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VESTIBULAR APPRATUSIntroductionVestibular apparatus is a complex sense organ of inner earPrimary organ for equlibriumIs required for- the movement of the eye that accompany the movement of the head detection of various movements and positions of the headInternal ear consists of-

Bony labyrinth- bony structure of inner ear

Membranous labyrinth Cochlea and vestibular apparatus are situated here

Perilymph- Fluid between bony and membranous labyrinth

Endolymph- Fluid within membranous labyrinths

Functional anatomy Lie in temporal bone on each side of the head

It consists of- three semicircular canal utricle and saccule (the otolith organ)

Semicircular canalAre three in numbers- horizontal (lateral) superior (anterior) inferior (posterior)

All three canals are at right angles to each otherAmpulla- each canal begins as a dilatation called ampulla

All three canals open into the utricle

Utricle opens into the saccule through a duct called Ductus endolymphaticus

Saccule communicates with cochleaCrista ampullaris and CupulaCrista is the receptor present in semicircular canalIn the ampulla there is a ridgeOn ridge are hair cells which give rise to hairsCupula- hairs and gelatinous substance

Crista- cupula and hair cells

MaculaMacula is the receptor present in utricle and sacculeIn utricle and saccule ridge like structure hair cells and hair is presentOtolith membrane- hairs and gelatinous material

Macula- Otolith membrane and hair cells

Hair cells Kinocilium- present at one end of hair cell large non motile hair

Stereocilia- present in the remaning hair cells are small with progressive increase in height

Hair cells are of two types- Type I flask shaped Type II-most common type

vEStibular pathwayNerve fibers from cristae and maculae Form vestibular division of 8th nerve Vestibular ganglion(Scarpa`s ganglion) Central axon proceeds towards the brain stem Enters brain stem and end in vestibular nucleusEfferent fibers from vestibular nucleus terminates in-

1.In the 3rd,4th,6th cranial nerve nuclei via median longitudinal fasciculus

2.Anterior horn of spinal cord via lateral vestibulospinal tract 3. In the cerebellum

4.Nuclei of reticular formation of the brain stem

5.Temporal lobe of opposite side via the opposite thalamus

6.Some fibers go back to the hair cells mode of action of Semicircular canalSemicircular canal detects angular acceleration during rotation of head along three perpendicular axes

Stimulus to each ampulla is rotation of the head

Posterior canal of one side and Anterior canal of other side form a pair Two horizontal canal of both sides make a pair

Semicircular canal signals changes in acceleration but are insensitive to constant rotatory movement

mode of action of utricle and sacculeSaccule and utricle gives information about linear acceleration and change in head position relative to the gravity

Saccule Vertical acceleration

Utricle Horizontal acceleration

Maculae of the saccule and utricle are stretch receptors

Stimulus for maculae is pull of the gravity on the otolith membrane

The hairs are deformed resulting in stimulation of the nerve fibres

Saccule are affected by lateral tilt of the head

Utricle are affected by nodding the head up and down

Movement of stereocilia towards the kinocilium increases action potential frequency

Role of vestibular apparatus in regulation of postureIt is a sense organ of balance

-Balance is regained by righting reflex

Vestibulo-ocular reflex

- When the head is moving towards a given direction, the eyes move to the opposite sideOtolith reflex-

prevent injuries when a person walks downstairs or jumps from a platfromFunctions Enable the erect position of the head and the normal posture of the body is maintained

Semicircular canal gives information about movement of the head

Otolith organ gives information about position of the head

Vestibular dysfunction 1. MOTION SICKNESS-

is a syndrome consisting of nausea, vomiting, headache, vertigo occurs during travelling due to overstimulation of vestibular apparatus 2. SEA SICKNESS type of motion sickness occuring during travelling by sea

results from irregular and repetitive motion of the ship 3. MENIERE`S DISEASE

Disorder involving vestibular apparatus

Episodes of dizziness (vertigo),tinnitus and hearing loss Due to distension of endolymphatic system

Drug treatment Bed rest Vestibular sedative Vasodilator like histamine

Surgical treatment Decompression of endolymphatic sac Sacculotomy Section of vestibular nerve

4. DRUG INDUCED DAMAGE -By streptomycin when used in heavy dose for treatment of tuberculosis

NYSTAGMUS Involuntary jerky oscillation of eyes Railwayman`s nystagmus- seen in normal person Cause of pathological nystagmus -Disease of cerebellum - Vestibular dysfunction - Weakness of extra-ocular muscles of eyes ( Myasthenia gravis)LABRYRINTHECTOMY

-Removal of labyrinth

a) Unilateral labrinthectomy - There is derangement of postural activity -Nystagmus b) Bilateral labyrinthectomy

- Individual behaves normally ,if allowed to use the vision

- Muscle tone is decreasedTest for vestibular functionRomberg`s sign -Test for loss of position sense

Barany`s Caloric test -Diagnostic purpose -Semicircular canal are stimulated -Water is at 30 C or 44 C - Causes nystagmus ,vertigo and nausea

Fistula test

- To induce nystagmus by producing pressure changes in the external canal which are then transmitted to the labyrinth -Normally the test is negative