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Tympanic Membrane

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Tympanic Membrane

• EAM terminates at the tympanic membrane TM.

• In newborns the TM is horizontal

• In adults , the TM sits at a 55 degree angle.

• Ossification of EAM causes changes in angle of TM until about age 5 when it reaches adult position.

infant

adult

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Tympanic Membrane

• Very thin and translucent (wax paper).

• Average thickness is 0.74 mm (.003 inches).

• Elliptically shaped– Vertically .9 cm– Horizontally .8 cm

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Tympanic Membrane

• Three Layers of TM– Ectoderm (cutaneous) - continuous with EAM

– Mesoderm (fibrous)• Radial Fibers

• Concentric Fibers

– Endoderm (mucous) - continuous with Tympanic Cavity.

• Pars Tensa contains all three layers.• Pars Flacida DOES NOT contain fibrous layer.

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Tympanic Membrane

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Middle Ear

• Tympanic Cavity

• Ossicles

• Eustachian Tube

• Middle Ear Muscles

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Tympanic Cavity

• About same size/shape as an aspirin tablet.– 15 mm (superior to inferior)– 15 mm (anterior to posterior)– 5 mm (lateral to medial)– For descriptive purposes the TC has been

compared to a six-sided room (4 walls + ceiling + floor).

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Tympanic Cavity

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Tympanic Wall

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Medial Wall

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Posterior Wall

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Aditus and Pyramidal Eminence

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Anterior Wall

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Tegmen

• Also known as the superior wall.• Paper thin.• Separates the tympanic cavity from the posterior

cranial fossa which houses the temporal lobe.• Inflamatory conditions of middle ear can pass

through the petrous-squamosal suture in children directly to the meninges of temporal lobe of cortex.

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Jugular Wall

• No landmarks.

• Jugular vein is found inferior to this wall.

• Glomus bodies can push up through this wall from jugular vein causing glomus jugularis.

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Ossicles

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Ossicles

• Connects tympanic membrane with the oval window.

• Smallest bones in the human body

• Connected via a series of joints.

• Held in place by a series of ligaments, tendons, and joints (see p. 455 of Zemlin).

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Ossicular Chain

• Function– Sound transmission to oval window– Protect cochlea from intense vibrations by

changing axis of rotation of stapes.

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Malleus

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Incus

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Stapes

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Eustachian Tube

• Function– Pressure equalization– Drainage

• Description– 35 mm and drops at about a 40 degree angle.– Cartilaginous portion is 2/3, osseous portion is 1/3– Osseous portion is open, Cartilaginous portion is

usually closed.– Begins at tympanic cavity and terminates in

nasopharynx.

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Eustachian Tube

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Eustachian Tube• Function

– Tensor palatini definitely involved in opening ET.

– Levator palatini role is not clear.

– Opening has been described as a milking action and also been described as the tensor palatini pulling on side of ET, opening the tube.

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Eustachian Tube

• Differences between adults and infants– Angle of ET

• Adults - about 40 degrees

• Children - more horizontal

– Length• Adults - about 35 mm• Children - shorter

– Flaccidity• More flaccid in

children

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Eustachian Tube

• Cleft Palate– Normally fibers from tensor palatini and levator

palatini insert into the velum.

– In cleft palate fibers from these two muscles insert into the levator palatini may insert into hard palate and tensor palatini may insert into lateral portions of velum.

– Number of fibers for these two muscles is often reduced in people with cleft palate.

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Middle Ear Muscles

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Tensor Tympani Muscle

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Stapedius Muscle

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Function of the middle ear

• The middle ear system that includes the tympanic membrane and the ossicles, acts as an impedance matching device between the air-borne sound waves and the fluids of the inner ear.

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Function of the middle ear

• Must consider tympanic membrane

• Impedance … opposition to the flow of energy.

• Impedance mismatch… occurs when you have two mediums of differing impedances.

• Impedance mismatch occurs between gas (air) in environment and fluid of inner ear.

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Example of impedance mismatch

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Areal Advantage

• 17:1 (55:3) areal advantage between tympanic membrane and oval window and yields

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Lever Advantage

• 1.3:1 lever advantage

QuickTime™ and aAnimation decompressor

are needed to see this picture.

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Resonance of the Middle Ear

• The middle ear ear system creates a gain of nearly 30 dB between 1000 and 2000 Hz.

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Resonance of the Middle Ear

• Effects of increased mass and stiffness.– increase in mass causes downward shift of

resonant frequency.– increase in stiffness causes upward shift in

resonant frequency.

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Summary