Hearing 2: Function of middle ear

Question 1

How does the middle ear couple sound energy to the cochlea?

Answer 1

Vibrating tympanic membrane → Vibrate middle ear bones (ossicles) → Vibrate cochlear fluid.

Question 2

What is the purpose of the physical protection of the cochlea in the auditory system?

Answer 2

Physical protection shields the delicate structures of the cochlea.

Question 3

Explain the concept of impedance matching in the middle ear.

Answer 3

Impedance Matching:
Amplifies sound pressure to match the impedance of air to the higher impedance of cochlear fluids.
Mechanism:
A. Force is concentrated over a smaller area of the stapes footplate, increasing pressure.
B. Ossicles act as a lever, further increasing pressure.
C. Result: 22 times increase in pressure exerted on cochlear fluid.

Question 4

Why does the ossicular system apply sound pressure to only one window of the cochlea?

Answer 4

To cause a differential pressure between the two windows, which is required for the movement of cochlear fluid.

Question 5

What are the functions of the middle ear muscles, and what is the acoustic reflex?

Answer 5

Functions:
A. Tenses the tympanic membrane for optimum vibration.
B. Acoustic Reflex:
Muscles contract in response to intense sounds.
Tensor tympani (innervated by cranial nerve V) pulls the malleus inward, and stapedius (innervated by cranial nerve VII) pulls the stapes away from the oval window, dampening sound transmission.
Significance:
A. Prevents auditory receptor damage.
B. Reflex reaction time: 40-160 ms.
C. Does not protect against brief intense sounds (e.g., gunshots).
D. Muscles contract before vocalization and chewing, reducing sound intensity.

Question 6

What is the function of the Eustachian tube?

Answer 6

Function:
Connects the middle ear with the nasopharynx.
Normal State:
Closed at rest.
During Swallowing, Chewing, or Yawning:
Opens to:
A. Renew air in the middle ear.
B. Equalize pressure on both sides of the eardrum.
C. Lined by ciliated epithelium, aiding drainage of middle ear secretions.
D. In children, the tube is short, wide, and straight, facilitating entry of nasopharyngeal secretions and fluids, potentially causing middle ear infection.