Vestibular System

[OVERVIEW] of Vestibular System:

• Function

• Sensors

• innervation

• Efferent?

Overview

• Function:

  • Detects position + motion of the head

• Sensors:

  • Saccule + Utricle (Otolith Organs)

    • Detect linear acceleration and head position relative to gravity

  • Semicircular Canals:

    • Detect angular acceleration

• Innervation:

  • by neurons in vestibular (Scarpa's) ganqlion

  • Central Processes → CNVIII (vestibular component)

• Efferent from vestibular complex:

  • Oculomotor Control System

  • Cerebellum

  • Spinal cord

Describe the anatomy of the vestibular system

• Location

• Blood Supply

• Components of

  • Outer bony labyrinth

  • Inner membranous Labyrinth

• Location: petrous portion of temporal bone

• Blood Supply:

  • Labyrinthine Artery (from basilar or AICA)

    • divides into cochlear and Vestibular Branches

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Outer bony labyrinth:

• vestibule

  • oval window

• semicircular canals

  • ampullae

• cochlea

  • Inner membranous

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Inner Membranous labyrinth

• utricle & saccule

• semicircular ducts

• cochlear duct

• endolymphatic sac and duct

• utriculosaccular duct

• ductus reuniens

Describe the degrees of the various planes

• Plane of horizontal (lateral) canal and utricle

  • 30 degrees from nasooccipital plane

• planes of anterior canal and posterior canal

  • 90 degrees apart

Describe the Hair Cells

• Stereocillia vs Kinocillum

• Supporting Cell

• NTs

• Efferent Nerves

  • Origin

  • Function

Hair Cells:

• Stereocillia vs Kinocilium

  • Stereocilia: 60-100 hairs project from each cell

    • progressively increasing in length

  • Longest cilium = Kinocilium

    • arises from Centriole

    • Function: Strain gauges; tranduces mech. stimulis

• Supporting Cell: Microvilli

• NTs:

  • Glutamate/Aspartate: Excitatory

• Efferent Nerves:

  • Origin: Reticular Formation

  • Function:

    • Releases AcH and CGRP → modulate activity of hair cell or vestibular afferent.

Describe the mechanism for transduction of these hair cells

Transduction:

• Sterocilli bends towards kinocilium → Depolarization:

  • Stereocilia contain K+ channels that open via tip links

    • K+ rushes in from endolymph

• Stereocilia bend away from kinocilium → hyperpolarizes

Describe the Static labyrinth

• Composition

• Describe the Otolithic Membrane:

  • What is it?

  • Function

• Describe the Orientation of Macula/Function

  • Orientation?

  • Location?

  • Relation to Striola

  • Function

Static Labyrinth

• Composition:

  • Utricle and Saccule

  • Each has Macula: small patch of hair cells

• Otolithic Membrane:

  • What is it?

    • stereocilia/kinocilia of macula = embedded in membrane containing small calcium carbonate crystals

  • Function:

    • creates pull on stereocilia in the direction of gravity

• Orientation of Macula/Function:

  • Utricle Macula: horizontally oriented

    • Parallel w/ base of the skull

    • kinocilia faces striola

  • Saccule Macula: vertically oriented

    • located on the medial wall

    • kinocilia face away from striola

  • Function:

    • detect static head position relative to gravity and linear acceleration

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NOTE: Columnar supporting cells of the maculae are continuous with the cuboidal epithelium lining the utricle and saccule

Describe how the static labyrinth provide Information about head position and linear acceleration

Describe the Kinetic Labyrinth

• Content

• Function

Kinetic Labyrinth

• Content:

  • Semicircular canals

  • Crista:

    • saddle-shaped ridge containing hair cells (3 pathches)

    • Found in Ampulla of each canal

• Function:

  • Movement of endolymph relative to cristae → detection of angular acceleration/deceleration in all 3 planes

Describe the Mechanism in which the Kinetic labyrinth encodes angular acceleration

• Cupula

  • What is it? Direction of movement in relation to head

• Cillia

  • Horizontal vs A/P Canal:

    • Kinocilium location

    • Depolarization event?

• Movement/Endolymph

Mech:

• Cupula

  • gelatinous accessory structure attached to epithelial lining.

  • Hair cells embedded here

  • Cupula pushed in direction opposite head movement → deflection of cilia.

• Cillia;

  • oriented to respond to movement of endolymph in relations to Utricle

    • Horizontal Canal:

      • kinocilium on side of utricle

      • Depolarization: Endolymph flow towards utricle

    • A/P Canals:

      • kinocilium on side away from utricle

      • Depolarization: Endolymph flow away from utricle

    • Both hyperpolarized when endolymph flows in the opposite direction mentioned above

• Movement and Endolymph:

  • NOTE: endolymph moves in oppposite direction of head due to inertia

  • NOTE: for A/P canals; towards utricle of one canal = away from the other canal;

    • EX: Looking up = Lymph moves Inferior/Anteriorly → Endolymph flows towards the Anterior canal; Endolymph flows away from Posterior canal → Excitation of Posterior/ Inhibition of Anteriorly

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NOTE:

• anterior and posterior canals are sensitive in the same fashion to roll and pitch movements.

Describe the Vestibular Nerve

• Aka?

• Location?

• Nt?

Ganglia:

• vestibular Ganglion = Scarpa’s

• Location:

  • lateral end of the internal acoustic meatus

• NT: Glutamate

Describe the Vestibular Nuclear Complex:

• Location?

• 4 main nucleus?

• Fate of Ascending vs Descending vestibular Nerve Fibers as it enters?

  • What about Kinetic vs Static Fibers

• Describe the function of commissural connections

Vestibular Nuclear Complex

• Location:

  • dorsally in pons and medulla

  • btw lateral part of 4th ventricle + inferior cerebellar peduncle

• 4 Main Nucleus

  • Lateral (Dieter) vestibular nucleus

  • Medial (Schwalbe) vestibular nucleus

  • Superior (Bechterew) vestibular nucleus

  • Inferior (descending or spinal) vestibular nucleus

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• Ascending vs Descending Fibers of vestibular nerve:

  • upon entering vestibular complex:

    • Ascending: → Superior Nucleus

      • NOTE: some goes to flocculonodular lobe of cerebellum (vestibulocerebellum)

    • Descending: → Medial + Inferior Nucleus

    • NOTE: Lateral nucleus: limited input from kinetic but some from static labvrinth

• Kinetic vs Static Labyrinth Fibers:

  • Kinetic: → superior + medial nuclei

  • Static: → medial + inferior

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commissural connections (vestibulovestibular)

• Function:

  • comparison of information between sides

  • important for vestibular compensation

    • Definition: recovery of postural reflexes after unilateral vestibular receptor loss due to trauma or disease

What does the Vestibular Nuclei receive INPUT from:

• Cerebellum

• SC

• Oculomotor System

Describe the Nuclei Output:

• Main

• Minor

INPUT:

• Cerebellum

  • Flocculonodular lobe: → superior, medial, inferior

  • Lateral vermis: → lateral

  • Fastigial (deep cerebellar) nucleus: → lateral and inferior

• Spinal cord

  • from spinovestibular tract

    • Proprioceptive information

    • processed information from reticular formation

• Oculomotor system

  • accessory optic system

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Output:

• Main: → cerebellum (ie., vestibulocerebellum), spinal cord, and oculomotor system

• Minor: parietal and insular cortex via thalamus

Describe the Output Systems of the Vestibular Nuclei

• Function?

• Compare and Contrast Medial/Lateral Vestibulospinal Tracts

  • Origins/Route

  • Travels

  • Innervation

  • Related to

  • Function

• Describe the Vestibulo-ocular System

  • Function

Functional Output System:

• Function:

  • Connections w/ cerebellum →

    • fine control over postural adjustments

    • eye movernents

  • Via vestibulospinal + vestibulo-ocular systems

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Medial/Lateral Vestibulospinal Tract

• Medial:

  • Origins:

    • Medially(mostly) + Inferior Nucleus

    • Bilaterally

  • Travels:

    • in MLF (descending component) → Cervical Spine

  • Innervation:

    • medial group (axial) of motorneurons in ventral horn

  • Related to:

    • flocculonodular lobe

  • Function:

    • integration of head and eye movements associated with changes in body position

• Lateral:

  • Origin:

    • Lateral Nucleus

    • uncrossed

  • Travels

    • Full length of SC

  • Innervation:

    • motorneurons medial(majority) and lateral groups in the ventral horn

  • Related to:

    • vermis of the cerebellum

  • Function:

    • Facilitates (axial extensor musculature)

      • maintain upright posture and balance

    • Adjusts trunk position + orientation in response from static + dynamic labyrinths

      • (balance)

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Vestibulo-ocular System

Function:

• Provides for maintenance of fixed gaze with on-going head and body movements

  • Semicircular canal stimulation → conjugate eye movernents in the plane as the canal

    • EX: (e.g., horizontal canal — horizontal conjugate eye movements)

Describe the Vestibulo-ocular reflex

• Components of Reflex Arc

• Pathway

  • X→X

  • Ipsi vs Contra

  • Most projections from?

• Function/Example

Vestibulo-ocular reflex

• 3 neuron reflex arc:

  • primary afferent neuron

  • vestibular nuclear neuron

  • oculomotor motorneuron (Ill, IV, and VI)

• Pathway:

  • vestibular nuclear neuron → Oculomotor Nuclei via MLF

    • Ipsilaterally: Inhibits

    • Contralterally: Excites

  • Most projections from Superior + medial nucleus

    • Lateral → Oculomotor via ascending tract of Dieters

• Function:

  • Co-operate functionally and link pairs of eye muscles

  • Example:

    • horizontal semicircular canal input →

      • excites contra (LR) + Ipsilateral medial rectus (MR);

      • inhibits ipsi LR + contra MR

Draw out the Vestibulo-ocular reflex

Describe the Thalamocortical System

• Function

• Pathway

Thalamocortical System

• Function:

  • Allows conscious perception of motion and spatial orientation

    • Via Combination of info from vestibular, visual and somatosensory systems

• Pathway

  • medial, superior, and inferior nuclei → VP + intralaminar nuclei of the thalamus (BiLat) →

    • → somatosensory cortex (BA 3a, 2v)

    • → somatosensory association cortex (BA 7)

    • → parieto-insular vestibuar cortex

      • (posterior insula extending into parietal operculum).

Describe Caloric testing

• Function

• Methodology

• Result

• Describe Nystagmus

  • Classifeid based on?

  • what is COWS

• Clinical

  • What happens in comatose patients

  • What is the doll’s eye Phenomenon

Caloric Testing

• Function: evaluate vestibular pathway

• Methodology:

  • Lateral Semicircular canal= in vertical plane

    • convection currents induced by irrigating external acoustic meatus w/ warm or cold water

• Result:

  • Warm:

    • conjugate gaze → opposite side (slow phase)

      • Then → eyes rapidly reposition to central position (fast phase)

        • This is called Nystagmus

  • Cold: Vice Versa

• Nystagmus:

  • Classified based on direction of the fast return phase

  • COWS

    • Cold water → Nystagmus to Opposite side

    • Warm water → Nystagmus to Same side

• Clinical Applications:

  • Comotose Patients:

    • fast phase (driven by the cortex) is absent

  • Doll's Eye Phenomenon:

    • When you turn your head, your eyes should go in opposite direction

    • If negative:

      • brainstem = damaged; eyes don’t move

Draw out the the fast/slow phase

What are the clinical signs that the vestibular system is damaged

Clinical manifestations of damage:

• V— vertigo — sensation or hallucination of rotation

• A — Ataxia —truncal ataxia where body position is difficult to maintain

• N— Nystagmus

• N— Nausea and Vomiting

  • (other autonomic signs may include palor and sweating)

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NOTE: Signs associated with pathology of both peripheral and central connections.