Anatomy of the Ear
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What are the three divisions of the ear?
External, middle and inner ear
What two components make up the external ear?
The auricle/pinna and external acoustic meatus
The auricle of the ear is mobile and can be orientated towards sound, which nerve controls the muscles responsible for this movement?
Facial nerve (cranial nerve VII)
What cartilage is found at the base of the auricular cartilage?
Annular cartilage - it anchors the auricle to the skull
The .. is a projection at the base of the ear?
Which side of the ear does skin adhere more firmly to?
The concave (inner) surface
Which side of the ear do the vessels run on?
The convex (outer) surface
What shape, roughly, is the external acoustic meatus?
Why is it difficult to examine the entire external acoustic meatus with an otoscope?
If you do not pull the ear straight then you only see to the angle of the ear canal with an otoscope. This can be irritating or painful so very often sedation is needed to see all the way down to the ear drum.
What components make up the middle ear?
- Tympanic membrane
- Tympanic cavity/bulla
- Auditory ossicles
- Auditory tube
What bone does the tympanic cavity lie within?
The petrous temporal bone
What normally fills the tympanic bulla?
In what species is the tympanic bulla divided?
How many auditory ossicles are there? What are they called?
3 - malleus, incus, stapes
What does the tympanic membrane look like when examined using an otoscope?
It appears as a blueish membrane at the distal end of the external ear canal
Which auditory ossicle is attached to the tympanic membrane/vestibular window?
- Tympanic membrane - malleus
- Vestibular window - stapes
What is the function of muscles attached to the auditory ossicles?
They have a protective function - when the muscles around the ossicles contract they reduce mobility of the bony chain and increase tension in the ear drum, therefore reducing oscillation. This reduces the transfer of pressure to the inner ear and therefore reduces the risk of damage to receptors in the inner ear.
What are the 3 ways sound can cross from the middle to inner ear?
- 1 - Sound waves travel down from the external ear, strike the tympanic membrane and cause it to vibrate. The vibrations are transferred through the ossicles to the vestibular window which creates pressure waves in the fluid contain in the inner ear. These then stimulate neuroreceptors within the inner ear.
- 2 - Vibrations through the skull are picked up by the inner ear. Snakes do most of their hearing through bone.
- 3 - Sound waves travel down, strike the tympanic membrane and cause vibration of air within the tympanic bulla. This is transferred to the cochlear window and causes vibrations in the inner ear.
Which structure connects the tympanic cavity to the nasopharynx?
The auditory tube
Where are the auditory tubes located in most domestic species? And in horses?
- Lateral wall of nasopharynx
- Gutteral pouches
What is the function of the auditory tubes?
Allows equalisation of pressure on either side of the tympanic membrane and allows drainage of material from the tympanic cavity
What structures make up the inner ear?
- Semicircular ducts
How many semicircular ducts are present in the inner ear?
What further receptors are present in the utriculus and sacculus?
Maculae (otolith organs)
Describe how the utriculus and sacculus determine the position of the head relative to gravity
The crystals on the surface of maculae are stuck onto the sensory hairs of these receptors. When the position of the crystals is affected by gravity these tug on the sensory hairs and affect nerve impulses, therefore determining the position of the head relative to gravity.
How are the semicircular ducts orientated?
At right angles to each other
At the end of each semicircular duct is a widening called the ...?
How do the semicircular ducts and ampullae detect head movement?
When you start spinning, inertia means that fluid in the ducts moves relative to the ducts - this movement stimulates sensory hairs which are present in the ampullae. This allows you to detect the head is moving. After you stop moving you still get stimulation of the sensory hairs due to inertia and movement of the fluid so this is why you feel dizzy and fall over.
What is nystagmus?
'Flicking' movement of the eyes in response to movement of fluid in semicircular canals
How does hearing occur in the inner ear?
Vibrations at the vestibular window and transferred to the cochlear window and transmitted through the perilymphatic space. This causes vibration of basement membrane and movement of sensory cilia which is registered by neuroreceptors and transmitted to the vestibulocochlear nerve (CN VIII)
What nerves supply the ear?
Vestibulocochlear nerve (CN VIII), Facial nerve (CN VII) and mandibular nerve V
What arteries supply the a) external b) middle and inner ear?
- a) External carotids
- b) Occipital artery
What is an aural haematoma?
Haematoma under the skin of the pinna
What nerves can be damaged by middle ear disease?
Facial nerve and the sympathetic supply to the eye
What imaging modalities are used to examine the ear?
Radiography (widely used) and CT/MRI (for clear imaging of inner and middle ear)
What should the inner ear look like on MRI?
A duck above the tympanic bulla
What is the term for inflammation of the ear? How does this differ for the external, middle and inner ear?
- Otitis externa, otitis media, otitis interna
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