Ear

Question 1

Name the 3 subdivisions of the ear.

Answer 1

External/outer ear
Middle ear
Internal/inner ear

Middle and internal ear are housed within the temporal bone in the petrous part, the thickened part.

Question 2

Describe the structure of the outer ear.

Answer 2

Auricle/pinna – the projecting outer portion of the ear

External acoustic meatus – canal leading for the base of the auricle to the tympanic membrane (ear drum)

Question 3

What are the properties of the auricle?

Answer 3

• Funnel shaped
• Wide distally to receive sound
• Narrow proximally to connect with external acoustic meatus
• Can be turned towards the direction of sound
• Supported by auricular cartilage

Question 4

What are the surface features of the auricle?

Answer 4

Medial border of helix
Antihelix
Lateral border of helix
Pretragic notch/incisure
Tragus
Intertragic notch/incisure – landmark for otoscope
Antitragus
Cutaneous pouch

Question 5

What is a lateral wall resection?

Answer 5

Side wall of vertical canal removed, for better drainage and ventilation.

Question 6

Why do white cats get squamous cell carcinoma?

Answer 6

Less melanin protection form UV

Question 7

Describe the structure of the external acoustic meatus.

Answer 7

• Canal from base of ear to tympanic membrane
• Vertical and horizontal parts of canal
• Cartilaginous and osseous parts
• Lined with skin containing sebaceous and ceruminous glands (ear wax)

Question 8

What is the middle ear?

Answer 8

• Housed in petrous part of the temporal bone
• Air filled space is the tympanic cavity

Question 9

Describe the structure and innervation of the tympanic membrane.

Answer 9

• Boundary between outer and middle ear
• Double layered epithelium with connective tissue in between
• Pain sensitive – pressure/trauma/infection

Auriculotemporal branch of V3 and auricular branches of X

Question 10

How is pressure equalised in the ear?

Answer 10

The middle ear communicates with the nasopharynx via the auditory tube. Allows equalisation of pressure across tympanic membrane. Salpingopharyngeal muscle does equalisation of pressure.

Question 11

What are the boundaries of the middle ear?

Answer 11

Lateral wall – tympanic membrane

Medial wall – formed by petrous part of temporal bone

Question 12

What is the function of the 2 fenestra at the boundaries of the middle ear?

Answer 12

Mechanical stimuli produced by sound waves pass through these to the inner ear in order to generate nerve impulses.

• Dorsal – vestibular/oval window
• Ventral – cochlear/round window

Question 13

Briefly state the path of sound in the middle ear.

Answer 13

1. Malleus
2. Incus
3. Stapes
4. Vestibular oval window

Question 14

How do the ossicles transmit sound waves to the inner ear?

Answer 14

• Sound waves cause physical movement of tympanic membrane
• Transmitted to malleus, whose handle is embedded in the tympanic membrane.
• Transmitted to vestibular window through chain of ossicles.
• Movement of stapes causes fluid in the internal ear to vibrate, stimulating neuroreceptors in membranous labyrinth, causing sound to be perceived.

Question 15

Describe the association between the middle ear and facial nerve.

Answer 15

• Facial nerve enters internal acoustic meatus with vestibulocochlear nerve
• Crosses temporal bone and emerges at stylomastoid foramen
• Chorda tympani branch leaves facial nerve distally and lies on the upper part of the tympanic membrane

Question 16

What is the function of the inner ear?

Answer 16

Converts mechanical stimuli for sound/positional movements of head into nerve impulses

Question 17

What is the innervation of the inner ear?

Answer 17

Vestibulocochlear nerve – cochlear branch to cochlear and vestibular branch to vestibular

Question 18

What does the inner ear consist of?

Answer 18

• Membranous labyrinth – continuous endolymph. Outer surface lined with perilymphatic cells. Within vestibule
• Enclosed within bony labyrinth – an excavation of the temporal bone. Lined with perilymphatic cells.

Question 19

Name the 3 chambers of the cochlea.

Answer 19

The cochlea is coiled 2.5 times round a central axis of bone – modiolus.

Upper chamber/scala vestibuli
Lower chamber/scala tympani
Middle chamber/cochlear duct/scala media

Question 20

Describe each cochlear chamber.

Answer 20

Upper - begins at vestibular window, continuous with vestibule contains perilymph

Lower camber – ends at cochlea window, contains perilymph, connected to upper chamber at apex of cochlea only

Middle chamber – contains endolymph

Question 21

Describe the mechanism of hearing.

Answer 21

1. Soundwave causes vibration of tympanic membrane
2. Transmitted to oval/vestibular window via chain of ossicles
3. Vibration of perilymph causes vibration of endolymph in cochlear duct: hair cells in duct stimulated
4. Cochlear nerve stimulated

Question 22

What is the role of the cochlear component of the vestibulocochlear nerve?

Answer 22

Has connections with the auditory cortex, the caudal colliculi and facial nucleus.

Question 23

What is the role of the vestibular component of the vestibulocochlear nerve?

Answer 23

Specialised receptors in the inner ear which detect:

• Rotational movements of the head – ampullary crests
• The position of head with respect to gravity and linear acceleration – maculae

Question 24

What are maculae and their function?

Answer 24

Hair project into gelatinous maternal which has supporting crystals of calcium carbonate (stataconia or otoliths)

When gelatinous layers of macula faces ground, membrane pulled by gravity and stimulates receptors.

Question 25

Based on the location to peripheral vestibular disease, the list of possible causes is listed using the mnemonic VITAMIND.

Answer 25

Vascular – not possible based on localisation Infectious/inflammatory – possible with history of otitis externa Trauma/toxic – possible. Some ear drops are ototoxic Anomaly – not possible Metabolic – unlikely. Hyperthyroidism is possible bit would expect other clinical signs. Idiopathic – possible although usually in older dogs Neoplastic – possible Degenerative – not consistent with sudden onset

Question 26

What are the diagnostic tests of vestibular disease?

Answer 26

- Otoscopy – visualising the ear canal./ take samples for cytology, culture and sensitivity. - Tympanic bullae radiography - Blood test to exclude underlying causes. Haematology and thyroid hormone T4 - Myringotomy – use a needle to take samples from the middle ear through the tympanic membrane. Not commonly done in practice but could be.