VESTIBULAR APPRATUS

INTRODUCTION Vestibular apparatus is a complex sense

organ of inner ear Primary organ for equlibrium Is required for- ▪the movement of the eye that

accompany the movement of the head ▪detection of various movements

and positions of the head

Internal 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 CANAL Are three in numbers- horizontal (lateral) superior (anterior) inferior (posterior)

●All three canals are at right angles to each other

Ampulla- 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 cochlea

CRISTA AMPULLARIS AND CUPULA Crista is the receptor present in

semicircular canal In the ampulla there is a ridge On ridge are hair cells which give rise to

hairs Cupula- hairs and gelatinous substance

Crista- cupula and hair cells

MACULA Macula is the receptor present in utricle

and saccule In utricle and saccule ridge like structure

hair cells and hair is present Otolith 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

nucleus

Efferent 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 CANAL Semicircular 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 SACCULE Saccule 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 POSTURE It 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 side

Otolith reflex-

prevent injuries when a person walks downstairs or jumps from a platfrom

FUNCTIONS

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 decreased

TEST FOR VESTIBULAR FUNCTION Romberg`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