Introduction to the Audio-Vestibular System Flashcards
1
Q
How is sound created?
A
By a variation in air pressure over time
2
Q
Which parts of the ear are filled with air and which are filled with fluid?
A
-The ear canal and the middle ear are filled with air
-The inner ear is filled with fluid
3
Q
Which structure separates the outer and the middle ear?
A
The tympanic membrane (eardrum)
4
Q
Which structure separates the middle and the inner ear?
A
The oval window
5
Q
What is the function of the pinna?
A
-Used more actively in animals
-In humans it gathers sound waves and protects
-Acts as acoustic filter- aids sound localisation and amplification
6
Q
How much does the pinna contribute to sound amplification?
A
-Amplifies sound approximately 10-15 dB for 1.5-7 kHz
-Each fold in the pinna is unique so the way sound is transferred differs in each individual
7
Q
What happens to sound once it enters the outer ear (middle ear function)?
A
-Sound is transferred to the tympanic membrane which vibrates and moves the ossicles
-The ossicles work together with lever action to amplify and enhance sound to the oval window
-Air is essential for this process
8
Q
What happens if you get fluid in the middle ear?
A
-Known as otitis media with effusion (collection of non-infected fluid in the middle ear space)
-Causes hearing loss
9
Q
What are the names of the ossicles in the order that they are found in the middle ear?
A
- Malleus
- Incus
- Stapes
10
Q
Describe the function of the middle ear (include an explanation for the impedance mismatch)
A
-There is an impedance mismatch between the air and the fluid in the cochlea
-There is a lever-action between the tympanic membrane and the oval window as well as an area ratio which provides the gain required to compensate for the impedance mismatch
11
Q
What is gain a function of?
A
Frequency
12
Q
What structures does the inner ear consist of?
A
-Cochlea (hearing portion)
-Semicircular canals (balance portion)
13
Q
How many fluid compartments does the cochlea contain?
A
2- one contains perilymph and one contains endolymph
14
Q
What are the two access points to the cochlea?
A
Oval window and round window which are in close proximity with each other
15
Q
What joins the oval window and round window?
A
Helicotrema
16
Q
In a elongated cochlea, where are the scala vestibuli, scala tympani and scala media?
A
-Scala vestibuli and scala tympani ‘sandwich’ the scala media
-The scala vestibuli is close to the vestibular system
-The scala tympani is close to the tympanic membrane
17
Q
What fluid does the scala vestibuli and scala tympani contain?
A
Perilymph which is low in K+
18
Q
What fluid does the scala media contain?
A
Endolymph which is high in K+
19
Q
What is the function of the oval window?
A
-The oval window is located at the footplate of the stapes
-When the footplate vibrates, the cochlear fluid is set into motion
20
Q
What is the function of the round window?
A
-Functions as a pressure relief port for the fluid that is set into motion
-Since fluid is incompressible it is squeezed out through the path of least resistance by the movement of the stapes in the oval window
21
Q
What are the three functions of the cochlea?
A
- Splits complex sounds into simple components (i.e. frequency or fourier analysis)
- Amplifies sound signal
- Converts mechanical vibrations into electrical signal (i.e. sensory/ mechanoelectrical transduction)
22
Q
What happens to the basilar membrane when the stapes vibrates?
A
-Waves are created in the cochlear fluids which travel along the basilar membrane, moving it up and down in a travelling wave
23
Q
What factors determine the optimal vibration frequency at each point along the basilar membrane?
A
-Stiffness
-Mass
24
Q
Describe how the optimal frequency changes along the basilar membrane in relation to the stiffness and mass of the membrane
A
At the base: Narrow and stiff with less mass
At the apex: Wider and floppier with more mass
25
What is the organ of Corti?
-The sensory organ of the cochlea
-Fluid runs in the organ of Corti so cells are more free to move than in other epithelia
26
What type of cells are found in the cochlea and in what arrangement?
-Two kinds of hair cells are found in the cochlea
-There are three rows of outer hair cells on the lateral side of the cochlea
-One row of inner hair cells
27
What happens to the hair cells as the basilar membrane vibrates?
-The hair cells are found along the whole length of the basilar membrane
-When the basilar membrane vibrates at a particular frequency the hair cells at those points in the basilar membrane get activated
28
What is the tectorial membrane?
-A ribbon-like strip of extracellular matrix that spirals the length of the cochlea
-Found at the top above the inner hair cells
29
What is the mechanism of transduction in the cochlea?
-Transduction relies on the movement of the organ of Corti
-Hair cells are the sensory transducers
30
Describe the anatomy of a hair cell
-Hair cells have a bundle of stereocilia projecting out of their apical surface
-Rows of stereocilia are bound together
-The longest stereocilium is known as the kinocilium
-Two proteins are coiled together in strand to form the tip link
-Individual rod membrane-bound filled with actin which makes it stiff
31
What kind of pattern is the hair bundle arranged in?
Staircase pattern
32
What fluid does the hair cell project into and what is its charge?
-Endolymph
-Positively charged (high in K+) (unusual)
33
What happens when the stereocilia are deflected?
1. K+ ions enter from endolymph
2. Pushing the stereocilia bundle towards the longest stereocilia depolarises the cell
3. Pushing the stereocilia bundle away from the largest stereocilia hyperpolarises the cell
34
When does deflection of stereocilia create an excitatory response and when does it create an inhibitory response? What happens to the transduction links?
Excitatory- towards longer stereocilium (transduction links stretched)
Inhibitory- towards shorter stereocilium (transduction links compressed)
35
What happens to the mV of the membrane when the hair cell bundle is pushed towards the tallest stereocilium vs in the other direction? What happens at the synapse?
-Resting at -50 mV
-If the hair cell bundle is pushed towards the tallest stereocilium the mV moves up (depolarisation)
-Increase in firing rate at synapse- saturates
-If the hair cell bundle is pushed in the other direction the mV decreases (hyperpolarisation)
-Activity shuts off at synapse
36
How many afferent nerve fibres does the cochlea have?
30,000
37
What is the most common type of afferent nerve fibre found in the cochlea?
Type I (95%)
38
Describe type I nerve fibres
-Myelinated
-Synapse with IHC
-One type I nerve fibre will only contact one inner hair cell
-Inner hair cells have up to 10-20 nerve fibres
39
What are type II nerve fibres?
-5% of the afferent nerve fibres in the cochlea
-Unmyelinated
-Synapse with outer hair cells
-1 type II nerve fibre per 5-100 outer hair cells
-Have cell bodies in the spiral ganglion
-Only really activated when there is a lot of noise
40
Describe the tuning curve of an auditory nerve
-Auditory nerve fibres are sharply tuned to a specific frequency
-Typical tuning curve has a V shape and the bottom is known as a characteristic frequency
41
What is phase-locking?
-Phase-locking is a property of auditory nerve fibres up to 2-3 kHz
-It means that the firing of a nerve fibre can synchronise to the temporal structure of a sound (it is also known as temporal coding)
-It only works at low frequencies because as the frequency increases the peaks get closer together and the nerve fibres cannot keep up
42
What structure comprise the auditory forebrain?
Cortex, medial geniculate nucleus
43
What structures comprise the auditory brainstem?
Inferior colliculus, lateral lemniscus, superior olive, cochlear nucleus
44
How does the cochlea amplify sound?
-Outer hair cells amplify membrane vibrations, enhancing sensitivity and frequency selectivity
-Enhancement about 40 dB
-Outer hair cells can contract and lengthen which removes dampening at the basilar membrane
-Activation of outer hair cells causes increased movement which leads to the activation of inner hair cells
45
What is the function of the vestibular system?
-Detect head movement and counter the movement with reflex eye movements and postural adjustments to maintain stable vision
46
How many sensory organs does the vestibular system have? What is the sensory receptor?
-Five sensory end organs (two types)
-Hair cells are the sensory receptors
47
What are the two types of vestibular hair cells?
1. Type I
-Flask shaped
-Enclosed in nerve
-Calyx
2. Type II
-Cylindrical
-Multiple bouton-like nerve endings
48
What are otolith organs?
-Otolith organs detect linear acceleration, gravity and changes in the head angle
-The utriculus and the sacculus are the 2 otolith organs in the human vestibular system
49
What is the function of the otolithic membrane?
The otolithic membrane is found over the surface of the epithelium and covers hair cell surfaces
50
What are otoconia?
-CaCO3 crystals which sit on top of the epithelium
-Otoconia have inertia and continue moving even if you stop the movement of the hair cells
51
What are the cristae organs?
-Semi-circular canal organs (ampullae) detect angular acceleration
-Three saddle- shaped cristae containing hair cells
-Gelatinous mass cupula at the surface which stereocilia project into
-Canals and organs perpendicular
52
What happens to the vestibular hair cells when the head is moved?
-Increases the depolarisation
-Inertia causes the fluid to lag behind the canal
53
What is the function of the outer ear?
Funnels and directs sounds to the middle ear
54
What are the main anatomical structures of the outer ear?
1. "Pinna" or "auricle"- different shapes in different people, different shapes in different people
2. Ear canal/ external acoustic meatus- cartilaginous tubular component about 4cm in length
3. Eardrum/ tympanic membrane- end of outer ear (lateral face)
55
What is the ear canal (external acoustic meatus) made of?
-Medial two thirds of the canal is bony
-The lateral third is cartilaginous
56
What lines the ear canal?
-Hair follicles and glands
57
What do the glands lining the ear canal produce?
A waxy oil called "cerumen" which is also known as earwax
58
What happens to wax once it forms?
-Most often makes its way to the opening of the ear where it will fall out or be removed by washing
-Wax may also harden within the ear canal and block the ear which is a common cause of hearing loss
59
What is known as inflammation of the ear canal?
-Otitis externa ("swimmer's ear")
-Causes ear pain, itchiness in the ear canal, discharge of liquid or pus from the ear and some degree of temporary hearing loss
-Caused by Pseudomonas aeruginosa
60
What is the tympanic membrane comprised of?
-3 layers of tissue (outer epidermal layer facing outwards, fibrous middle layer full of collagen and connective tissue, mucous membrane on its innermost surface)
61
What holds the tympanic membrane in place?
A thick ring of cartilage
62
How many quadrants does the tympanic membrane have?
4- antero-superior, antero-inferior, postero-inferior, postero-superior
63
Which quadrant is intervention preferred in and why?
The postero-inferior because it does not have the cone of light, no blood vessels and no nerves
64
What is myringotomy?
-A procedure where a hole is cut in the postero-inferior quadrant in order to relieve pressure caused by fluid buildup in the middle ear
-Ear tubes called grommets can be inserted into this hole which allows air to enter the middle ear
65
What is the function of the middle ear in normal hearing?
-The middle ear contains three bones called ossicles: malleus, incus and stapes
-In the middle ear air vibrations are converted to fluid vibrations to ensure that sound energy does not get lost due to the impedance mismatch between the air in the ear canal and the fluid in the cochlea
-The middle ear provides amplification of 20-30 dB
66
How does the tensor tympani muscle work?
-Pulls malleus forward
-Flattens the tympanic membrane which changes the way the information is going to be transferred through the middle ear
-Increases the tension across the tympanic membrane by stiffening it
-This dampens the sound of chewing and talking
67
What is the role of the stapedius muscle?
-The stapedius muscle is activated by loud sounds (>70-100 dB)
-Sound goes into the inner ear, then the brain and motor facial nerve is activated to stop too much energy coming into the ear
-Stapedius muscle retracts the stapes from the oval window to avoid damaging the cochlea
-Acoustic reflex takes 40ms to take effect
68
What kind of loud sounds is the acoustic reflex ineffective against?
Very sudden loud sounds such as gunshots- since it takes 40ms to take effect
69
What are the most basic tests of middle ear function?
The tuning fork tests- Weber's test and Rinne's test
70
How is the Weber's test carried out?
1. Base of the struck tuning fork is placed on the bridge of the forehead, nose, or teeth
-If hearing normal: no lateralisation of sound, heard in both ears
-If unilateral conducive loss: sound lateralises towards the affected ear
-If unilateral sensorineural hearing loss: sound lateralises to normal or better-hearing side
71
How is the Rinne's test carried out?
1. Struck tuning fork placed on the mastoid bone behind the ear
2. Patient indicates when the sound is no longer heard
3. Move the fork (held at base) beside ear and as if now audible
-If normal test: the air conduction will be better than the bone conduction so the patient hears the fork at their ear
-If conductive hearing loss: bone conduction better than air conduction so the patient cannot hear the fork at their ear
72
What is the name of the test now more commonly used in clinic to test middle ear function?
-Tympanometry
-Objective test of middle ear function
-Measures the "admittance" or "compliance" of the middle ear
-226 Hz tone is generated by the probe tip and inserted into the external ear canal
-When the sound strikes the tympanic membrane it causes vibration of the middle ear
-Energy is reflected out from the tympanic membrane
73
What does a normal tympanogram look like?
Has a sharp peak usually around 1
74
What kind of problems can low compliance, excess compliance and negative ME pressure indicate?
Low compliance: shows flat/ shallow curve- can be caused by fluid, membrane not moving inwards, scarring, rip
Excess compliance: very high peak- can be bone disarticulation, bones not connected to each other properly or moving too freely
Negative ME pressure: Normal shape but shifted away from 0- indicates infection, Eustachian tube problem
75
What is microtia?
-Condition where ear does not develop fully during the first trimester of pregnancy
-More common in males
-90% of time it only affects one ear
-Associated with absent ear canals ("canal/ aural atresia") or narrow ear canals ("canal stenosis")
-Not genetically inherited in most cases
-Sometimes associated with other craniofacial syndroms e.g. Treacher Collins Syndrome
76
What are the four types of microtia?
Type 1: Ear smaller than normal, key features present but may have minor alterations
Type 2: Some features are missing, pinna misshapen, much of the lower 2/3 of the ear is still present, ear canal may be present but frequently narrow
Type 3: Most common, only small peanut-shaped remnant of the ear lobe remains, ear canal usually absent
Type 4: Complete absence of an external ear, also called "anotia", rare
77
What is otosclerosis?
-Abnormal bone growth in the middle ear
-Commonly affects stapes footplate and reduces its mobility
-Causes mild to severe hearing loss
-Starts to affect people in their 20s and 30s
-Can spread to the inner ear
-80-90% can successfully be treated with hearing aids or surgery
78
What ae the two surgeries used to treat otosclerosis?
1. Stapedectomy- where a prosthetic device is used to replace the abnormal stapes (whole stapes is removed and replaced)
2. Stapedotomy: Stapes is laser-adapted for better movement (no removal of stapes)
79
What is the evidence that otosclerosis has a genetic component?
-Crompton et al (2019)
-Patients with a family history (40%) found to have an earlier age of onset and a higher incidence of bilateral disease and vertigo than non-familial subjects
80
What is meant by mechanosensitivity?
-Sensory patches of the inner ear are mechanosensitive which means that they are able to detect mechanical signals/ vibrations
-These are movements of the fluid in the cochlea
-The energy is then converted from vibrations to something the brain can understand
81
What is the name of the cochlear sensory epithelium?
Organ of Corti
82
How does the organ of Corti differ in its function to vestibular sensory epithelia?
-The organ of Corti detects fluid motions in the cochlea created by sound waves and converts them to electrical signals
-The vestibular sensory epithelia detect movements created by changes in the position of the head such as linear acceleration/ deceleration or rotational acceleration
83
Is the kinocilium (tallest stereocilium) present on hair cells for our whole lives?
Yes and no
Yes- in the vestibular hair cells
No- present on the organ of Corti hair cells during development and then absent from mature organ of Corti hair cells
84
What kind of organisation do hair bundles display?
Polarised organisation- all of the hair bundles point in the same direction
85
What is kinocilium made of?
Tubulin instead of actin like the stereocilia
86
How are the stereocilia connected to each other in a hair bundle? How does this aid their function?
-Through tip links, top connectors and shaft connectors
-The tip links gate the mechano-electrical transduction (MET) channels-key to how hair cells sense vibrations
-The tip link gets stretched when it moves over which open ion channels to allow K+ influx into the cell
-Shaft connectors keep the structure together
-Top connectors are protein links between stereocilia to keep them cohesive
87
What is the function of the cupula in the vestibular system?
-Hair bundles are embedded in the cupula
-Cupula will move when the epithelia have been stimulated
-Hair bundles oriented in the same direction
-Movement of the cupula will signal a single stimulus
-All sending information to the brain at the same time
88
How are maculae polarised?
-Along the striola
