Topic Four - The Middle Ear

Question 1

Identify the different middle ear structures

Answer 1

Kahoot/Quizlet Quiz

Question 2

What is the purpose of the middle ear?

Answer 2

The middle ear are crucial for impedance matching.

Question 3

What is impedance?

Answer 3

A sound wave does not transfer easily into a medium of different properties (elasticity and density) than it was already traveling through (e.g., sound does not travel well from air into water).

The more dissimilar the density and elasticity of the mediums, the more sound if reflected at the boundary.

Without the middle ear, 99.9% of sound energy would be reflected off the cochlear.

Question 4

What are the three ways in which the middle ear helps with impedance matching?

Answer 4

The middle ear acts as an acoustical transformer in three ways:

1. Buckling of the tympanic membrane
2. The lever ratio of the ossicles
3. The area ratio of the tympanic membrane to the oval window

Question 5

Order the different impedance matching processes from most to least impactful?

Answer 5

1. Area ratio of the tympanic membrane to the oval window (+17 Pa)
2. Lever ratio of the ossicles (+1.3 Pa)
3. Buckling of the TM

Altogether, these processes result in an increase in 22.7 Pa or approx. 27 dB.

Question 6

Describe how the lever ratio of the ossicles works?

Answer 6

As the long crus of the incus is 1.3 times shorter than the manubrium of the malleus, there is an increase in force.

Question 7

Describe how the area of the TM to the oval window works?

Answer 7

The area of the TM is 17 times larger than the oval window.

Question 8

How are the middle ear bones connected?

Answer 8

They are connected by synovial joints - this allows for a degree of slippage and the ability to conduct sound vibrations

Question 9

How much does the malleus weigh?

Answer 9

Around 23mg

Question 10

How much does the incus weigh?

Answer 10

Around 27mg

Question 11

How much does the stapes weigh?

Answer 11

Around 2.5mg

Question 12

What is the joint between the malleus and incus called?

Answer 12

The incudomalleal joint

Question 13

What is the joint between the incus and stapes called?

Answer 13

The incudostapedial joint

Question 14

What are the two main middle ear muscle?*

Answer 14

• Tensor tympani
• Stapedius tendon

Question 15

What nerve innervates the two main middle ear muscles?*

Answer 15

Tensor tympani
- Trigeminal nerve (CN V)

Stapedius tendon
- Facial nerve (CN VII)

Question 16

What causes the middle ear muscles to contract and what does this result in?*

Answer 16

Tensor tympani
- Contracts prior to vocalisation

Stapedius tendon
- Contracts in response to loud sounds

Both muscles have a protective effect and reduce the flow of energy

Question 17

Describe the ipsilateral reflex pathway*

Answer 17

Peripheral:
- Right Cochlear
- Auditory nerve CN VIII

Central:
- Ventral Cochlear nucleus
- Medial superior olive
- Facial nerve nucleus
- Facial nerve CN VII

Peripheral
- Right stapedius muscle

Question 18

Describe the contralateral reflex pathway*

Answer 18

Peripheral
- Right cochlear
- Auditory nerve CN VIII

Central
- Ventral cochlear nucleus
- Decussation to left side
- Left medial superior olive
- Left facial nucleus
- Facial nerve CN VII

Periperal
- Left stapedius muscle

Question 19

What is the function of the eustachian tube?*

Answer 19

• Equalizes middle ear pressure
• Aerate middle-ear space
• Prevent nasopharyngeal reflux

Question 20

Describe the anatomy of the eustachian tube*

Answer 20

• Runs from middle ear cavity to the pharynx
• Tympanic 1/3 is bony and cone shaped, remains open during swallowing
• Distal 2/3 fibrocarilaginous, usually closed
• Surrounded by c-shaped cartilage
• Two muscles are attached: tensor veli palatini, levator veli palatini (contracts during swallowing and opens ET)