Middle Ear

Question 1

Anatomy of middle ear

Answer 1

Mucous layered but filled with air. Jugular vein is right at the bottom of the middle ear under bone.
The ossicular chain is made up of Malleus, incus and stapes. They move together as a chain and are held in place by ligaments. Incus ligaments: posterior and superior. Malleus ligaments: superior, anterior and lateral.

Question 2

Anatomy of the Eustachian tube

Answer 2

Starts at the bottom of the middle ear and leads all the way to the Nasopharynx. At the end of the ET there is the Tenasor Palatini muscle - closed in resting state but open when yawning or swallowing.
- 36 mm long
- 1/3 bone and 2/3 cartilage
- mucous membrane layered continuous with TM
- runs from middle ear to nasopharynx
- tiny hair caller cilia run along the tube waving - drifts mucous towards tensor palatini

Question 3

What is the stapes footplate

Answer 3

This is attached to the oval window- this is how sound gets to the inner ear, ligaments are attached to stapes to assure it is always pulled back into position at the oval window ready to respond to the next sound

Question 4

Function of middle ear: transducer

Answer 4

• the tympanic membrane transducers acoustical energy into mechanical energy

Question 5

Function of middle ear: conduction

Answer 5

• this can only happen if there is equal pressure between the outer and middle ear. To allow free movement of TM. Eustachian tube ensures efficient conduction by:
• equalising pressure either side of TM
• ventilation; tensor palatini muscles operate like a valve - opening when yawning/ swallowing
• drainage : it connects middle ear to nasopharynx and drains the mucous lining.

Question 6

Functions of ME: amplification

Answer 6

There is low impedance at the TM but a high impedance at the oval window due to the cochlea fluids in inner ear. The middle ear has to compensate for this by using the impedance mismatch mechanism.

Question 7

Impedance mismatch mechanism: areal ratio

Answer 7

• stiletto heel affect: TM is 14 times larger than oval window. So sound pressure is condensed into smaller area = force per unit area increases at oval window.

Question 8

Impedence mismatch mechanism: leverage action

Answer 8

Malleus is 1.15x longer than incus. This is 1:13 pressure increase. So low pressure large movement at TM = small movement high pressure at stapes- this further amplifies pressure.

Question 9

Impedance mismatch mechanism: tm characteristics

Answer 9

Is concave in shape and concentrates signal at the Umbo. Unequal tension at different parts of TM respond to different frequencies in different ways.

Question 10

Functions of middle ear; protection

Answer 10

The middle ear has two different ways it protects through the two different muscles: the stapedius muscle and the tensor tympani muscle.

Question 11

Acoustic reflux

Answer 11

The stapedius muscle connects the stapes to the ME wall. It is responsible for acoustic reflux. This protects the inner ear from being damaged by loud sounds. The stapedius muscle contracts in response to loud sounds. This is not instantaneous- ear hears sound, feeds to brain, brain feed to ear and muscle contracts. This equals reduced movement of ossicles and tm which stops the conduction of sound being so efficient. Always bilateral and only happens when sound is 70dB above hearing threshold

Question 12

Non acoustic reflux

Answer 12

Tensor tympani muscle is attached to Malleus. Muscle contacts upon a puff of air to the cornea. reduces Malleus movement. Tense muscles = stiff ossicles. This is to protect from trauma.