Chapter 123: Middle and Inner Ear

Question 1

List the components of the tympanic cavity:

Answer 1

• Epitympanum (dorsal component) - Smallest, largely occupied by incus and part of malleus
• Mesotympanum (middle component) - True tympanic chamber. Bound laterally by the tympanic membrane and posteriorly by the cochlear membrane. Promontory on medial aspect
• Hypotympanum (ventral component) - Largest, sitting within the tympanic bulla

Question 2

What are the main components of the tympanic membrane?

Answer 2

• Pars flaccida dorsally
• Pars tensa ventrally, much larger. Manubrium of malleus is embedded in dorsal part of pars tensa

Question 3

Where is the aural opening of the auditory (eustachian) tube located?

Answer 3

Rostral mesotympanic chamber.

Question 4

List the ossicles of the middle ear:

Answer 4

• Malleus - articulates with the pars tensa laterally and the incus medially
• Incus - articulates with the malleus laterally and stapes medially
• Stapes - articulates with the incus medially and the fibrocartilaginous ring around the oval window medially

Question 5

What ligaments hold the ossicles in place?

Answer 5

• Lateral ligament of the malleus
• Dorsal ligament of melleus and incus
• Annular ligament of the stapes

Question 6

What muscles are responsible for movement of the ossicles?

Answer 6

• Tensor tympani muscle - pulls malleus medially, increasing the tension and convexity of the tympanic membrane
• Stapedius muscle - Tenses the stapes to limit its movement

both act to dampen sound vibrations

Question 7

What is the major anatomical difference between a cat tympanic bulla and a dog?

Answer 7

Double chamber:
- Larger ventral cavity
- Smaller rostrolateral compartment
Separated by a bony septum.

**Tympanic plexus on bony promontory is more exposed/more sensitive iatrogenic trauma

Question 8

What muscle causes opening of the auditory tube during swallowing?

Answer 8

Tensor veli palatini

Question 9

What is the bony labrinth?

Answer 9

A perilymph filled cavity in the temporal bone that communicates with the middle ear through the vestibular and cochlear windows.
Contains the membranous labyrinth.

Question 10

What are the three sections of the membranous labyrinth?

Answer 10

Vestibule
- Middle portion containing the saccule and utricle

Cochlea
- Bony spiral ‘seashell’ structure containing the cochlear coil
- Coil originates at the cochlear window and is divided by the cochlear duct into the scala vestibuli and the scala tympani
- Floor of the duct contains the Organ of Corti

Semicircular canals
- Anterior, posterior and lateral
- Each has an ampulla arranged at right angles to each other
- The saccule and utricle are found at the confluence of the canals

Question 11

How does the inner ear function in relation to sound perception?

Answer 11

Ossicles transmit sound waves to inner ear causing movement of endolymph within the cochlea. Soundwaves are converted to nerve impulses by the hair cells in the organ of Corti and transmitted to the brain via the cochlear nerve fibers.

Question 12

What structures within the inner ear contribule to vestibular function?

Answer 12

• Semicircular canals
• Saccule
• Utricle

Question 13

What is BAER testing?

Answer 13

Brainstem Auditory Evoked Responses

Question 14

What nerve and artery should be avoided during a VBO?

Answer 14

• Hypoglossal nerve
• Lingual artery

Question 15

What can be palpated externally to aid in the ventral approach to the bulla in a dog?

Answer 15

The paracondylar (jugular) process
- Bulla is 5-10mm rostral and medial to this prominence

Question 16

What is the tympanic cavity?

Answer 16

A bony shell situated caudal and medial to the zygoma and TMJ. It is formed by the tympanic component of the temporal bone and lined with respiratory mucosa.

Question 17

True or false? The tympanic cavity is NOT contiguous with the auditory tube and nasopharynx.

Answer 17

False - all are connected.

Question 18

What two membranes are found in the mesotympanic chamber?

Answer 18

Tympanic membrane

Cochlear membrane

Question 19

What are the most common complications after TECABO in cats (due to anatomy)?

Answer 19

Horner’s in up to 42% from damage to postganglionic sympathetic fibers in middle ear

Facial nerve paralysis in up to 56% with 28% permanent

Question 20

Why is sound transmission across the air/fluid interface of the middle/inner ear better than expected?

Answer 20

Normally poor impedance is compensated for by focusing of sound waves by ossicles onto the oval window.

Question 21

Swallowing opens the auditory tube to allow what?

Answer 21

Clearance of debris, equilibration of atmospheric pressure.

Question 22

What structures are associated with the vestibular nerve?

Answer 22

Semicircular canals

Question 23

Where does the cochlear nerve terminate?

Answer 23

The Organ of Corti.