CSD 3103
anatomy of speech and hearing
mechanisms
Hearing mechanisms
Fall 2008
The Middle Ear
The middle ear
Important Structures:





Epitympanic recess
Tympanic cavity
Aditus ad antrum
Mastoid air cells
Ossicles
The middle ear
Schematic
view of the
middle ear
boundaries
and
landmarks
The superior face
 Tegmen
Tympani
The inferior face


Tympanic plate
Jugular fossa
The medial face
 Oval window
 Footplate of the
stapes
 Round window
 Promontory
The lateral face
 Eardrum
The posterior face

Mastoid wall

Tympanic aditus

Pyramidal eminence
Chorda tympani
nerve

The anterior face
 Carotid wall
 Eustachian
tube
General structures of the
middle ear

Eardrum

Ossicular chain


Eustachian tube
Middle ear muscles
The ossicles
 Malleus (hammer)
 Incus (anvil)
 Stapes (stirrup)
The malleus

Manubrium

Neck

Head

Lateral process
The malleus
The incus

Short process

Long process

Lenticular process

Incudostapedial joint
The incus
The stapes

Head

Neck

Anterior crus

Posterior crus

Footplate
The stapes
Articulated ossicular
chain
Ossicles on a dime
Ligaments of the ossicular
chain




Superior malleal
ligament
Anterior malleal
ligament
Lateral malleal
ligament
Posterior incudal
ligament
The ossicular chain in
place
Purpose of the ossicuar
chain

Impedance matching

Protection
Ossicular motion
Vibratory motion of The
stapes
Impedance matching of
the middle ear
a sound wave traveling in a medium of
certain physical properties, namely density
and elasticity, will not pass readily into a
medium with different properties
 the more different the characteristics of the
two media are, the more sound energy will
be reflected at the boundary

Impedance matching of
the middle ear
Acoustic resistance of air: 41.5 ohms
 Acoustic resistance of cochlear fluid: 161,000
ohms
 This represents a ratio of 3880:1
 Without the impedance matching capabilities of
the middle ear, only 1/10 of 1% of the energy of
an incoming sound wave would make it into the
cochlea--99.9% of the energy would be reflected
at the boundary

Area advantage

The area of the
tympanic membrane is
17x the oval window

As the area decreases,
the pressure increases
Impedance matching of
the middle ear
Area advantage
 Curved membrane buckling

Curved membrane
buckling
Notice how the eardrum
curves from its rim at both
ends to its attachment
with the malleus in the
middle. This point of the
eardrum (V1) doesn’t
move as far. This causes
an increase in force.
Impedance matching of
the middle ear
Area advantage
 Curved membrane buckling
 Lever action

Lever action advantage
 The advantage is
increased in (B) when
the fulcrum is moved
closer to the mass to
be lifted.
Purpose of the ossicuar
chain

Impedance matching

Protection
Purpose of the ossicuar
chain
The acoustic reflex
Tensor tympani muscle
Stapedius muscle
The tensor tympani

Larger of the two
tympanic muscles

Tendon leaves the bony
wall via the
cochleariform process
The stapedius


The smaller of the two tympanic muscles
Tendon leaves the bony wall via the apex of the pyramidal
eminence
The acoustic reflex

It is a reflex

Bilateral

Occurs in response to sound intensities delivered
to either ear at 80-90 dB above threshold
The eustachian tube






35-38 mm long
Oriented downward,
forward, medialward
Osseous portion
Cartilaginous portion
Isthmus
Tensor palatini muscle