Vestibular Function Vestibular Function and Anatomyand Anatomy

UTMB Grand Rounds UTMB Grand Rounds

April 14, 2004April 14, 2004

Gordon Shields, MDGordon Shields, MD

Arun Gadre, MDArun Gadre, MD

System of balanceSystem of balance Membranous and bony labyrinth Membranous and bony labyrinth

embedded in petrous boneembedded in petrous bone 5 distinct end organs5 distinct end organs

– 3 semicircular canals: superior, lateral, 3 semicircular canals: superior, lateral, posteriorposterior

– 2 otolith organs: utricle and saccule2 otolith organs: utricle and saccule

Semicircular canals sense angular Semicircular canals sense angular accelerationacceleration

Otolithic organs (utricle and saccule) Otolithic organs (utricle and saccule) sense linear accelerationsense linear acceleration

EmbryologyEmbryology

3rd week of 3rd week of embryonic embryonic developmentdevelopment

Otic placode Otic placode formed from formed from neuroectoderm neuroectoderm and ectodermand ectoderm

Otocyst or otic Otocyst or otic vesicle 4th weekvesicle 4th week

EmbryologyEmbryology

Endolymphatic duct forms Endolymphatic duct forms Utricular chamber becomes Utricular chamber becomes

utricle/semicircular canalsutricle/semicircular canals Saccular chamber becomes Saccular chamber becomes

saccule/cochleasaccule/cochlea Separation of saccule and cochlea-Separation of saccule and cochlea-

ductus reuniensductus reuniens

EmbryologyEmbryology

Week 3 sensory epithelia develop from Week 3 sensory epithelia develop from ectodermectoderm

3 cristae, 2 maculae 3 cristae, 2 maculae Vestibulocochlear ganglion starts as one Vestibulocochlear ganglion starts as one

then spits into superior and inferior then spits into superior and inferior divisionsdivisions– Superior division: Superior/lateral canals, Superior division: Superior/lateral canals,

utricleutricle– Inferior division: saccule, posterior canal (via Inferior division: saccule, posterior canal (via

singular nerve)singular nerve)

Semicircular canals Semicircular canals are orthogonal to are orthogonal to each othereach other

Lateral canal Lateral canal inclined to 30 inclined to 30 degreesdegrees

Superior/postereor Superior/postereor canals 45 degrees canals 45 degrees off of sagittal planeoff of sagittal plane

Utricle is in Utricle is in horizontal planehorizontal plane

Saccule is in Saccule is in vertical planevertical plane

AnatomyAnatomy

There are five There are five openings into area openings into area of utricleof utricle

Saccule in Saccule in spherical recessspherical recess

Utricle in elliptical Utricle in elliptical recessrecess

45% from AICA45% from AICA 24% superior 24% superior

cerebellar arterycerebellar artery 16% basilar16% basilar Two divisions: Two divisions:

anterior vestibular anterior vestibular and common and common cochlear arterycochlear artery

Superior vestibular Superior vestibular nerve: superior nerve: superior canal, lateral canal, canal, lateral canal, utricleutricle

Inferior vestibular Inferior vestibular nerve: posterior nerve: posterior canal and sacculecanal and saccule

Membranous labyrinth is surrounded Membranous labyrinth is surrounded by perilymphby perilymph

Endolymph fills the vestibular end Endolymph fills the vestibular end organs along with the cochleaorgans along with the cochlea

PerilymphPerilymph– Similar to extracellular fluidSimilar to extracellular fluid– K+=10mEQ, Na+=140mEq/LK+=10mEQ, Na+=140mEq/L– Unclear whether this is ultrafiltrate of Unclear whether this is ultrafiltrate of

CSF or bloodCSF or blood– Drains via venules and middle ear Drains via venules and middle ear

mucosamucosa

EndolymphEndolymph– Similar to intracellular fluidSimilar to intracellular fluid– K+=144mEq/L, Na+=5mEq/LK+=144mEq/L, Na+=5mEq/L– Produced by marginal cells in stria Produced by marginal cells in stria

vascularis from perilymph at the cochlea vascularis from perilymph at the cochlea and from dark cells in the cristae and and from dark cells in the cristae and maculaemaculae

– Absorbed in endolymphatic sac which Absorbed in endolymphatic sac which connected by endolymphatic, utricular connected by endolymphatic, utricular and saccular ductsand saccular ducts

Sensory structuresSensory structures

Ampulla of the semicircular canalsAmpulla of the semicircular canals Dilated end of canalDilated end of canal Contains sensory neuroepithelium, Contains sensory neuroepithelium,

cupula, supporting cellscupula, supporting cells

Cupula is gelatinous Cupula is gelatinous mass extending mass extending across at right angleacross at right angle

Extends completely Extends completely across, not across, not responsive to gravityresponsive to gravity

Crista ampullaris is Crista ampullaris is made up of sensory made up of sensory hair cells and hair cells and supporting cellssupporting cells

Sensory cells are either Sensory cells are either Type I or Type IIType I or Type II

Type I cells are flask Type I cells are flask shaped and have chalice shaped and have chalice shaped calyx ending shaped calyx ending

One chalice may One chalice may synapse with 2-4 Type I synapse with 2-4 Type I cellscells

Type II cells – cylinder Type II cells – cylinder shaped, multiple shaped, multiple efferent and afferent efferent and afferent boutonsboutons

Hair cells have 50-100 stereocilia and a single kinocilium.

stereocilia are not true cilia, they are graded in height with tallest nearest the kinocilium.

Kinocilium is located on one end of Kinocilium is located on one end of cell giving each cell a polaritycell giving each cell a polarity

Has 9+2 arrangement of microtubule Has 9+2 arrangement of microtubule doubletsdoublets

Lacks inner dynein arms, and central Lacks inner dynein arms, and central portion of microtubules not present portion of microtubules not present near ends – may mean they are near ends – may mean they are immobile or weakly mobileimmobile or weakly mobile

Each afferent neuron has a baseline Each afferent neuron has a baseline firing ratefiring rate

Deflection of stereocilia toward Deflection of stereocilia toward kinocilium results in an increase in kinocilium results in an increase in the firing rate of the afferent neuronthe firing rate of the afferent neuron

Deflection away causes a decrease in Deflection away causes a decrease in the firing ratethe firing rate

kinocilia are located closest to utricle kinocilia are located closest to utricle in lateral canals and are on in lateral canals and are on canalicular side in other canalscanalicular side in other canals

Ampullopetal flow (toward the Ampullopetal flow (toward the ampulla) excitatory in lateral canals, ampulla) excitatory in lateral canals, inhibitory in superior/posterior canals inhibitory in superior/posterior canals

Ampullofugal flow (away from the Ampullofugal flow (away from the ampulla) has opposite effectampulla) has opposite effect

Semicircular canals Semicircular canals are pairedare paired– Horizontal canalsHorizontal canals– Right superior/left Right superior/left

posteriorposterior– Left superior/right Left superior/right

posteriorposterior– Allow redundant Allow redundant

reception of movementreception of movement– Explains compensation Explains compensation

after unilateral after unilateral vestibular lossvestibular loss

Otolithic organsOtolithic organs

Utricle and saccule sense linear Utricle and saccule sense linear accelerationacceleration

Cilia from hair cells are embedded in Cilia from hair cells are embedded in gelatinous layer gelatinous layer

Otoliths or otoconia are on upper Otoliths or otoconia are on upper surfacesurface

Calcium carbonate Calcium carbonate or calciteor calcite

0.5-30um0.5-30um Specific gravity of Specific gravity of

otolithic membrane otolithic membrane is 2.71-2.94is 2.71-2.94

Central region of Central region of otolithic membrane otolithic membrane is called the striolais called the striola

Saccule has hair Saccule has hair cells oriented cells oriented awayaway from the striolafrom the striola

Utricle has hair Utricle has hair cells oriented cells oriented towardstowards the striola the striola

Striola is curved so Striola is curved so otolithic organs are otolithic organs are sensitive to linear sensitive to linear motion in multiple motion in multiple trajectoriestrajectories

Central connectionsCentral connections

Scarpa’s ganglion is in the internal Scarpa’s ganglion is in the internal auditory canalauditory canal

Contains bipolar ganglion cells of first Contains bipolar ganglion cells of first order neuronsorder neurons

Superior and inferior divisions form Superior and inferior divisions form common bundle which enters common bundle which enters brainstembrainstem

No primary vestibular afferents cross No primary vestibular afferents cross the midlinethe midline

Afferent fibers terminate in the Afferent fibers terminate in the vestibular nuclei in floor of fourth vestibular nuclei in floor of fourth ventricleventricle– Superior vestibular nucleusSuperior vestibular nucleus– Lateral vestibular nucleusLateral vestibular nucleus– Medial vestibular nucleusMedial vestibular nucleus– Descending vestibular nucleusDescending vestibular nucleus

Vestibular nuclei project toVestibular nuclei project to– CerebellumCerebellum– Extraocular nucleiExtraocular nuclei– Spinal cordSpinal cord– Contralateral vestibular nucleiContralateral vestibular nuclei

Senses and Senses and controls motioncontrols motion

Information is Information is combined with that combined with that from visual system from visual system and proprioceptive and proprioceptive systemsystem

Maintains balance Maintains balance and compensates and compensates for effects of head for effects of head motionmotion

Vestibulo-ocular reflexVestibulo-ocular reflex– Membranous labyrinth Membranous labyrinth

moves with head moves with head motionmotion

– Endolymph does not Endolymph does not causing relative motioncausing relative motion

– Cupula on right canal Cupula on right canal deflected towards deflected towards utricle causing increase utricle causing increase in firing rate, left in firing rate, left deflects away causing a deflects away causing a decrease in firing rate.decrease in firing rate.

– Reflex causes Reflex causes movement of eyes to movement of eyes to the left with saccades the left with saccades to rightto right

– Stabilizes visual imageStabilizes visual image

If acceleration stops, and spin to If acceleration stops, and spin to right is at constant velocity, right is at constant velocity, sensation of motion stops after 14-20 sensation of motion stops after 14-20 seconds as does nystagmusseconds as does nystagmus

Cupula only takes 8-10 seconds to Cupula only takes 8-10 seconds to return to equilibrium positionreturn to equilibrium position

Vestibular integrator is the term for Vestibular integrator is the term for the prolongation and is mediated by the prolongation and is mediated by the vestibular nuclei and cerebellumthe vestibular nuclei and cerebellum

Vestibulospinal ReflexVestibulospinal Reflex

Senses head movement and head Senses head movement and head relative to gravityrelative to gravity

Projects to antigravity muscles via 3 Projects to antigravity muscles via 3 major pathways:major pathways:– Lateral vestibulospinal tractLateral vestibulospinal tract– Medial vestibulospinal tractMedial vestibulospinal tract– Reticulospinal tractReticulospinal tract

How do calorics work?How do calorics work? Patient is lying down with Patient is lying down with

horizontal canals oriented horizontal canals oriented vertically (ampulla up)vertically (ampulla up)

Cold water irrigation Cold water irrigation causes endolymph in causes endolymph in lateral portion to become lateral portion to become dense and fall causing dense and fall causing deflection of cupula away deflection of cupula away from utricle with a from utricle with a decrease in the firing ratedecrease in the firing rate

This causes nystagmus This causes nystagmus with fast phase (beat) with fast phase (beat) away from the stimulusaway from the stimulus

With warm water irrigation With warm water irrigation column of endolymph column of endolymph becomes less dense, rises becomes less dense, rises and causes deflection of and causes deflection of cupula toward the utriclecupula toward the utricle

Results in increase firing Results in increase firing rate and nystagmus which rate and nystagmus which beats towards the beats towards the stimulationstimulation

COWS (cold opposite, COWS (cold opposite, warm same)warm same)

SourcesSourcesShepard NT, Solomon D. Functional Operation of the Balance

System in Daily Activities. Otolaryngologic Clinics of North America 2000;33(3):455-468.

Minor LB. Physiological principles of vestibular function on earth and in space. Otolaryngology-Head and Neck Surgery 1998;118(3 part 2):S5-S15.

Abdel Razek OA. Anatomy of the Vestibular System. www.emedicine.com

Hoffman R, Strunk C. Vestibular Anatomy and Physiology. Department of Otolaryngology Grand Rounds University of Texas Medical Branch December 9, 1992.

Baloh RW. Dizziness, Hearing Loss, and Tinnitus. Philadephia, F.A. Davis Company, 1998.

Jahn AF, Santos-Sacchi J. Physiology of the Ear. Second edition. San Diego, Singular, 2001.

Friedman I, Ballantyne J. Ultrastructural Atlas of the Inner Ear. London, Butterworth & Co., 1984.

Janfaza P, Nadol JB. Temporal Bone and Ear. In: Janfaza P ed. Surgical Anatomy of the Head and Neck. Philadelphia, Lippincott Williams & Wilkins, 2001:419-479.

Wall C, Vrabec JT. Vestibular Function and Anatomy. In: Head & Neck Surgery-Otolaryngolog. Philadelphia, Lippincott Williams & Wilkins, 2001:1641-1650.