to left what does this mean?
zero phase shift
How do hair cells of semi-circular canals and otoliths transmit neural discharge to vestibular nuclei?
Via the Vestibular Nerve (CN VIII)
Where does the superior vestibule nerve receive inout from?
- Utricle
- Anterior & lateral semicircular canals
Where does the inferior vestibular nerve receive input from?
- Saccule
- Posterior semicircular canal
How does primary sensory neurons convey angular & linear acceleration information?
Through vestibular division of CN VIII to vestibular nuclei
What are the 4 major nuclei in the pons/rostral medulla and what is the function?
- Superior – relay for VOR (through Medial Longitudinal Fasciculus)
- Medial – largest of the nuclei; relay forVOR (through MLF); medial vestibulospinal tract
- Lateral – principal nucleus for lateral vestibulospinal reflex
- Inferior – connected to all nuclei &cerebellum (lateral nucleus fibers run over it)
What are the vestibular nuclei connections?
- Distributes to other CN through Medial Longitudinal Fasciculus (MLF) (VOR)
- Flocculonodular node (cerebellum)
- Vestibulospinal tracts
- Medial and Lateral
- Cerebral cortex
- Reticular formation for autonomic control and arousal
What is the nucleus of origin of the Medial longitudinal fasciculus (MLF)?
Primarily medial vestibular nucleus and superior vestibular nucleus
What is the projection of the medial longitudinal fasciculus?
- The MLF connects the vestibular nuclei to ocular nuclei (III, IV, VI) for eye muscle control
- MLF: heavily myelinated tract runs near midline bilaterally
What is the function of the medial longitudinal fasciculus?
Vestibulo-ocular reflex (VOR) - Adjust eyes in response to head movement
Where does the cerebellum project?
- Vestibulocerebellum (through inferior nuclei or directly)
- Flocculonodular lobes
- Cerebellar vermis
What is the function of the cerebellum?
Vestibulo-ocular reflex (VOR) – adjust eyes in response to head movements
- Contributes to posture control for balance
- Influences coordination of limb movements and magnitude of motor response based on vestibular stimul
For the medial vestibulospinal tract what is the:
Nucleus of origin?
Projections?
Level of termination?
Function?
- Nucleus of origin: Medial andInferior vestibular nucleus
- Projection: Descends bilaterally in medial motor system
- Level of Termination: Cervical and Upper Thoracic Cord
- Function: Position of Head and Neck
For the lateral vestibulospinal tract what is the:
Nucleus of origin?
Projections?
Level of termination?
Function?
- Nucleus of origin: Lateral vestibular nucleus
- Projection: Descends in medial motor system ipsilaterally
- Level of termination: entire spinalcord – terminate in ventral horn
- Function: Balance
What is the specific part of the cortex has vestibular nuclei connections/ pathways?
Parietal association cortex and lateral temporoparietal junction
In regards to the cortex, what is the projection, function and where does it give input to?
- Projection: to ventral posteriornucleus of thalamus → cortex
- Function: Control neck & headposition
- Input to Corticospinal tracts
What is the vascular supply to the peripheral vestibular system?
Labyrinthine artery (branch of AICA)
* Common cochlear artery
* Anterior vestibular artery
What is the vascular supply to the central vestibular system?
- PICA: inferior portion of cerebellar hemispheres, dorsolateral medulla
- Basilar artery: pons
- AICA: cerebellum, lower pons, peripheral vestibular system
What are some examples of inner ear pathology?
- Vestibular Neuritis (UVH)
- Benign Paroxysmal Positional Vertigo
- Meniere’s Disease
- Unilateral or Bilateral vestibular hypo function
What can central lesions be a result of?
- Head Trauma
- Meningitis
- Brainstem or Cerebellar Stroke
- Multiple Sclerosis
What are some impairments that can be seen with vestibular pathology?
- Vertigo
- nystagmus
- unsteadiness
- postural instability
- ataxia
- nausea
- vision blurring with head movements
What is vertigo?
illusion of movement
- Room is spinning
- Most common cause is a Peripheral dysfunction
What can lightheadedness be from?
- Hypotension
- Hypoglycemia
- Anxiety
What is disequilibrium and what can it be due to?
- Disequilibrium: sense of being off balance
- Could also be due to weakness and somatosensory dysfunction
What is oscillopsia?
Motor of objects in the environment that are known to be stationary
What can other dizziness be due to?
light-headedness/faintness, nausea, unsteadiness
What are some common causes of central vestibular disorders?
- Ischemia
- tumors
- cerebellar degeneration or hemorhage
- brainstem stroke
- Multiple Sclerosis
- Arnold-Chiari malformation
What are the signs and symptoms of central vestibular disorders?
- Somatosensory and/or motor loss
- double vision
- Horner’s syndrome
- ataxia
- dysarthria
- verticality
Central vestibular disorder are caused by?
Damage to vestibular nuclei or to connections within the brain
