Anatomy and Physiology of Vestibuler System

7/30/2019 Anatomy and Physiology of Vestibuler System

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Dr. Dini Sri Damayanti,MKes

VESTIBULER SYSTEM

r. Dini Sriamayanti,MKes


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The vestibular system is the system of balance. It isalso involved in the function of maintaining visualfixation during head movement and in maintainingposture and lower muscular controlThe vestibular system is made of five sensory organson each side of the head embedded in the petrous

portion of the temporal bone.There are the superior, posterior, and lateralsemicircular canals as wells as the utricle and saccule.


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Embryology

The vestibular apparatus begins development aroundthe third week of life.

On the lateral portion of the Rhombencephalon thesurface ectoderm thickens to form the otic placode.This deepens to become the otic pit and by the fourth week encloses to become the otic vesicle or otocyst.


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The otic vesicle elongates and differentiates to form adorsal utricular portion and a ventral saccular portion.The utricular portion becomes the semicircular canalsand the utricle. The superior semicircular canal formsfirst, followed by the posterior and lateral canals.The saccular portion becomes the saccule and thecochlear duct.


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The communication between the saccule and

membranous cochlea narrows to form the ductusreuniens.Also around the third week the sensory epitheliadevelop from ectoderm in the cristae of the formingsemicircular canals and in the maculae of the formingotolithic organs. About week 6 the central vestibular nuclei are formedand project to the endorgans. Hair cells are mature by the 9 th week.


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Anatomy

There are three semicircular canals, superior, posterior,and lateral (or horizontal).

The semicircular canals are orthogonal to each otherthat is, at right angles.There is a dilated end of each semicircular canal calledthe ampulla where the sensory organ is located.

The lateral canal is inclined from the horizontal planeabout 30 degrees. The superior and posterior canalsare at roughly 45 degree angles to the sagittal plane.


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Each semicircular canal is maximally sensitive tomotion in the plane of that canal.The canals also act in functional pairs with both canalslying in the same plane. Rotation in that plane will beexcitatory to one canal and inhibitory to the other.

The left and right lateral canals are one pair. Thesuperior canal on one side is paired with the posteriorcanal on the opposite side.


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The otolithic organs are the utricle and saccule whichare sensitive to linear acceleration.The saccule lies in the spherical recess on the medial wall of the vestibule. It is oriented in the verticalplane.

The utricle lies in the elliptical recess on the medial wall of the vestibule and is oriented in the horizontalplane.There are five openings into the area of the utricle

from the semicircular canals. The superior andposterior canals share a common crus.


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The membranous labyrinth is enclosed in a

bony labyrinth within the petrous portion of the temporal bone.The membranous labyrinth is surrounded

by perilymph and contains endolymph.Perilymph may be formed from anutrafiltrate of blood or cerebrospinal fluid.

It is similar in ionic concentration toextracellular fluid with low potassium andhigh sodium.


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The endolymph within the membranouslabyrinth is produced by the marginal cells

of the stria vascularis in the cochlea and by cells called dark cells in the maculae of theutricle and saccule.Endolymph is absorbed in theendolymphatic sac. It has ionicconcentrations similar to intracellular fluid with high potassium and low sodium.


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The vestibule is situated between the internalauditory meatus anteromedially and the middleear laterally.The cochlea sits anterior to the vestibule andconnected via the ductus reunions.

Posteriorly and laterally are the mastoid air cells.Medially is the posterior cranial fossa.The seventh and eight cranial nerves enter the

vestibule and cochlea from the internal auditory meatus.


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The vestibulocochlear nerve splits into the vestibularand the cochlear nerves. The vestibular ganglion hastwo parts. The superior vestibular nerve supplies thesuperior and lateral semicircular canals as well as theutricle.The inferior vestibular nerve supplies the posteriorcanal and the saccule.


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The blood supply to the vestibular organs is throughthe labyrinthine artery (internal auditory artery) which arises from the anterior cerebellar artery(45%),superior cerebellar artery (24%), or basilar artery (16%).There are two branches of the labyrinthine artery.The first branch, the anterior vestibular artery suppliesthe utricle, superior and lateral ampullae and a smallportion of the saccule.


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The second branch, the common cochlear artery, alsosplits in two divisions. The cochlear artery suppliesthe cochlea. The vestibulocochlear artery divides intoa cochlear ramus and the posterior vestibular artery.This artery supplies the posterior ampulla, most of thesaccule, part of the utricle, and the superior and lateralampullae.


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The sensory portion of the semicircular canals is theampulla.It contains the neuroepithelium called the cristaampullaris, the cupula, supporting cells, connectivetissue, blood vessels and nerve fibers.The crista extends perpendicularly across the canal in

a saddle shape. The sensory hair cells and thesupporting cells are modified columnar epithelial cells with microvilli.


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There are two types of hair cells. Type I hair cells areflask shaped and have a chalice shaped nerve endingon them. One calyx nerve ending can synapse with just one or two to four hair cells.Type II hair cells are cylinder shaped and have multipleefferent and afferent bouton nerve synapses. Thesehair cells have 50-100 stereocilia and a singlekinocilium. These stereocilia are not true cilia but aremicrovilli which are graded in height with the tallestnearest to the single kinocilium.


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The kinocilium is located on one end of the cell givingit a morphological polarization.The movement of the hair bundle toward thekinocilium causes an increase in the firing rate of the

hair cell, while deflection away causes a decrease in thefiring rate.In the lateral semicircular canals the kinocilium islocated on the side nearest the utricle while in in the

superior and posterior semicircular canals thekinocilium is away from the utricle.


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The hair bundle extends upwards towards thegelantinous cupula. This structure extends completely across the membranous labyrinth and moves with theflow of endolymph. It has a specific gravity close tothat of the endolymph and is not responsive to gravity.


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The vestibular neurons are bipolar with their cell bodieslocated in Scarpas ganglion in the internal auditory meatus.The superior and inferior vestibular nerves join to form acommon bundle which enters the brainstem.These first order neurons do not cross the midline. These

afferent fibers terminate in the vestibular nuclei in thefloor of the fourth ventricle.The nuclei are the superior vestibular nucleus, the lateral

vestibular nucleus, the medial vestibular nucleus, and thedescending vestibular nucleus.From the vestibular nuclei projections go to thecerebellum, extraocular muscle nuclei, antigravity muscles,and opposite vestibular nuclei.


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Physiology

The functions of the vestibular system are to sense angularacceleration, linear acceleration and to coordinate headand eye movements as well as maintain the antigravity and

lower body muscles in relation to the head.The semicircular canals provide sensation for angularacceleration. The membranous labyrinth moves with headmotion while the endolymph does not, causing a relativeflow of endolymph and deflection of the cupula.

Each hair cell has a resting potential and its associatedneuron has a spontaneous discharge. Movement will causean increase in the discharge rate on one side and a decreasefrom the paired canal on the opposite side.


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The otolith organs are sensitive to linear acceleration.Movement of the otoconia mass deflects the hair cellsattached to it. The maculae have hair cells oriented inmany directions so linear acceleration in any directionmay be sensed. The saccule is most sensitive to gravity since it is in the vertical plane.


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Vestibulo-ocular reflex

Membranous labyrinth moves with headmotionEndolymph does not causing relativemotionCupula on right canal deflected towardsutricle causing increase in firing rate, leftdeflects away causing a decrease in firing

rate.Reflex causes movement of eyes to the left with saccades to rightStabilizes visual image


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

Cupula only takes 8-10 seconds to return toequilibrium position Vestibular integrator is the term for the prolongationand is mediated by the vestibular nuclei and

cerebellum


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The VOR


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Vestibulospinal ReflexSenses head movement and head relative to gravity Projects to antigravity muscles via 3 major pathways:

Lateral vestibulospinal tractMedial vestibulospinal tractReticulospinal tract


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Selection based on Availability Accuracy Value for the task at hand

Processing of inputs from the periphery

SENSORY ORGANIZATION

Compare, Select& Combine Senses

Determination of Body Position

Visual Vestibular

Somatosensory


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C A L O R I C T E S T I N G

Thermal convectivetheory:

Heating or cooling theexternal ear canal causesconvection current in theendolymph andsubsequent movement ofthe cupula.


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With warm water irrigationcolumn of endolymph becomesless dense, rises and causesdeflection of cupula toward theutricleResults in increase firing rateand nystagmus which beatstowards the stimulationCOWS (cold opposite, warmsame)


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Anatomy and Physiology of Vestibuler System