Middle Ear

3 main parts of the middle ear

1. Ossicular chain, or middle ear ossicles

•Connect the tympanic membrane to the oval window of the cochlea

2. Middle ear muscles

•Tightens ossicular chain in response to loud sounds

3. Eustachian tube

•Equalizes air pressure in middle ear cavity

Middle ear functions

• Impedance matching

• Protect the inner ear from overload

The Ossicular Chain

3 bones (ossicles) of the middle ear

1. Malleus

2. Incus

3. Stapes

The stapes is the smallest bone in the body!

Malleus

HAMMER

Attaches to the tympanic membrane at manubrium

Incus

ANVIL

Middle bone in ossicular chain

Stapes

STIRRUP

Attaches to the cochlea at oval window

The Ossicular Chain Function

1. Connect the tympanic membrane to the oval window of the cochlea

2. Transfer energy efficiently from the air filled ear canal to the fluid filled cochlea

• Need to transfer the sound pressure wave from air to fluid

3 ways to get sound energy to the inner ear

1. Bone conduction

2. Air pressure changes in the middle ear cavity

3. Vibration through the ossicular chain

Bone conduction

• Sound that travels via vibration of the bones of the skull, bypassing the middle ear and going directly to the inner ear

• Not significant for normal hearing

Air Pressure Changes in Middle Ear Cavity

• Sound that travels through the middle ear without encountering the ossicles and stimulate the oval and round windows directly

• Inefficient for normal hearing because of impedance mismatch

Vibration through the Ossicular Chain

• Sound pressure waves are converted into mechanical vibration of the malleus, the incus, and the stapes, which transmits the energy directly to the oval window (and not the round window)

• Main mode for normal hearing to overcome impedance mismatch

Acoustic Impedance (Z)

Opposition to the flow of acoustic energy

High impedance

Resistance to movement

•High impedance (“hard to move”) [e.g., fluid in a filled tube]

•Small movement in response to a given input (pressure)

•Large input (pressure) needed for a given movement

Low impedance

•Low impedance (“easy to move”) [e.g., air filled tube]

•Large movement in response to a given input (pressure)

•Small input (pressure) needed for a given movement

Impedance Matching in the Middle Ear

•The middle ear serves to match the impedance between the air-filled ear canal (low impedance)and the fluid filled cochlea (high impedance)

•The ossicles (bones of the middle ear; malleus, incus, stapes) transfer energy to one of the cochlear windows

How Does Impedance Matching in the Middle Ear work?

The middle ear uses three mechanisms to provide an impedance match between the ear canal and the cochlea:

1. Area Transformer (~x17)

2. Lever Transformer (~x1.3)

3. Buckling motion (~x2)

1. Area Transformer (~x17)

•Main contributor to middle ear impedance matching

 

•Remember: F = p*A

•Force remains constant, so p1A1 = p2A2

•Since A2 is much smaller, p2 will be much larger

2. Lever Transformer (~x1.3)

• The lever action of the middle ear bones also helps to amplify incoming vibrations

3. Buckling motion (~x2)

• The buckling motion of the ear drum also helps to amplify incoming vibrations

Middle Ear Transfer Function (filter)

How much gain does the middle ear provide?

•The anatomy of the middle ear provides pressure gain through three distinct mechanisms

1. Area Transformer:    20*log(17) = 25 dB

2. Lever Transformer:    20*log(1.3) = 2 dB

3. Buckling Motion:          20*log(2) = 6 dB

 

•Overall, these mechanisms serve to increase the sound vibrations by a maximum factor of x44!

•20*log(44) ~= 33 dB gain

The frequency dependence of impedance is determined by three factors

1. Mass

2. Stiffness

3. Damping (friction)

Mass

•Air has mass (so do ossicles, etc.)

•Requires force (F) to move the air’s mass (Newton’s laws of motion)

•Most relevant at high frequencies

Stiffness

•Air has stiffness (i.e., springiness – it pushes back when compressed)

•Requires force to compress the air

•Most relevant at low frequencies

•Application: some middle-ear pathologies involve increased stiffness, which creates low-frequency hearing loss

Damping (friction)

•Movement of air through a tube must overcome several sources of friction (walls, other air molecules)

•Requires force to overcome friction

•Most relevant at medium frequencies

-Mass and stiffness cancel each other

-Occurs at resonance

Middle Ear Muscles

•The ossicular chain is connected to the middle ear cavity by ligaments and muscles

 

•When the middle ear muscles contract, they stiffen the ossicular chain and alter middle ear function

• The stapedius muscle (attached to stapes) is part of an acoustic reflex

• This middle ear muscle reflex provides (limited) protection from loud sounds

  –Only for low frequency sounds and sounds >90 dB SPL

  –Fairly slow response (10-150 ms), so gun shots and other transient sounds are too fast to be attenuated

  –Can be activated prior to self vocalizations

Eustachian Tube

The Eustachian tube connects the middle ear space with the nasopharynx (back of nose/mouth)

• The Eustachian tube opens occasionally to equalize inside/outside pressurethrough the nasopharynx

  •Example: flying in an airplane

  •After take off (on ascent), air pressure decreases (outside P < inside P)

  •Ear drum pushed outward

  •Hearing is reduced (sometimes painful)

  •Ear pops (swallow or yawn) = Eust. tube opening

  •Now inside P = outside P

  •Hearing returns to normal
  

• Also provides air flow to middle ear cavity and exit route for mucus or fluid

Several pathologies can affect middle ear function

• Otitis Media

• Otosclerosis

• Cholesteatoma

Otitis Media

•Fluid in middle ear space

•Increases stiffness (smaller air space, reduces compliance), so creates low-frequency hearing loss

Otosclerosis

•Bone overgrowth around stapes footplate; “locks” the stapes in place

•Increases stiffness, so creates low-frequency hearing loss

Cholesteatoma

•Benign skin growth in the middle ear cavity

•Bad cases can destroy ossicles (or require surgery that destroys ossicles)

•Loss of ossicles can create a ~60-dB hearing loss

•Loss of ~35-dB gain from ossicle transforms

•Loss of ~25-dB gain from lack of differential pressure input to the oval and round windows of the cochlea

2 main classes of hearing loss

1. Conductive hearing loss

2. Sensorineural hearing loss

1. Conductive hearing loss

•Damage to or blockage of the external ear and/or middle ear

•Prevents sound conduction to inner ear

2. Sensorineural hearing loss

•Damage to the sensory or neuralcomponents of the inner ear (cochlea)