Vestibular System Flashcards
What is the most common origin of vestibular issues causing dizziness?
Peripheral vestibular system
What are the components of the outer ear?
- Auricle/pinna
- Auditory canal
- Tympanic membrane (boundary between outer ear and middle ear)
What are the components of the middle ear?
Ossicles: malleus, incus, stapes
What are the components of the inner ear?
- Cochlea
- Labyrinth
- Vestibule (utricle and saccule)
What are the functions of the peripheral vestibular system?
- Maintain postural stability
- Stabilizing visual images during head movement to allow clear vision
- Provides information used for spatial orientation (head position relative to gravity)
What is the bony labyrinth?
- Structure in the inner ear made of compact bone
- Filled with perilymph, which is a fluid similar to CSF
- Contains membranous labyrinth
What is the membranous labyrinth?
- Structure suspended within the bony labyrinth
- Filled with endolymph, which is a fluid similar to intracellular fluid
- Contains the 3 semicircular canals and 2 otolith organs
What are the three semicircular canals?
- Horizontal semi-circular canal
- Posterior semi-circular canal
- Anterior semi-circular canal (superior semi-circular canal)
They detect angular acceleration around 3 axes
Describe the horizontal semicircular canal
- Innervated by the superior vestibular nerve
- Excited with transverse plane movements
- Right horizontal semicircular canal is excited when turning to the right, left horizontal semicircular canal excited when turning to the left
Describe the posterior semicircular canals
- Innervated by the inferior vestibular nerve
- Inferior to the anterior semicircular canal
- Bilateral semicircular canals are excited with posterior sagittal movements (looking up)
Describe the anterior semicircular canals?
- Innervated by superior vestibular nerve
- Superior to posterior semicircular canal
- Bilateral semicircular canals are excited with anterior sagittal movements (looking down)
What structures are within the semicircular canals?
- Endolymph
- Ampulla: widened end of canal
- Inside ampulla: gelatinous cupula, sensory hair cells (stereo cilia and kinocilia)
How does a semicircular canal become excited?
- Endolymph moves in response to angular head movement and moves in the opposite direction of the perilymph
- Causes cupula to move thus causing hair cells to move
- Stereocilia cells are deflected towards kinocilia causing depolarization
How does a semicircular canal become inhibited?
- Endolymph moves in response to angular head movement and moves in the opposite direction of the perilymph
- Causes cupula to move thus causing hair cells to move
- Stereocilia cells are deflected away from kinocilia causing inhibition
What is the push-pull mechanism?
- The brain can detect the direction of head movement by comparing the input between the two vestibular systems (left and right)
- Inputs should be equal and opposite
What is tonic firing rate?
- Occurs when the head is stationary
- Labyrinth is constantly firing action potentials to the vestibular nuclei
- We know that our head is still and balanced
What are the otolith organs?
- Utricle (horizontal movement and head tilt) and saccule (vertical movement)
- Detect linear acceleration
- Contain maculae
What structures are inside the utricle and saccule?
- Maculae inside that contain otoconia, which are calcium carbonate crystals
- Hair cells (stereocilia and kinocilia) sit on the medial wall of the saccule and the floor of the utricle
How are utricles and saccules excited?
- Head tilt or acceleration causes a pull on the otoconia (crystals)
- The pull of crystals causes hair cells to move toward each other causing excitation
What is the vestibular ocular reflex?
- Maintains stability of an image on the fovea of the retina during rapid head movements
- Eyes move opposite to the head
- Peripheral vestibular system connects to central vestibular pathways
Describe what occurs in the VOR pathway when you shake your head no
- Primary vestibular neurons from the horizontal semicircular canal will synapse in the ipsilateral medial vestibular nuclei
- Secondary ipsilateral vestibular neurons from the labyrinth travel in the medial longitudinal fasciculus and decussate to the contralateral abducens nucleus
- The abducens nucleus sends motor neurons to two places : contralateral lateral rectus muscle, cross again to ipsilateral oculomotor nucleus
- The ipsilateral oculomotor nucleus sends neurons to the ipsilateral medial rectus muscle
What direction does each eye move when you turn your head to the right?
- Both eyes move left
- The right eye adducts via cranial nerve 3 and the medial rectus
- The left eye abducts via cranial nerve 6 and the lateral rectus
What are the extraocular eye muscles, their innervations, and their functions?
- Medial rectus (CNIII): adducts eye
- Superior rectus (CNIII): looks up
- Inferior rectus (CNIII): look down
- Inferior oblique (CNIII): looks up and out
- Superior oblique (CNIV): looks down and in
- Lateral rectus (CNVI): abducts eye
What is VOR gain?
- The relationship of eye velocity to head velocity
- Gaze stability with smooth pursuits using eye muscle movements with slower head movements (60 degrees per second) (VOR not solely responsible)
- The VOR is solely responsible when the head is turned faster than 60 degrees per second
What is VOR phase?
- The relationship of amplitude between the eyes and the head
- If the head moves 10 degrees to the right the eyes should also move 10 degrees to the right
True or False
Hair cells of the semicircular canals and otoliths transmit neural discharge to the vestibular nuclei through the vestibular nerve (CN VIII)
True
Which nerve receives input from the utricle, anterior semicircular canal, and horizontal semicircular canal?
Superior vestibular nerve
Which nerve receives input from the saccule and posterior semicircular canal?
Inferior vestibular nerve
What are the four main vestibular nuclei found in the pons/rostral medulla?
- Superior vestibular nuclei: relay for VOR
- Medial vestibular nuclei: largest of the four, relay for VOR
- Lateral vestibular nuclei: lateral vestibulospinal reflex
- Inferior vestibular nuclei: connected to other nuclei and cerebellum
Vestibular nuclei make connections between the vestibular system and other structures such as….
- Other cranial nerves via the medial longitudinal fasciculus
- Flocculonodular node in the cerebellum
- Vestibulospinal tracts
- Cerebral cortex
- Reticular formation for autonomic control and arousal
How does the medial longitudinal fasciculus contribute to the vestibular system?
- Connects the medial and superior vestibular nuclei to the ocular nuclei (cranial nerves 3, 4, and 6) for eye muscle control
- Heavily myelinated tracts that run near midline bilaterally
- Part of VOR to adjust eyes in response to head movements
How does the cerebellum develop reciprocal connections in the vestibular system?
- Information is sent to the vestibulocerebellum (flocculonodular lobes, cerebellar vermis)
- Adjusts eyes in response to head movement (VOR)
- Contributes to postural control
- Influences coordination of limb movements and magnitude of motor response based on vestibular stimuli
How does the medial vestibulospinal tract contribute to the vestibular system?
- Signals from the medial and inferior vestibular nucleus descend bilaterally in the medial motor system to terminate in the cervical and upper thoracic cord
- Controls position of head and neck
How does the lateral vestibulospinal tract contribute to the vestibular system?
- Signals from the lateral vestibular nucleus descend in the medial motor system ipsilaterally down the entire cord, terminating in the anterior horn
- Controls balance
How does the cortex contribute to the vestibular system?
- The ventral posterior nucleus of thalamus projects to the parietal association cortex and the lateral temporal junction
- Controls head and neck position
- Sends input to corticospinal tracts
What is the vascular supply to the peripheral vestibular system?
- Labyrinthine artery, which is a branch of the anterior inferior cerebellar artery
- The labyrinthine artery divides into the common cochlear artery and anterior vestibular artery
What is the vascular supply to the central vestibular system?
- Posterior inferior cerebellar artery: inferior portion of the cerebellar hemispheres and dorsolateral medulla
- Basilar artery: pons
- Anterior inferior cerebellar artery: cerebellum, lower pons
What inner ear pathologies may cause peripheral lesions?
- Vestibular neuritis (UVH)
- Benign paroxysmal positional vertigo (BPPV)
- Meniere’s disease
- Unilateral or bilateral vestibular hypofunction
What pathologies may cause central lesions?
- Head trauma
- Meningitis
- Brainstem or cerebellar stroke
- Multiple sclerosis
What impairments can be associated with the vestibular system?
- Vertigo
- Nystagmus
- Unsteadiness
- Postural instability
- Ataxia
- Nausea
- Vision blurring with head movements
What is vertigo?
- Illusion of movement
- The room feels like its spinning
- Most common cause is a peripheral dysfunction
What is light-headedness?
- Feeling faint
- Can be caused by hypotension, hypoglycemia, or anxiety
What is dysequilibrium?
- Sense of feeling off balance
- May also be due to weakness or somatosensory dysfunction
Describe benign paroxysmal positional vertigo
- Due to otoconia is semicircular canals
- Acute condition
- Vertigo lasts around 2 minutes or less
- Can resolve itself over weeks or months but can be quickly treated with particle repositioning maneuvers
- Symptoms aggravated with to change in head positions
Describe vestibular neuritis
- Due to infection
- Acute condition
- Sever symptoms for 2-3 days with gradual improvement over 2 weeks
Describe Meniere’s disease
- Chronic condition
- Symptoms last 30 minutes - 24 hours
- May have hearing loss with bouts of vertigo
- Tinnitus as wells as a feeling of fullness in the ear
Describe central vestibular disorders
- Caused by damage to the vestibular nuclei or to connections in the brain
- Rarely only see vestibular signs
- Common symptoms: somatosensory and/or motor loss, double vision, Horner’s syndrome, ataxia, dysarthria, verticality
Describe nystagmus for inner ear pathologies
- Delayed
- Fatiguing of nystagmus, becomes less intense
- Horizontal or rotary, not vertical
- Does not change directions
- Only present when vertigo is present
Describe nystagmus for central patholgies
- Immediate or delayed
- Horizontal, rotary, or vertical
- May change directions
- May occur in the absence of vertigo
