Middle Ear

Question 1

Structures of middle ear

Answer 1

• Tympanic membrane
• Middle ear cavity
• Ossicular chain
• Eustachian tube
• Middle ear muscles

Question 2

Middle Ear Cavity

Answer 2

Air filled space that is irregular in shape

Question 3

Sections of middle ear

Answer 3

Tympanum (main cavity) and Epitympanum (attic)

Question 4

Walls of Middle Ear Cavity

Answer 4

• Lateral
• Inferior
• Anterior
• Superior
• Medial
• Posterior

Question 5

Lateral Wall (middle ear cavity)

Answer 5

Landmarks are:

• Tympanic membrane
• Chorda tympani (if severed, lose sense of taste)

Question 6

Tympanic Membrane

Answer 6

Thin, elliptical shape, transparent, primary landmark of lateral wall

3 layers:

• cutaneous
• fibrous
• membranous

Question 7

Layers of Tympanic Membrane

Answer 7

Cutaneous- lateral most layer made of epithelial tissue
Fibrous- middle layer, strong, contains 2 sets of fibers
Membranous- medial most layer, mucus membrane

Question 8

Medial Wall (middle ear cavity)

Answer 8

Prominence of semicircular canals

Question 9

Anterior Wall (middle ear cavity)

Answer 9

Eustachian tube!

Question 10

Eustachian Tube

Answer 10

Air in the middle ear is refreshed by ET, helps with drainage from middle ear, opens fraction of a second at a time

When ET fails to open, there is negative pressure

Question 11

Ossicular Chain

Answer 11

Suspended in middle ear cavity and held in place by a series of tendons and ligaments

• Malleus
• Incus
• Stapes

Question 12

Middle Ear Muscles

Answer 12

• Tensor tympani: innervated by CNV (Trigeminal)

- Stapedial muscle: innervated by CNVII (Facial)

Question 13

Optimal ME Conditions

Answer 13

• Equalized air pressure on both sides of TM

- Mobility of the moving parts of the middle ear (TM and Ossicular chain)

Question 14

Tympanic Membrane Physiology

Answer 14

• Lies at angle at medial end of EAC
• Incredibly sensitive
• TM vibratory motion is different depending on the frequency of the incoming signal (lower freq moves more as a unit, higher freq moves segmentally)
• Sound undergoes transformation from acoustic energy to mechanical energy

Question 15

Physiological Functions of the Middle Ear

Answer 15

1. Overcome impedance mismatch
2. Middle ear muscle reflex
3. Ossicular chain movement

Question 16

Impedance mismatch

Answer 16

Mismatch in impedance between the tissue filled and fluid filled inner ear and air filled outer ear and middle ear

Question 17

Overcoming the impedance mismatch

Answer 17

1. Area ratio hypothesis
2. Lever ration hypothesis
3. Curvature of the tympanic membrane

-With all 3, total amplification to overcome the IM is 33 dB

Question 18

Area-ratio Hypothesis

Answer 18

Concentration of acoustic energy from a large area to a small area

-The pressure at the oval window is 17 times greater than at the tympanic membrane

Question 19

Lever Ratio Hypothesis

Answer 19

Transfer of force from TM to stapes relies on ossicle movement

• Ossicles work as lever
• Pressure increase of 17 times greater from the area ration is multiplied by 1.3 from the lever ratio

Question 20

Curvature of the TM

Answer 20

The TM tends to buckle as it moves, which causes the malleus to move with 2 times more force

Question 21

Middle Ear Reflex

Answer 21

Clinical implications of acoustic reflex that helps protect inner ear- but reflex isn’t perfect