15 - Vestibular System Flashcards
Endolymph Composition, Location
High K+ (+80 mV)
In membranous vestibular system
Perilymph Composition, Location
High Na+, like CSF
Bathes membraneous vestibular system in bony labrynth of inner ear (scala vestibuli, scala tympani)
Vestibular System output
- Eye position 2. Axial posture
Components of Vestibular System
- Peripheral apparatus (static & kinetic labryinths)
- Vestibular (Scarpa’s) ganglion and nerve (CN 8)
- Vestibular Nuclei Complex in brainstem
- Outputs to cerebellum, SC, and brainstem motor neurons AND cortex
Vestibular Hair Cell stereocilia organization
Arranged by increasing length with a single tall kinocilium
Vestibular Hair Cell depolarization
When stereocilia are deflected toward the kinocilium
Vestibular Hair Cell hyperpolarization
When stereocilia are deflected away from the kinocilium
Peripheral Apparatus of Vestibular System
Sensory hair cells are found in 5 regions of the membranous labyrinth
- Utricle (linear acceleration)
- Static labrynth/otolith organs
- Saccule (linear acceleration)
- Static labrynth/otolith organs
- 3 ducts of semicurcular canals
- Kinetic labyrinth
Static Labyrinth
- Utircle
- Saccule
- Detect linear accelerations of the head
- AKA otolith organs
- Major component is otolithic maculae
Ampulla Anatomy
- Raised, dilated region at base of semicircular canal
- Contains crista: a patch of hair cells
- Cilia from the crista project into cupula, a gelatinous matrix that spans the ampulla, interrupting endolymphatic space
Vestibular (Scarpa’s) Ganglion
- Peripheral processes of the bipolar cells
- Located in internal acoustic meatus
- Central process forms the vestibular component of cranial nerve VIII
- Efferent component of vestibular nerve (small number of axons from the vicinity of the lateral vestibular nucleus) innervate the hair cells.
4 Main Vestibular Nuclei and their Vasculature
- Lateral vestibular nucleus: AICA
- Medial vestibular nucleus: PICA
- Superior vestibular nucleus: AICA
- Inferior vestibular nucleus: PICA
All 4 cannot be seen in the same cross section; they are present for significant rostrocaudal distance from rostral medulla to middle of pons
Output of Vestibular Nuceli
Main:
- Spinal cord (medial and lateral vestibulospinal tracts)
- LVN, MVN
- Oculomotor system (medial longitudinal fasiculus)
- MVN, SVN
- Cerebellum
Minor:
- Travel via thalamus to parietal cortex, where balance reaches conscious perception
- MVN
3 Vestibular Reflexes
- Vestibulo-spinal reflexes (VSR)
- Act to stabilize the limbs and body when perturbed
- Vestibulo-collic reflexes (VCR)
- Act on neck musculature to stabilize the head
- Vestibulo-ocular reflexes (VOR)
- Act to maintain stable vision during head motion
Lateral Vestibular Nucleus
- Main source of information going into cerebellum
- NOTE: the cerebellum also just gets into from the vestibular nerve itself!
- Generates lateral vestibulo-spinal tract which controls the vestibulo-spinal reflex (VSR)
Inferior Vestibular Nucleus
Only communicates with other side
Medial Vestibular Nucleus
- Generates the medial vestibulo-spinal tract which drives the vestibulo-collic reflex (VCR)
- Project up to extraocular motor nuclei, axons get to targets via medial longitudinal fasciculus
- Projects to the ventral posterior intermediate nucleus of the thalamus
- Feeling of being dizzy/out of balance
Medial Longitudinal Fasciculus
- Midline fiber bundle that connects CN 3, 4, and 6 with themselves AND with the vestibular nuclei inferiorly
- Important tract that “yolks” together nuclei that drive eyeball movement
- Important for vestibulo-ocular reflex (VOR)
- Vasculature = basilar paramedians
Superior Vestibular Nucleus
Project up to extraocular motor nuclei, axons get to targets via medial longitudinal fasciculus
Vestibulo-ocular Reflex (VOR)
- Serves to maintain a stable image on the retina during rapid head rotation when the eyes are fixed on a target
- VOR constitutes the direct control system
- VOR is a 3-neuron reflex arc:
- Primary afferent neuron
- Vestibular nuclear neuron
- Oculomotor (3, 4, and 6) motoneuron
Medial Vestibulospinal Tract
- Used in VCR
- Descending pathway
- BEGINS IN THE MEDULLA
- Supplied by ASA
- Arises mostly from the medial vestibular nucleus on both sides
- Each tract contains both crossed and uncrossed fibers (decussate immediately)
- Fibers descend bilaterally in the midline, supplying the medial (axial) lower motor neurons in the ventral horn of the cervical and upper thoracic spinal cord
- Main function: adjust position of the head in response to changes in posture (VCR)
Lateral Vestibulospinal Tract
- VSR
- Vasculature: AICA > PICA > ASA
- Arises mostly from the lateral vestibular nucleus on the same side (this is an uncrossed tract)
- Travel from LVN, descend through IVN to the end, then “pop up” ventrally at motor decussation in pyramids, then through ventral funiculus of SC.
- Descends the full length of the spinal cord, innervating lower motor neurons in both medial (majority) and lateral (small number) groups in the ventral horn
- Activation of this pathway facilitates axial extensor muscles and inhibits axial and appendicular flexors
- Primary function is to maintain upright and balanced posture in response to inputs from the static and kinetic labyrinths (VSR)
Clinical Signs Associated with Damage to Vestibular System
- Vertigo: a sensation of motion when one is stationary, a type of “dizziness”
- Ataxia: truncal ataxia (body position is difficult to maintain)
- Nystagmus: rapid, oscillating conjugate eye movements, generally horizontal
- Nausea & vomiting: other automatic signs may also include sweating, excessive salivation, etc.
Benign paroxysmal positional vertigo (BPPV)
- BPPV
- Most common cause of vertigo
- Caused by otoconia (ear crystal) displaced from the otolithic membrane in the utricle, drifting into the semicircular ducts (the posterior duct is most commonly affected)
- Benign: not life-threatening
- Paroxysmal: comes in sudden, brief spells
- Positional: triggered by certain head positions or movements specific to directional sensitivity of the effected canal
- Vertigo: false sense of rotational movement
- Two forms:
- Canalithiasis (most common)
- Otoconia are moving within semicircular duct
- Vertigo, nystagmus resolve within 1 min
- Cupulothiasis
- Occurs when otoconia adhere to cupula
- Vertigo, nystagmus persist for longer periods
- Canalithiasis (most common)
Canalithiasis
- Most common type of BBPV
- Occurs when otoconia are moving within the semicircular duct
- Vertigo and nystagmus resolve within 1 min
Cupulolithiasis
- Less common form of BPPV
- Occurs when otoconia adhere to the cupula
- Vertigo and nystagmus persist for longer periods
Kinetic Labyrinth
- Semicircular ducts
A. Utricle
B. Saccule
C. Otolith organs
D. Vestibule
E. Semicurcular ducts
Linear Accesleration of Head centers
Utricle, saccule (otolith organs, static labyrinth)
Angular acceleration of head center
3 ducts of semicircular canal (kinetic labyrinth)
Gravito-interal acceleration
- Otolithic (utricle, saccule) maculae (patches of hair cells) detect static head position relative to gravity
- Mass of otoliths above the hair cell-containing gelatinous layer shears the stereocilia.
- Detects linear movement of the head
Otoliths
- Calcium carbonate crystals
- Mass of otoliths above the hair cell-containing gelatinous layer shears the stereocilia forthe deteciton of linear accerelation
What is this?
Kinetic Labyrinth
A. Ampulla
B. Utricle
C. Vestibular nerve fibers
D. Semicircular canal
E. Endolymph
F. Hair cells of crista
G. Cupula
Cranial Nerve 3 Function
Medial Rectus
Cranial Nerve 6 Function
Lateral Rectus
Cristae
Patch of hair cells; considered a sensory receptor
