Ear Flashcards
Name the 3 subdivisions of the ear.
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.
Describe the structure of the outer ear.
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)
What are the properties of the auricle?
- 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
What are the surface features of the auricle?
Medial border of helix
Antihelix
Lateral border of helix
Pretragic notch/incisure
Tragus
Intertragic notch/incisure – landmark for otoscope
Antitragus
Cutaneous pouch
What is a lateral wall resection?
Side wall of vertical canal removed, for better drainage and ventilation.
Why do white cats get squamous cell carcinoma?
Less melanin protection form UV
Describe the structure of the external acoustic meatus.
- 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)
What is the middle ear?
- Housed in petrous part of the temporal bone
- Air filled space is the tympanic cavity
Describe the structure and innervation of the tympanic membrane.
- 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
How is pressure equalised in the ear?
The middle ear communicates with the nasopharynx via the auditory tube. Allows equalisation of pressure across tympanic membrane. Salpingopharyngeal muscle does equalisation of pressure.
What are the boundaries of the middle ear?
Lateral wall – tympanic membrane
Medial wall – formed by petrous part of temporal bone
What is the function of the 2 fenestra at the boundaries of the middle ear?
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
Briefly state the path of sound in the middle ear.
- Malleus
- Incus
- Stapes
- Vestibular oval window
How do the ossicles transmit sound waves to the inner ear?
- 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.
Describe the association between the middle ear and facial nerve.
- 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
What is the function of the inner ear?
Converts mechanical stimuli for sound/positional movements of head into nerve impulses
What is the innervation of the inner ear?
Vestibulocochlear nerve – cochlear branch to cochlear and vestibular branch to vestibular
What does the inner ear consist of?
- 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.
Name the 3 chambers of the cochlea.
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
Describe each cochlear chamber.
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
Describe the mechanism of hearing.
- Soundwave causes vibration of tympanic membrane
- Transmitted to oval/vestibular window via chain of ossicles
- Vibration of perilymph causes vibration of endolymph in cochlear duct: hair cells in duct stimulated
- Cochlear nerve stimulated
What is the role of the cochlear component of the vestibulocochlear nerve?
Has connections with the auditory cortex, the caudal colliculi and facial nucleus.
What is the role of the vestibular component of the vestibulocochlear nerve?
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
What are maculae and their function?
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.
Based on the location to peripheral vestibular disease, the list of possible causes is listed using the mnemonic VITAMIND.
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
What are the diagnostic tests of vestibular disease?
- 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.
