ANATOMY AND PHYSIOLOGY OF MIDDLE EAR

EMBRYOLOGY OF MIDDLE EAR

PHARYNGEAL POUCH:-PROXIMAL NARROW PART=EUSTACHIAN TUBEDISTAL DILATED PART=             1.TYMPANIC CAVITY             2.ANTRUM             3. ATTIC             4.MASTOID AIR CELLSOSSICLES=   1st ARCH CARTILAGE=1.HEAD OF MALLEUS                                             2.BODY OF INCUS    2nd ARCH CARTILAGE=1.HANDLE OF MALLEUS                                               2.LONG PROCESS OF INCUS                                               3.CRURA OF STAPES   OTIC CAPSULE=FOOT PLATE OF STAPES

Middle ear spaces Mucous membrane lines the bony

walls of tympanic cavity and it extends to cover the ossicles and supporting ligaments

Mucosal folds separate middle ear space in to compartments

Major mucosal folds1. Superior

malleolar2. Lateral malleolar3. Lateral incudal4. Medial incudal 5. Superior incudal6. Inter ossicular

COMPARTMENTS AND FOLDS OF TYMPANIC CAVITY: attic compartment:

1. anterior compartment2. posterior compartment                       a)  superior incudal space                        b) medial incudal space

  compartment of mesotympanum     1.anterior malleolar fold      2.posterior malleolar fold                                      

ISTHMUS TYMPANI ANTICUS: It lies between tensor

tympani fold and crura of stapes.

ISTMUS TYMPANI POSTICUS: Located near tip of short

process of incus between posterior incudal ligament ,medial incudal fold , pyramid and stapedius muscle

Anterior pouch of vontroltsch: Between anterior malleolar

fold and tympanic membrane anterior to handle of malleus

Posterior pouch of vontroltsch: b/w post malleolar fold and

T.M posterior to handle of malleus

Spread of cholesteatoma Posterior epitympanic

cholesteatoma: Orginates at prussack’s space –

lateral to ossicles – superior incudal space – aditus and antrum

Posterior mesotympanic cholesteatoma :

Orginate at post-sup quadrant of pars tensa – lies medial to ossicles – spread to involve facial recess and sinus tympani –reaches mastoid via posterior tympanic isthmus

Anterior epitympanic cholesteatoma:

Orginate from epitympanic retraction anterior to head of malleus – reaches mesotympanum via anterior pouch of von troltsch – involve supra tubal recess – geniculate ganglion at risk

TYMPANIC CAVITY(MIDDLE EAR):

DIVIDED IN TO 3 PARTS:1. EPITYMPANUM (ATTIC)2. MESOTYMPANUM3. HYPOTYMPANUM

LATERAL WALL OF MIDDLE EAR: FORMED BY   1.BONY LATERAL WALL OF EPITYMPANUM  2.TYMPANIC MEMBRANE   3.BONY LATERAL WALL OF HYPOTYMPANUM

PETROTYMPANIC FISSURE:       RECIEVES:1.  ANTERIOR MALLEOLAR LIGAMENT2.  ANTERIOR TYMPANIC BRANCH OF

MAXILLARY ARTEY

CANAL OF HUGIER:MEDIAL WALL OF PETROTYMPANIC FISSURE THROUGH WHICH CHORDA TYMPANI RUNS OUT

COURSE OF CHORSA TYMPANI

ROOF OF THE MIDDLE EAR: TEGMEN TYMPANI:  FORMED BY

PETROUS AND SQUAMOUS PORTION OF TEMPORAL BONE

PETOSQUAMOUS SUTURE:ROUTE OF INFECTION IN TO EXTRA DURAL SPACE IN CHILDREN

  

ANTERIOR WALL OF MIDDLE EARlower 1/3rd:- plate of bone covering carotid arterymiddle 1/3rd:- 1.eustachian tube 2.canal for tensor  tympani 

upper 1/3rd:-has anterior epitympanic sinus anterior to ossicle heads&can hide cholesteatoma in canal wall up surgeries                                

POSTERIOR WALL:- aditus processus pyramidalis vertical portion of

facial nerve facial  recess

(suprapyramidal recess)

sinus tympani (infra pyramidal recess)

FLOOR OF MIDDLE EAR: formed by :-pneumatised bone

separates from jugular bulb

tympanic branch of glossopharyngeal nerve enters at junction between floor and medial wall

MEDIAL WALL OF THE MIDDLE EAR:

three depressions:

1. oval window 2. round window3. sinus tympani facial nerve canal dome of the lateral

scc:-posterior and lateral to facial canal

MUCOSA OF TYMPANIC CAVITY:- mucous secreting resporatory ciliated

epithelium ventilation route from mesotympanum and

epitympanum is via two small openings1. anterior isthmus tympani2. posterior isthmus tympaniprussak's space:- site for cholesteatoma      laterally=pars flaccida      medially  = neck of malleus      inferiorly=lateral process of malleus      superiorly=lateral malleolar fold

COMPONENTS OF TYMPANIC CAVITY:EAR OSSICLES:

malleus= head,neck, anterior process, lateral process ,handle incus=body, short process, long process, lenticular process stapes=head,neck, anterior crura, posterior crura,foot plate long axis of stapes is

always horizontal

MUSCLES OF MIDDLE EAR:

1. tensor tympani2. stapedius:-supplied by

a branch of mandibular nerve

nerves:3. chorda tympani4. tympanic plexus

BLOOD SUPPLY OF MIDDLE EAR:-

from branches of :1. middle meningeal artery2. maxillary artery3. ascending pharyngeal artery4. stylomastoid branch of posterior

auricular artery nerve supply:-sensory:-ixth cranial nerve

through tympanic plexus motor:-1. tensor tympani=mandibular division of vth nerve2. stapedius=facial nerve

intensity Defined as the power transmitted by

sound wave through a unit area Intensity is dependent on pressure and

velocity Intensity= peak pressure x peak velocity/2 Displacement produced by sound wave

vary with frequency if intensity is constant Low frequency vibration produces greater

displacement

Sound is alternate compression and rarefraction of air

2 properties of sound: intensity and frequency

IMPEDANCEImpedance is defined as resistance

offered by a medium for transmission of sound

DECIBEL Measurement of sound level It is logarithmic unit of

measurement that express the magnitude of physical quantity of sound ,relative to reference level

A difference of 1 db is the minimum perceptive change in volume of sound

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FUNCTIONS OF MIDDLE EAR

IMPEDENCE MATCHING

ATTENUATION

PHASE DIFFERENCIAL EFFECT

MIDDLE EAR FUNCTIONS Couples sound energy to cochlea

Provide physical protection for cochlea

It serves as an acoustic transformer to match the impedance of air to the much higher impedance of the cochlear fluids

It couple sound preferably to only one window of the cochlea , thus producing differential pressure between windows ,required for the movement of cochlear fluids

MODE OF VIBRATING MIDDLE EAR STRUCTURES

TYMPANIC MEMBRANE: It moved to and fro, it is buckled in

the regions between manubrium of malleus and the , anterior and posterior edges.

Inferior edges T.M – vibration greatest

in high frequencies above 6kHz, the pattern becomes much more complex, vibration breaks up in to many small zones with the reduction in efficiency of transfer of vibration.

The axis of rotation of the ossicles and the axis of suspension by their ligaments coincide with their centre of rotational inertia , so bones vibrate with very little loss through suspending ligaments

Movement of stapes up to 1 kHz – stapes foot plate

moves primarily like a piston

Higher frequencies : vibration become more complex , with rotatory motion along both the long and short axis of the foot plate

IMPEDANCE TRANSFORMER middle ear acts as

impedance transformer

It transfers the incoming vibrations from large low impedance T.M to much smaller, higher impedance ,oval window.

Impedence transformer will change the low pressure , high displacement vibration of air into high pressure , low displacement vibrations suitable for driving the cochlear fluids

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Impedence mismatch

IF THERE WAS NO MIDDLE EAR SYSTEM ,99% OF SOUND WAVES WOULD HAVE REFLECTED BACK FROM OVAL WINDOW

MIDDLE EAR BY ITS IMPEDENCE MATCHING PROPERTY ALLOWS 60% OF SOUND ENERGY TO DISSIPATE IN INNER EAR

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(a) HYDRAULIC ACTION OFTYMPANIC MEMBRANE

Total effective area of tympanic membrane 45mm2

Area of stapes footplate is 3.2mm2 Effective areal ratio is 14:1 Thus by focusing sound pressure

from large area of tympanic membrane to small area of oval window the effectiveness of energy transfer between air to fluid of cochlea is increased

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(b) Lever action of ossicles Handle of malleus is 1.3 times

longer than long process of incus

Overall this produces a lever action that converts low pressure with along lever action at malleus handle to high pressure with a short lever action at tip of long process of incus

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(c) Action of tympanic membrane Eustachian tube equilibrates the

air pressure in middle ear with that of atmospheric pressure, thus permitting tympanic membrane to stay in its most neutral position.

A buckling motion of tympanic membrane result in an increased force and decreased velocity to produce a fourfold increase in effectiveness of energy transfer

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PHASE DIFFERENTIAL EFFECT Sound waves striking the tympanic

membrane do not reach the oval and round window simultaneously.

There is preferential pathway to oval window due to ossicular chain.

This acoustic separation of windows is achieved by intact tympanic membrane and a cushion of air around round window

This contributes 4dB when tympanic membrane is intact

INFLUENCE OF MIDDLE EAR MUSCLES

2 muscles in the middle ear1. Tensor tympani muscle2. Stapedius muscle

Tensor tympani muscle : Insertion : It inserts on to the top of

manubrium of malleus medially Action : it pulls malleus medially and anteriorly

, nearly at right angles to the normal direction of vibration

STAPEDIUS MUSCLE:Insertion : inserts on the posterior aspect of

stapesAction : pulls stapes posteriorly

Muscles affect the transmission of sound in 2 ways

1. By increasing the stiffness of ossicular chain2. By changing the direction of vibration of

ossicles so that movement is less effectively coupled to cochlea

middle ear muscles contract in response to sounds

In humans only stapedius can be driven acoustically , unless the sound is loud enough to give a startle reflex

To start a reflex, the stimuli be 80 db above subject’s absolute threshold in the frequency range from 250 kHz to 4 kHz

FUNCTION OF MIDDLE EAR MUSCLE

1. Middle ear muscle reflex protects from noise damage ,but the reflex is too slow to protect the ear from sudden impulsive noise ,but have effect on longer lasting noise

2. Reflex causes selective attenuation of low frequency components in high intensity speech thereby improving intelligibility of speech

3. Reflex reduces the influence of resonances in the middle ear

Thank you