Anatomy of the Ear
Describe the pathway of sound as it enters the ear and becomes a neural impulse.
1. Sound waves cause physical movement of the tympanic membrane. 2. Transmitted to malleus, whose handle is embedded in the tympanic membrane. 3. Malleus --> incus --> stapes --> vestibular/oval window. 4. Movement of stapes causes vibrations of perilymph, which causes vibrations of endolymph in cochlear duct, which causes vibrations of the basilar membrane. Hair cells embedded in the basilar membrane are stimulated. 5. Hair cells are depolarised and they release neurotransmitter that synapses with the fibres of the cochlear nerve. 6. Cochlear nerve makes connections with auditory cortex (conscious perception), caudal colliculi (reflex head movement) or facial nucleus (reflex ear movement).
All three parts of the ear - external, middle and internal - are concerned with hearing (cochlear), which part is also concerned with balance (vestibular apparatus)?
Inner
What feature makes it harder to view the ear with an otoscope and can reduce drainage of the ear?
The external acoustic meatus has a vertical part, as well as a horizontal part.
What do the specialised receptors in the inner ear (vestibular apparatus)?
- The maculae record the position of the head with respect to gravity - The cristae (ampullary crests) record the rotational movements of the head.
Which structures is the middle ear composed of?
- Tympanic cavity (air-filled pace) - Auditory ossicles (located dorsally) - Audiotory tube
What symptoms are associated with vestibular disease?
- Abnormal nystagmus - Head tilt/circling Can be central or peripheral.
What receptors are associated with the vestibular component of the vestibulcochlear nerve?
- Ampullary crests for movement of the head - Maculae for position of the head
What does the cochlear component of the vestibulocochlear nerve make connections with?
- Auditory cortex - Caudal Colliculi - Facial nucleus
Which structures compose the external ear?
- Auricle/pinna - External acoustic meatus
What is the innervation of the tympanic membrane?
- Auriculotemporal branch of V3 (mandibular, trigeminal) - Auricular branch of vagus (an otoscope can cause syncope, AKA fainting)
What is the function of the inner ear?
- Converts mechanical stimuli for sound --> nerve impulses - Converts positional movements of head with respect to gravity --> nerve impulses
What clinically inspecting the eardrum, what is important to note?
- Curvature - Integrity - Colour
What are the 3 subdivisions of the ear?
- External/outer ear - Middle ear - Internal/inner ear
What reflexes are associated with the cochlear component of the vestibulcochlear nerve?
- Reflex movement of head towards auditory stimuli (caudal colliculi) - Reflex movement of the ears towards auditory stimuli (facial nucleus)
The external acoustic meatus is lined with skin contain what glands (2 types)?
- Sebaceous - Ceruminous (waxy secretion)
One end of each semicircular duct is ampullated/expanded close to the utriculus, what is found here?
Ampullary crests - detect movement of the head.
What structure connects the middle ear and the nasopharynx?
Auditory tube A throat infection can spread to the middle ear.
The cartilage of the auditory tube of a horse is deficient ventrally and the mucosa forms a pouch, what is this pouch called?
Auditory tube diverticulum (gutteral pouch)
Where does the external acoustic meatus begin and end?
Base of the auricle --> tympanic membrane
The external acoustic meatus has has cartilaginous and osseous parts, where do they attach?
Cartilaginous -> pinna Osseous -> skull
What is the function of the tensor tympani?
Dampens the sound of chewing When tensed, the muscle pulls the malleus medially, tensing the tympanic membrane and damping vibration in the ear ossicles and thereby reducing the perceived amplitude of sounds. It is innervated by the mandibular division of the trigeminal nerve (same as muscles of mastication).
In which window does the stapes bone sit?
Dorsal - vestibular/oval
The medial wall (petrous part of temporal bone) of the middle ear has 2 fenestra, what are the called?
Dorsal - vestibular/oval Mechanical stimuli produced by sound waves is amplified by the ossicles and passes through to the inner ear for conversion into nerve impulses. Ventral - cochlear/round Allows fluid in the cochlea to move, which in turn ensures that hair cells of the basilar membrane will be stimulated.
Describe the structure of the tympanic membrane:
Double layered epithelium with connective tissue in between.
What is the alternative name for the tympanic membrane?
Eardrum
What are the ampullary crests bathed in?
Endolymph When it moves due to head movements it stimulates the ampullary crests.
What does the membranous labyrinth contain?
Endolymph - high potassium and low sodium makes it more excitable and sensitive to movement (hair cells).
As well as the vestibulocochlear nerve, which cranial nerve has a close relationship with the middle ear?
Facial nerve The facial nerve enters the internal acoustic meatus with the vestibulocochlear nerve, crosses the temporal bone and emerges at the stylomastoid foramen. The chords tympani branch (provides taste sensation to rostral 2/3 of tongue) leaves the facial nerve distally and lies on the upper part of the tympanic membrane. The function of the facial nerve can be impaired by an ear infection.
What do the stereocilia of the hair cells of the maculae receptors project into?
Gelatinous matrix It has supporting crystals of calcium carbonate that makes it heavy (otolithic membrane). When the head moves the effect of gravity pulls on the structures and causes the cells to fire.
What receptors are associated with the cochlear component of the vestibulcochlear nerve?
Hair cells in the cochlear duct
What is the function of the auditory tube?
Important for equalisation of pressure across the tympanic membrane.
What is the function of the tympanic bulla?
It helps detect and improve the perception of sounds of the highest and lowest frequencies Rodents have relatively large tympanic bulla.
What is the main fuction of the auricle?
It is wide distally to receive sound and can be turned toward the direction of sound. It helps channel sound to the auditory apparatus. Some animals ears are also very expressive and can be used for communication.
What are the lateral and medial boundaries of the middle ear?
Lateral - tympanic membrane Medial - petrous part of temporal bone
What is the surgery called that involves cutting down into the ear canal and removing cartilage to help improve drainage and ventilation?
Lateral wall resection, or Zepp procedure.
What are the 3 ossicles?
Malleus, incus and stapes
Which nerves and blood vessels are found in the auditory tube diverticulum?
Medial Compartment: - Cranial nerves IX, X, XI, XII. - Continuation of the sympathetic trunk beyond the cranial cervical ganglion. - Internal carotid artery. Lateral Compartment: - Cranial nerve VII - limited contact with the dorsal part of the compartment. - External carotid artery crosses the lateral wall of the lateral compartment in its approach (as maxillary artery) to the atlas canal. - The external maxillary vein is also visible. Fungal spores can enter via the pharyngeal opening and cause a mycotic infection, which can erode through blood vessels (nosebleeds) and nerves.
Where are the ossicles located?
Middle ear, dorsally
What is a potential problem with pendulous ears?
More predisposed to ear problems. As the ear folds, it covers the ear canal and prohibits air from entering and drying the canal. The result is a moist, warm ear canal that is a perfect environment for organisms to grow.
Which part of the ear are infections most serious?
Otitis interna An animal can develop otitis external, otitis media and otitis interna. Otitis interna is the most serious as it will affect both hearing and balance, and if it spreads deeper it can cause meningitis.
What is the tympanic membrane the boundary between?
Outer and middle ear
If the facial nerve is damaged will the ear stay erect?
Partially Even thought the facial nerve innervates the auricular muscles, the ears will stay partially direct due to the cervical nerves.
What does the bony labyrinth contain?
Perilymph
What reflex is the tendon of stapedius m. involved in?
Sound attenuation reflex Innervated by the facial nerve. If there is a loud sound it will contract and brace the stapes and prevent it from passing the signal onto the the auditory apparatus, where it could cause damage. A sudden gunshot, or similar, is too fast for the reflex , which could lead to damage.
What is the Organ of Corti?
Receptor organ for hearing It is located in the mammalian cochlea.
What type of cancer is common on the tips of white cats ears?
Squamous-cell carcinoma
What type of joint is found between the ossicles?
Synovial
Which bone are the middle and inner ear housed in?
Temporal bone, petrous part (not squamous part)
What is embedded in the tympanic membrane?
The first ossicle, the malleus. When looking at the eardum with an otoscope the stria mallearis can be seen, the 'hammer stripe'.
Where does the spiral cochlear duct arise from?
The saccule
What do the semicircular canals extend from?
The utricle
Maculae are receptors that monitor the position of the head with respect to gravity, where are they located?
The walls of the utriculus and sacculus (within the vestibule)
What structure supports the auricle?
Thin auricular cartilage Helps keep the ear partially erect.
Why are waxy secretions important in the external acoustic meatus?
Traps particles that come into the ear and stops them reaching the ear drum. However, too much can reduce hearing and can act as a nidus for infection. Ear mites will feed on ear wax.
What is the bulbous expansion of the temporal bone that houses the ventral part of the middle ear called?
Tympanic bulla Prominent in cats and dogs, but not so much in horses.
What reflex is associated with the vestibular component of the vestibulcochlear nerve?
Vestibulo-ocular reflex Elicits eye movement by stimulating the vestibular system to stabilize images on the retinas.
Whats is the auricle/pinna?
Visible part of the ear
Is the ear drum sensitive to pain?
Yes Pressure (airplane), trauma and infection.