stucture of ear

7/30/2019 Stucture of Ear

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Structure of ear:-

The ear has 3 parts

1.External ear2.Middle ear3.Internal ear

External ear consist of 2 parts

a) Pinnab) External auditory meatus

Pinna or auricle consists of a

fibrocartilagenous plate is covered by connective tissue &

skin. The plate is characteristically folded & ridged. The skin

is thin & contains fine hairs & sebaceous gland. On posterior

surface sweat glands are present. Pinna collects & reflects

sound waves into auditory meatus. The depression in the

pinna forming the orifice of auditory meatus is called

concha.

The external auditory meatus is a long

tortuous canal abt 2.54cm(55mm) extending from concha &

it is medially shut off by tympanic membrane. The initial

part is cartilaginous & is covered by thick skin containing


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stiff hairs, sebaceous gland and ceruminous gland. The

glands are coiled & tubular and opens on the surface of the

skin& its columnar epithelial cells contains brown pigment

granules & fat droplets. The secretion of ceruminous gland ,

sebaceous gland & the desquamated epithelial cells form

the earwax or cerumen. The inner part is bony as it adheres

closely to the periosteum. This portion is also covered by

skin which contains sebaceous gland & small fine hairs(on

the superior wall of the canal). The skin covering this

portion is continuous with the cuticular layer of tympanic

membrane.

The external auditory meatus transmits the sound

waves perpendicularly to the membrane. The tortuosity of

the canal prevents any mechanical injury to the tympanic

membrane from outside.

Middle ear or tympanic cavity or tympanum is

small, narrow,irregular & laterally compressed. It is situated

within the temporal bone.

The middle ear contains

1.Air2.Auditory ossicles

a.Malleusb.Incusc.stapes

3.Auditory musclesa.Tensor tympani muscle


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b.Stapedius muscle

The tympanic membrane or eardrum forms the lateral wallof middle ear cavity. It is thin, semitransparent, elliptical in

shape & lies obliquely , downwards,inwards& forwards.

Size 10mm *9mm. it is concave from outside. The point of

maximum concavity is called umbo & to it the handle of the

malleus is attached on the inner side. The periphery of the

membrane is attached to the tympanic sulcus in thesurrounding bony ring by means of the fibrocartilage. The

tympanic membrane is formed by 3 layers

1.Lateral cutaneous layer continuous with the skin ofauditory meatus.

2.Intermediate fibrous layer formed of collagenousfibres

3.Medial mucous layer (tympanic mucosa) composed ofsingle layer of cuboidal epithelial cells.

In the medial wall there are 2 windows

Oval window (fenestra ovalis)to which the foot plate

of the stapes is attached & sealed by means of a

membrane.

Round window (fenestra cochlea or rotunda)- which is

closed by a membrane.

The posterior wall communicates with air cavities in the

mastoid process & the largest air cavity is mastoid antrum.


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In the anterior wall there are 2 canals, the upper one

lodges the tensor tympani & the lower one is called

Eustachian tube(auditory tube) which opens into the

nasopharynx. Its upper part is surrounded by bony wall &

lower part by fibrocartilagenous plate. The Eustachian tube

connects the middle ear with posterior part of nose and

forms the passage of air between middle ear & atmosphere.

Normally it remains closed but opens during chewing ,

yawning, and sneezing. The main function is to equalise the

pressure on both sides of the ear drum. Any imbalance in

the static pressure affects the transmission of middle ear

structure. Eg:- diving, landing an aircraft,going up in an

aeroplane. In these conditions the change in the pressure

produce considerable stress on the tympanic membrane if

the pressure in the middle ear is not equalised with theoutside pressure.

Auditory ossicles are 3 minute bones arranged in the

form of a chain or complex system of levers extending

across the middle ear from tympanic mm to oval window.

From outside inwards they are

Malleus (hammer) attached to TM with a

handle(manubrium), neck & head (capitullum) articulating

with the anterior surface of the body of incus.

Incus (anvil ) looking like premolar tooth with a body, a

short process to which a ligament is attached & a long


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process, to its tip (lenticular process) the head of stapes is

attached.

Stapes(stirrup) is the smallest bone in the body. It has abody, neck, anterior crus , posterior crus & a foot plate

which is attached to the oval window. The base of stapes

produces a rocking rather than the push pull movement.

The 3 bones are bound by ligaments & when the sound

waves impinge on the TM these chains of ossicle vibrates as

a single unit. Thus inner side of tympanic membrane is

linked to the fluid filled cochlea at the oval window. The

mechanism of joint & bearing systems of the single unit of

the bony ossicles are such that displacements at the

tympanic membrane by air will be 3 times higher in the oval

window. Thus it acts like a hydraulic press. The force in the

oval window is spread over an area of 3.2 sqmm but at the

TM this spread is over 65 sqmm. Thus the pressure changes

in the liquid of inner ear just inside the oval window will be

greater than that of outside the TM.

The auditory muscles include

Tensor tympani arising from the cartilagenuos portion of ET,

adjacent to greater wing of sphenoid bone & osseous canal

& its tendon attached to head of malleus. Supplied by the

mandibular division of trigeminal nerve. Tensor tympani

pulls & keeps the TM stretched or tensed constantly which

is essential for the transmission of sound waves reaching


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the TM. So paralysis of this muscle causes hearing

impairment.

Stapedius is the smallest muscle in the body(1mm length).It lies in the conical bony cavity on the posterior wall of the

middle ear cavity. It arises from interior pyramid of

tympanic cavity &its tendon is attached to the posterior

surface of the neck of stapes. Supplied by facial nerve. It

prevents excess movement of stapes. when stapedius

contracts it pulls the neck of stapes backwards & reducesthe movement of footplate against the fluid in cochlea.

Tensor tympani & stapedius muscles modify the

transmission of sounds of different intensities & provide a

protective mechanism.

A loud sound causes reflex contraction of tensor tympani& stapedius muscles after a latent period of 40 to 80

miiliseconds. It is called the tympanic reflex. The tympanic

reflex protects the TM from being ruptured by loud sound.

It also prevents fixation of footplate of stapes, against

oval window during exposure to loud sound. When both the

muscles contract, the manubrium of malleus moves inward& stapes is pulled outward. This leads to stiffness of

auditory ossicles so that the transmission of sound is

decreased. Thus helps to protect the inner ear

(cochlea)from damaging effects of loud sounds.


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Internal ear or labyrinth is a membranous structure,

enclosed by a bony labyrinth in petrous part of temporal

bone. It consist of the

a) sense organ of hearing - cochlea in frontb)sense organ of equilibrium vestibular apparatus,

including the semicircular canals & otolith organ

behind.

Cochlea is a coiled structure resembling the

snails shell. It consist of a bony (osseous) canal or

tube arranged spirally around a central conical bony

(spongy bone) pillar, the modiolus through the axis

of which the auditory nerve passes. It makes about2 turns extending from the base of the cochlea to

the apex. The end is called cupula. The base forms

the bottom of the internal auditory meatus through

which the cochlear nerve fibres enter the modiolus.

From the modiolus a bony ridge called osseous

spiral lamina projects into the canal & winds aroundthe modiolus like the thread of a screw. The spiral

lamina follows the spiral turns of cochlea & ends at

the cupula in a hook shaped process called hamulus.

The bony canal is divided into 3 spiral canals

by 2 partition , namely basilar membrane &


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Reissners membrane (vestibular membrane). They

are

a)Scala vestibulib)Scala tympanic)Scala media

The basilar membrane is a connective tissuemembrane . It extends from the tip of the osseous

spiral lamina to spiral ligament ( tough dense fibrous

band) which lines the outer wall of the canal. It is

called membrous spiral lamina. Along the basilar

membrane there are 20,000 to 30,000 tiny fibres

called basilar fibres (auditory fibres). Each fibre isdifferent size & shape. The fibres near the oval

window are short & stiff. While reaching towards the

helicotrema the basilar fibres gradually become longer

& soft.

Reissners membrane extends obliquely from the

upper surface of spiral lamina to the upper part ofspiral ligament.

Scala vestibuli lies above the reissners

membrane. Scala vestibuli ends at the basis of cochlea

at oval window which closed by the foot plate of

stapes. It is filled with Perilymph.


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Scala tympani lies below the basilar membrane.

It lies parallel to scala vestibule & at the base it ends

at the round window which is covered by the

membrane called secondary tympanic membrane. It

also is filled with perilymph.

At the apex scala vestibuli & scala tympani

communicates through a small opening called

helicotrema.

Scala media also called cochlear duct (canal or

partition) or membranous cochlea or otic cochlea is

enclosed between basilar membrane& vestibular

membrane. It is a triangular fluid filled space. It is

called space of Nuel. It extends b/w the spiral lamina

& spiral ligament. Its upper wall is formed by

vestibular membrane, outer wall by spiral ligament &lower wall (tympanic wall) by basilar membrane &

bony spiral lamina. The scala media is filled with the

endolymph. It ends blindly at apex & but at the base it

communicates with the saccule of the otolith organ of

of vestibular apparatus through duct of Hensen or

ductus ( canalis ) reuniens.

On the basilar membrane lies the receptor

organ of hearing called organ of corti. It is

neuroepithelial structure in the cochlea. The organ of

corti extends throughout the cochlear duct. It is made


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up of sensory elements ,hair cells &supporting cells.

They include, from medial to lateral

1.Border cells2.Inner hair cells3.Inner phalangeal cells4.Inner pillar cells5.Outer pillar cells6.Outer phalangeal cells7.Outer hair cells8.Cells of Hensen9.Cells of Claudius10.Tectorial membrane & Reticular lamina

The outer & inner pillar cells projects like rods

from the basilar membrane and are called rods of corti.

Each pillar cell has a broader base, elongated body or

pillar & head at the tip of pillar. The head of the rods

unite to enclose a triangular tunnel called the inner

tunnel or tunnel of corti.

Medial to inner pillar cells there is a single

row of inner hair cells & lateral to outer pillar cellsthere are 3 or 4 rows of outer hair cells. There are

about 3500 inner hair cells & about 20,000 outer hair

cells in the cochlea. They are the receptors of

auditory sensations. Inner hair cells are flask shaped

& have a broader base, numerous mitochondria & a

larger nucleus situated at the base. The outer hair


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cells are columnar cells with centrally placed nucleus

& aggregates of mitochondria at the base. The base

of the inner hair cell & the outer hair cells rests on

supporting cells called inner phalangeal & outer

phalangeal cells respectively.The inner phalangeal

are arranged in a row along the inner surface of inner

pillar cells on the basilar membrane. The outer

phalangeal cells are tall columnar cells & are called

cells of deiters and are arranged in 3-5 rows. From

the phalangeal cells stiff phalangeal processes

projects upwards b/w the hair cells to form a part of

reticular lamina (lamina reticularis). A triangular

fluidspace is present b/w the innermost outer

phalangeal process & outer pillar cells called space of

Nuel.O n the surface of the hair cell there are shortstiff hairs called stereocilia. The hairs of the hair cells

projects through the pores of the reticular lamina &

gets embedded in the tectorial membrane

(membrane tectoris). The movement of tectorial

membrane in relation to the vibrations in endolymph

stimulates the hair of hair cells. The tectorialmembrane is made up of gelatinous substance. It is

attached to the outer border of the spiral lamina at

one end & to the cells of Hensen at the other end.

They are tall columnar cells arranged in several rows

on the basilar membrane lateral to outer phalangeal

cells. The cells of Hensen also send off processes


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which help in the formation of reticular lamina. The

space b/w the outer phalangeal cells & the cells of

Hensen is called the outer tunnel. On the outer side

of the cells of Hensen there are cuboidal cells called

cells of Claudius. In certain areas some group of cells

are present b/w cells of Claudius & basilar membrane

called Boettcher cells. Along the inner side of inner

hair cells there is a single layer of slender columnar

cells called Border cells. Auditory impulses reaching

the hair cells are transmitted by the auditory nerve.

At the base of each hair cells two types of

cochlear nerve endings (sensory nerve fibres) are

present. They are the smaller & larger nerve

endings. The smaller nerve endings are thinner

and less granulated. The larger ones are thickerand densely granulated. The cochlear nerve fibres

after losing their sheaths enter the organ of corti.

The nerve fibres from the outer hair cells cross the

tunnel corti &meet the nerve fibres from the inner

hair cells & reach the bipolar cells of the spiral

ganglion situated on the osseous spiral lamina.

Auditory pathway:-

The bipolar cells of spiral ganglion

constitutes the First order Neurons. The dendrites

of bipolar cells terminate around the hair cells of


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the organ of corti and carry the auditory impulses

to the ganglion cells. The axons of these cells

constitute the cochlear division of the 8th

cranial

nerve and leaves through the internal auditory

meatus and enters the medulla oblongata. In the

medulla at the lower border of pons the nerve

fibres divide into two parts . One part end in the

ventral cochlear nucleus and other part in the

dorsal cochlear nucleus lying ventral & dorsal to

the restiform body respectively.

From these nuclei the second order neurons

arise. From the ventral nucleus a transverse band

of fibres arises called corpus trapezoideum. Its

fibres run in 3 different directions.

a)A few fibres end in the superior olivary nucleusand trapezoid body of the same side & some

cross midline and enters the trapezoid body &

the superior olivary nucleus of opposite side.

b)Majority of fibres cross the midline and entersthe lateral lemniscus and the joins the nucleus

of lateral lemniscus.

c)Some of fibres remains uncrossed & enterslateral lemniscus of same side& terminate in the

nucleus of lateral lemniscus on same side.

The fibres from the dorsal nucleus pass

along the floor of the 4

th

ventricle towards the


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midline & forms a band of fibres called the

medullary striae or acoustic striae. These fibres

also have same course.

Therefore , the lateral lemniscus on each

side contains auditory fibres of both sides. The

fibres then ascends through the pons &midbrain

and ends in the

a)Inferior colliculus of tectum of midbrain.b)Medial geniculate body of thalamus. The

medial geniculate body forms the

subcortical auditory centre. The two medial

geniculate bodies remain united by

commissural fibres , passing through the

optic chiasma & forms Guddens

commissure.From the medial geniculate body the third order

neuron arises and constitute the Auditory radiations. The

fibres pass through the posterior limb of internal capsule

and end in the auditory centre in the temporal lobe of

cerebral cortex. The centre is situated in the Heschls gyrus

and in the superior temporal gyrus. Areas 41 & 42 (situated

in the anterior transverse gyrus & lateral surface of superior

temporal gyrus ) and wernickes area (in the upper part of

superior temporal gyrus posterior to areas 41&42) are the

auditory areas. Area 41 & 42 are the primary auditory areas.

There is a point to point representation of basilar

membrane at first in the medial geniculate body and then in


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the auditory centre. The apex of cochlea in the anterior

end,basal part in the posterior end & the intermediate

region in the middle of the centre. The audiopsychic centre

lies close to it occupying the superior, middle & inferior

temporal gyrus area 20,21,22. The cortical auditory centres

are concerned with

a)Perception of auditory impulsesb)Analysis of pitch & intensity of soundc)Determination of source of sound.

The area 41 & 42 are concerned with the perception of

auditory impulses only. The analysis & interpretation

of the full meaning of a particular sound occurs in the

audiopschyic area along with the wernickes area. The

auditory impulses from each ear project to the

auditory centres of both side. So unilateral removal ofone auditory cortex does not produce deafness.


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stucture of ear

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