Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

The tympanic membrane (TM)/eardrumThe tympanic membrane (TM)/eardrum

1.1. Is composed of three layers of tissue.Is composed of three layers of tissue.

2.2. Separates the external ear from the Separates the external ear from the

middle ear.middle ear.

3.3. Is connected to the malleus and Is connected to the malleus and

reflects the “cone-of-light”.reflects the “cone-of-light”.

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

The three layers of tissue of the TM are:The three layers of tissue of the TM are:

1.1. The cutaneous layerThe cutaneous layer (same layer as the EAM) (same layer as the EAM)

2.2. The fibrous layerThe fibrous layer (the “compliance” layer) (the “compliance” layer)

3.3. The mucous layerThe mucous layer (same as the inner liner of the (same as the inner liner of the middle ear cavity)middle ear cavity)

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

The three middle ear bones are:The three middle ear bones are:

1.1. The malleus The malleus (attached to the TM)(attached to the TM)

2.2. The incus The incus (attached to the malleus and stapes)(attached to the malleus and stapes)

3.3. The stapes The stapes (attached to the oval window)(attached to the oval window)

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

Two muscles attached to middle ear bones.Two muscles attached to middle ear bones.

A tendon attaches the A tendon attaches the stapedius musclestapedius muscle to the to the stapes. stapes. (innervated by the VII cranial/facial nerve)(innervated by the VII cranial/facial nerve)

A tendon attaches the A tendon attaches the tensor tympani muscletensor tympani muscle to the malleus.to the malleus. (innervated by the V cranial nerve) (innervated by the V cranial nerve)

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

The movement of the three middle ear The movement of the three middle ear bones creates a “mechanical amplifier”.bones creates a “mechanical amplifier”.

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

The Eustachian tube The Eustachian tube

It equalizes the air pressure between the It equalizes the air pressure between the middle ear cavity and the external auditory middle ear cavity and the external auditory meatus. Usually opens and closes every meatus. Usually opens and closes every time we swallow.time we swallow.

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

The areal ratioThe areal ratio

The eardrum (TM) has a surface area 17 The eardrum (TM) has a surface area 17 times larger than the footplate of the times larger than the footplate of the stapes (the footplate is attached to the stapes (the footplate is attached to the oval window). An areal ratio can be oval window). An areal ratio can be calculated based upon the difference in calculated based upon the difference in size between the two surface areas.size between the two surface areas.

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

Impedance transformerImpedance transformer

The low impedance of the air medium The low impedance of the air medium moving the eardrum is defeated by the moving the eardrum is defeated by the high impedance of the inner ear fluids.high impedance of the inner ear fluids.

This impedance mismatch is overcome by This impedance mismatch is overcome by the mechanical movement/energy of the the mechanical movement/energy of the middle ear bones.middle ear bones.

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

This mechanical amplifier coupled with This mechanical amplifier coupled with the areal ratio difference between the the areal ratio difference between the eardrum and the oval window create the eardrum and the oval window create the middle ear transformermiddle ear transformer..

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

Transduced energy/Middle ear transformerTransduced energy/Middle ear transformer

The acoustic sound information presented to the The acoustic sound information presented to the eardrum is transduced from sound wave energy eardrum is transduced from sound wave energy through an air medium into mechanical energy. through an air medium into mechanical energy. This coupled with the areal ratio complete the This coupled with the areal ratio complete the transformation of sound energy into the high transformation of sound energy into the high impedance of the inner ear fluids. The impedance of the inner ear fluids. The movement of the middle ear bones creates the movement of the middle ear bones creates the impedance matching function. This transformer impedance matching function. This transformer results in approximately 33 decibels of gain. results in approximately 33 decibels of gain.

Becoming Familiar with the Middle EarBecoming Familiar with the Middle Ear

For more specific information regarding For more specific information regarding the function of the middle ear muscles and the function of the middle ear muscles and biologic frequency modification, please biologic frequency modification, please refer to Lass & Woodford page 67.refer to Lass & Woodford page 67.