Middle Ear

Question 1

what is the middle ear composed of?

Answer 1

tympanic membrane
3 auditory ossicles: malleus, incus, stapes
2 tympanic muscles: stapedius and tensor tympani

Question 2

tympanic membrane

Answer 2

marks the boundary between outer and middle ear
oval shaped
placed at 55 degree angle
ear drum

Question 3

malleus

Answer 3

provides point of attachment with TM at Umbo

• manubrium*: a long process attaching to TM along its length
• anterior process*: attached by the anterior ligament of the malleus

Question 4

stapes

Answer 4

head articulates with the long process of incus
the neck bifurcates (branches into two)
footplate rests in the oval window of the temporal window

Question 5

incus

Answer 5

provides the intermediate communicating link of the ossicular chain
body articulates with the head of the malleus
long process is parallel with long process of malleus, articulating with the stapes
short process is attached by the posterior ligament of the incus

Question 6

stapedius

Answer 6

when it contracts, the stapes is rotated backwards
innervated by the stapedial branch of the VII facial nerve
embedded in the bone of the posterior wall of the middle ear
inserts into the posterior neck of the stapes

Question 7

tensor tympani

Answer 7

contraction pulls the malleus forwards
innervated by the V trigeminal nerve
inserts into the upper manubrium

Question 8

what happens when both tympanic muscles contract?

Answer 8

sound transmissions in the middle ear are reduced

Question 9

non-protective function of eustachian tube

Answer 9

aeration–brings oxygen to middle ear cavity

connects middle ear cavity to nasopharynx

Question 10

2 functions of eustachian tube

Answer 10

aeration

homeostasis

Question 11

protective function of eustachian tube

Answer 11

equalizes pressure across TM
prevents loud sound from entering the nasopharynx
drainage into nasopharynx

Question 12

2 functions of the middle ear

Answer 12

sound transmission

protection of hearing

Question 13

sound transmission function in the middle ear

Answer 13

amplification of middle frequencies

Question 14

3 mechanisms for impedance mismatch problems

Answer 14

area ratio
lever system
buckling of TM

Question 15

purpose of impedance matching

Answer 15

to match the level of resistance in the outer ear canal and eustachian tube to the fluid in the cochlea
so the signal released from the TM will maintain its intensity when it passes through the fluid in the cochlea

Question 16

area (impedance matching)

Answer 16

approx 25 dB gain –> 17:1 ratio
area of TM is larger than area of stapes, so the pressure that comes from the TM to the stapes will cause the fluid in the cochlea to vibrate in a way that matches the surface of the TM

Question 17

lever (impedance matching)

Answer 17

almost 2 dB gain –> 1.3:1 ratio
movement of stapes is greater than that of the malleus even though the malleus is longer than the stapes
force on incus and stapes will be higher than malleus

Question 18

total impedance in middle ear

Answer 18

31 to 33 dB

Question 19

buckling of TM (impedance matching)

Answer 19

4-6 dB increase
when the TM buckles, the arm of the malleus moves a shorter distance than the surface of the TM
buckling reduces velocity of the malleus and doubles force

Question 20

protection of hearing (2 functions)

Answer 20

Eustachian tube equalizes pressure across TM

acoustic reflex

Question 21

acoustic reflex

Answer 21

happens by contraction of stapedius and tensor tympani
main function is protection, but protection is never perfect
triggered by 80 dB (on average)