1. CSD 3103 anatomy of speech and hearing mechanisms Hearing mechanisms Fall 2008
The Middle Ear

2. Important Structures:
The middle ear
Important Structures:
Epitympanic recess
Tympanic cavity
Aditus ad antrum
Mastoid air cells
Ossicles
The structures of the middle ear lie in a cavity within the petrous portion of the temporal bone. Altho most diagrams are 2 dimensional, you need to get a feel for the middle ear cavity as a volume.
Fig 10.1 middle ear in cross section--very simplified drawing
The volume is narrow--only 2mm to 4mm wide, but very high with the vertical dimension about 15 mm.
Total volume is 2cc--if you filled it with water, this is how much water you would use.
The cavity is divided into 2 parts--the attic or epitympanic recess, which is the portion extending upward beyond the superior border of the eardrum.
Tympanic cavity or middle ear space--which is the portion lying medially to the eardrum.
The epitympanic recess is largely occupied by the head of the malleus and the bulk of the incus, two of the middle ear ossicles.
The tympanic antrum communicates with the mastoid air cells. Has implications regarding pathways for infection.

3. The middle ear
Schematic view of the middle ear boundaries and landmarks
Many authors view the middle ear cavity as a box--to get a perspective regarding where the middle ear structures are found.
Fig 1-2 shows the names of the six faces of the box.
• superior face (ceiling)
• inferior face (floor)
• posterior face (left side wall)
• anterior face (right side wall)
• lateral face (front wall)
• medial face (back wall)
Let’s talk about some structures associated with these faces.

4. The superior face Tegmen Tympani Show Fig 6-43 at the same time
• The superior face--paper thin plate of bone called the tegmental wall or tegmen tympani separates the cavity from the cranium and the meningeal coverings of the brain. Has implications regarding pathways for infection

5. The inferior face Tympanic plate Jugular fossa
the inferior face---formed by the tympanic plate of the temporal bone which separates the cavity from the jugular fossa (large groove than accommodates the jugular vein).

6. The medial face Oval window Footplate of the stapes Round window
Promontory
• the medial face---on the other side is the cochlea or inner ear. Important landmarks
- the oval window--a kidney shape opening into the vestibule of the inner ear. Occupied by the footplate of the stapes
- the round window--a circular opening into the basal turn of the cochlea which is covered by a thin membrane
- promontory--a rounded prominence projecting into the middle ear cavity. Caused by the first turn of the bony labyrinth of the inner ear

7. The lateral face Eardrum
• the lateral face---most of the lateral wall is formed by the eardrum.

8. The posterior face Mastoid wall Tympanic aditus Pyramidal eminence
Chorda tympani nerve
the posterior face---sometimes called the mastoid wall. Landmarks include the tympanic aditus which is the entrance to the tympanic antrum. The pyramidal eminence is here and contains the stapedius muscle. A major landmark is the chorda tympani nerve runs along this wall. The chorda tympani is a branch of the facial nerve that courses thru the middle ear.

9. The anterior face Carotid wall Eustachian tube
the anterior face---sometimes call the carotid wall. The internal carotid artery runs behind this wall. Major auditory landmark is the eustachian tube.

10. General structures of the middle ear
Eardrum
Ossicular chain
Eustachian tube
Middle ear muscles
Structures and function of the middle ear:
Fig 4.3--general structures of middle ear seen in a coronal (front-back) view.
• eardrum--we’ve already discussed
• ossicles or ossicular chair---major auditory structure of the middle ear
• Eustachian tube
• middle ear muscles
Take each in turn.

11. The ossicles Malleus (hammer) Incus (anvil) Stapes (stirrup)
The ossicles: the three tiny bones that make up the ossicular chain transmit the sound-induces vibrations of the eardrum to the cochlea through the oval window.
The ossicles are the smallest bones in the body and are at their adult size at birth--they don’t grow.
The three bones include the malleus (hammer), incus (anvil) and stapes (stirrup). Instead of being attached to other bones the ossicles are suspended within the cavity by a series of ligaments and tendons. We’ll look at these later.

12. The malleus Manubrium Neck Head Lateral process
• The malleus--Show fig 10.2 approximately 8-9 mm long. Landmarks include the manubrium (handle)--which in life is firmly embedded between the fibrous and mucous membrane layers of the tm. This is the most lateral attachment of the ossicular chain.
The neck is a narrowing between its manubrium and head.
The lateral process is what produces the bulge on the eardrum when we look otoscopically.
The head of the malleus connects with the body of the next middle ear ossicle--the incus.

13. The malleus
Show photo of malleus

14. The incus Short process Long process Lenticular process
Incudostapedial joint
• The incus--show fig About 7 mm long and is the middle bone.
Landmarks include the short process, the long process, and the lenticular process. This is the point of attachment with the stapes.
The connection is thru a true ball-and-socket kind of joint called the incudostapedial joint.

15. The incus
Show photo of incus

16. The stapes Head Neck Anterior crus Posterior crus Footplate
• the stapes---show fig 10.4 only about 3.5 mm long. This is THE smallest bone in the human body. Landmarks include the head, the neck, the anterior and posterior crus or crura, and the footplate. The footplate is embedded into the oval window by the annular ligament. This is the most medial attachment of the ossicular chain.

17. The stapes
Show photo of stapes

18. Articulated ossicular chain
articulated ossicular chain--fig 6-52 showing various views of an articulated ossicular chain. Very much moves as a unit

19. Ossicles on a dime
Fig 6-53 is a photo of the three middle ear bones sitting on a dime. Notice how much room is left.

20. Ligaments of the ossicular chain
Superior malleal ligament
Anterior malleal ligament
Lateral malleal ligament
Posterior incudal ligament
The ligaments that suspend the ossicular chain--show fig The ossicles are suspended in the middle ear cavity by four ligaments.
• the superior malleal ligament---runs from the superior face of the cavity (tegmen tympani) to the head of the malleus
• the anterior malleal ligament--runs from the anterior face of the cavity to the anterior process of the malleus
• the lateral malleal ligament--runs from the bony wall to the neck of the malleus
• the posterior incudal ligament---runs from the mastoid (posterior wall) to the short process of the incus

21. The ossicular chain in place
Because of the way the ossicular chain is suspended by these series of ligaments, its inertia is very small and they are well balanced. As a result, once sound vibrations have ceased, the vibrations of the ossicles also terminate abruptly. If this were not the case, the ossicular chain would tend
to swing like a pendulum and would continue to vibrate after the sound had ceased.
Fig 2-10 the ossicular chain in place--review major landmarks

22. Purpose of the ossicuar chain
Impedance matching
Protection
Putting it all together---ossicular chain movement.
The ossicles have two main purposes
• to deliver sound vibrations efficiently to the inner ear fluids (impedance matching)
• to help the inner ear from being overdriven by excessively strong vibrations (protection)

23. Ossicular motion Fig 1-5: schemata of ossicular motion.
When positive pressure is exerted against the tm, the manubrium of the malleus is moved medially and the head of the malleus swings laterally, pulling the head of the incus with it.
In turn the stapes is rocked into the oval window, causing displacement of the cochlear fluids.
Fluid is incompressible, round window provides pressure relief.
Negative pressure applied to the TM shows opposite movements.

24. Vibratory motion of The stapes
Fig 2-14 shows the vibratory motion of the footplate of the stapes. Don’t view it like a little piston, altho the motion of the stapes changes when higher sound pressures are exerted on the TM.
When sounds higher than 70 dB SPL are introduced, the stapes vibrates in a way that impedes the transmission of energy to the cochlea---possible protection mechanism.

25. 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
The need for the middle ear as an impedance matcher.
A basic principle of acoustic states that a sound wave traveling in a medium of certain physical properties (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.

26. 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
In the case of moving sound energy from air (outer ear, middle ear) to a fluid (cochlea)---the properties of these two media are very different. To put a number on it--if you define the acoustic resistance (synonymous with the term impedance--for our purposes) (in ohms) as how resistant the media is to energy flow:
air has an acoustic resistance of 41.5 ohms
cochlear fluid has a resistance of 161,000 ohms
the ratio of these resistances is 3880.
What this means is that if the ear had no way of matching these resistances (making the differences smaller) , one-tenth of 1% of the sound in air will pass to fluid, while the remaining 99.9% would be reflected back thru the outer ear.
This is one of the primary functions of the ossicular chain. To act as a mechanical transformer that boosts the original signal so the energy can be efficiently transmitted to the cochlea.

27. Area advantage
The area of the tympanic membrane is 17x the oval window
As the area decreases, the pressure increases
This transformer function is accomplished by the combination of three mechanisms
• Area advantage---largest contributor of the effect. Fig The area of the TM is about 17 times that of the oval window. P=F/A. As A decreases, the force stays constant, so P has to increase. This is the same principle as a thumbtack---show fig 6-68

28. Impedance matching of the middle ear
Area advantage
Curved membrane buckling

29. 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.
• Curved membrane buckling ---show fig The eardrum curves from its rim at both ends to its attachment with the malleus in the middle. As a result, the point of the TM attached to the malleus
doesn’t move as far (displacement is less) as the ends. There is an increase in force, then at this point because the product of Force and Displacement have to be the same along all points of the TM (F1 x D1 = F2 x D2)

30. Impedance matching of the middle ear
Area advantage
Curved membrane buckling
Lever action

31. Lever action advantage
The advantage is increased in (B) when the fulcrum is moved closer to the mass to be lifted.
• Lever action of the ossicular chain---fig 10.7 Modeling the ossicular chain as a lever system. In lever systems, the closer a mass is to the fulcrum (the pivot), the more force is generated at the shorter end. It’s the old F x D thing. If the distance between the fulcrum and the mass is shorter at one end, then the force at that end is higher. Show fig 6-66 for ossicular chain as a lever system.
All three of these mechanisms together increase the pressure about 46 times.

32. Purpose of the ossicuar chain
Impedance matching
Protection
Recall the second function of the ossicular chain: to help the inner ear from being overdriven by excessively strong vibrations (protection)

33. Purpose of the ossicuar chain
The acoustic reflex
Tensor tympani muscle
Stapedius muscle
A primary way this is accomplished is thru the two middle ear muscles (the tympanic muscles) and the acoustic reflex.
The two tympanic muscles include the tensor tympani and the stapedius, the smallest striated muscles in the body. Both muscles are unique in that they are completely encased in bony canals and only their tendons enter the tympanic cavity. This reduces muscular vibrations that might interfere with sound transmission.

34. The tensor tympani Larger of the two tympanic muscles
Tendon leaves the bony wall via the cochleariform process
The tensor tympani muscle---Fig The larger of the two. You can see where the muscle originates. It leaves its bony wall thru the cochleariform process and the tendon inserts on the head of the malleus. Contraction of this muscle draws the malleus medially and anteriorly.

35. The stapedius The smaller of the two tympanic muscles
The stapedius muscle---fig Much smaller than the tensor tympani.
The muscle originates within a bony canal running on the posterior wall of
the tympanic cavity. The tendon emerges thru an aperture at the apex of the pyramidal eminence. The tendon inserts on the head of the stapes. When contracted, the stepedius draws the stapes posteriorly and at right angles to the direction of the movement of the ossicular chain.
The smaller of the two tympanic muscles
Tendon leaves the bony wall via the apex of the pyramidal eminence

36. The acoustic reflex It is a reflex Bilateral
Occurs in response to sound intensities delivered to either ear at dB above threshold
The acoustic reflex: contraction of the tympanic muscles are reflexive and in response to rather loud sounds coming into the ear.
The reflex is bilateral and occurs when sound intensities are on the order of dB above threshold.
When the middle ear muscles contract, the ossicles work less efficiently (they become more stiff) and less force is delivered to the cochlea.
This has led to the postulation that the acoustic reflex is present as a way to protect the cochlea from damage that could result from excessive sound levels.
One drawback, tho, is the latency of contraction is at least 10 ms--so there’s no protection against transient bursts of high intensity noises like explosions.

37. The eustachian tube 35-38 mm long
Oriented downward, forward, medialward
Osseous portion
Cartilaginous portion
Isthmus
Tensor palatini muscle
In order for the middle ear to work effectively, the air pressure within the middle ear cavity must equal the air pressure in the outer ear. The way this is accomplished is thru the eustachian tube.
So altho the eustachian tube doesn’t contribute directly to the transmission
of sound across the middle ear to the cochlea, it does contribute to the health of the middle ear.
Fig 2-11 The eustachian tube---establishes communication between the middle ear and the nasopharynx.
In adults, it’s about mm long and directed downward, forward, and medialward.
Divided into two sections. Starting from the middle ear and working our way to the nasopharynx, they are
• the osseous portion---begins in the anterior wall of the middle ear cavity.
• cartilaginous portion--most of the tube is cartilage (about 20 mm).
Where the two portions meet is called the isthmus. The tube at the isthmus end is normally open all the time. The tube at the nasopharynx end is normally closed.
The tensor palatini muscle is what causes the tube to open briefly to equalize the pressure.
This is done when we yawn or chew.
Eustachian tubes in children are only half as long, are more horizontal than vertical and also usually always open. Gives rise to more middle ear problems.