Sound Conduction, Transduction and Treatments Flashcards
What scale is used to measure how loud a sound is?
Decibels (logarithmic scale)
What is the audible range for humans in terms of frequency?
20-20,000 Hz
What is the name given to the wing shaped flap skin and cartilage that makes up the outer ear?
Pinna
Describe the shape of the outer ear and its importance.
It is conical – starts off wide at the external auditory meatus and narrows to the tympanic membrane.
This focuses the noise and increases the pressure on the tympanic membrane.
Amplify some frequencies by resonance effect in the canal
Protects the ear from external threats (hair and wax- has chemical that kill bacteria)
Is the tympanic cavity fluid-filled or air-filled?
Air-filled
State 2 ways in which the ossicles increase the pressure of vibration of the tympanic membrane.
Focussing the vibrations from the large surface area of the tympanic membrane to the small surface area of the oval window – this decrease in surface area means that the pressure is increased .
The incus has a flexible joint with the stapes, such that the ossicles use leverage to increase the force on the oval window.
This amplifies the sound by 30 dB. Hence the fucntion of middle ear is AMPLIFICATION
What is the point of the middle ear? Why isn’t the tympanic membrane continuous with the cochlea?
The cochlea contains fluid, in which you are trying to induce a pressure wave.
If the tympanic membrane was continuous with the cochlea, you would go straight from air to fluid and 99% of the energy will bounce back due to impedance.
Sound waves require more energy to travel through fluid than air so the increase in pressure of vibration allowed by the ossicles is crucial for this conduction
What 2 muscles are involved in making sure that the ossicles aren’t damaged by excessive vibration due to loud noise?
Tensor Tympani.
Stapedius
What is the name given to this reflex (tensor tympani)?
Auditory reflex
What is the latency period of this reflex?
50-100 ms
What is hyperacusis?
Painful sensitivity to low intensity sounds – can occur in conditions that lead to flaccid paralysis of the auditory reflex muscles (e.g. Bell’s Palsy)
Which test is used to determine the site of damage to the auditory system, that is causing hearing loss?
Weber Test
What are the 2 specialised membranes of the cochlea? i.e. where sound enters and leaves
Oval Window.
Round Window
What are the three compartments of the inner ear?
Scala Vestibuli- bony structure
Scala Media- membranous structure
Scala Tympani- bony structure
Which types of fluid do each compartment contain?
Scala Vestibuli + Scala Tympani = perilymph
Scala Media = endolymph
What structure connects the two perilymph compartments?
Helicotrema
Describe how the cochlea functions.
The vibration of the tympanic membrane is conducted and amplified to a vibration of the oval window by the footplate of the stapes.
This vibration induces a pressure wave in the perilymph in the scala vestibuli.
This vibrates the scala media leading to vibration of the basilar membrane.
The round window vibrates as well to equalise the pressure in the cochlea.
Describe the difference in sensitivity of different parts of the basilar membrane.
Higher frequency sounds = base
Lower frequency sounds = apex
What is the Organ of Corti?
The sense organ of the cochlea, which converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve
Where is the Organ of Corti found? and explain how it can discern amplitude and frequencuy of sounds
It lies on top of the basilar membrane and beneath the tectorial membrane.
Higher frequency causes vibration and oscillation at the BASE
Low Freq- apex vibrate
L A B H (Low Ass Breast High)
What are the two types of cell in the organ of corti?
Inner and outer hair cells
Describe the features and function of inner hair cells.
Found on their own.
Not in contact with the tectorial membrane.
Help in transduction of sound into nerve impulses. They have stereocilia that move in response to the movement of endolymph in the scala media.
they have 95% afferent function
Roughly 3500 in the body
Describe the features and function of outer hair cells.
Found in groups of three.
They are in contact with the tectorial membrane. They deflect and allow contact of IHC wiht tectorial membrane
They receive input from the brain (95% efferent). Electromotile so can expand and contract to amplify the amount of vibration (this is the basis of the cochlear amplifier). Hence they are involved in modulation of sensitivity
Damage can result in sensorineural hearing loss.
Roughly 20,000 in the body
Which compartment of the cochlea does the stereocilia of the hair cells project into?
Endolymph (base is in the perilymph)
What internally generated sounds are the outer hair cells responsible for?
Otoacoustic emissions - can be checked in infants
What are stereocilia connected by?
Tip links
What bony conical structure is found at the middle of the cochlea?
Modiolus
Describe what happens when the basilar membrane is displaced upwards.
Draw
Depolarisation.
Stereocilia move away from the modiolus.
K+ channels open. K+ enters from the endolymph.
Ionic change depolarises the cell and nneurotransmitters (glutamate) is released.
Higher amplitudes (louder) of sound will cause greater deflection of stereocilia and K+ channel opening
Describe what happens when the basilar membrane is displaced downwards.
Hyperpolarisation.
Stereocilia move towards the modiolus.
K+ channels close
Describe the difference in K+ and Na+ concentration in the different compartments of the cochlea.
what maintains the different concentrations?
Scala Media = High K+ and Low Na+.
Scala Tympani = High Na+ and Low K+
NOTE: stria vascularis maintains this concentration
Describe the auditory pathway from the cochlea to the primary auditory cortex.
draw it and show where they cross over
what nucleus is responsible for what plane of hearing
Spiral ganglion -> cochlear nuclei -> superior olive -> inferior colliculus -> medial geniculate nucleus -> primary auditory cortex
Cochear- vertical plane
Superior olive- horizontal plane; hence can locate sound due to difference in time
Up to what point is the auditory pathway from one ear ipsilateral?
Cochlear nuclei.
Beyond this point there is bilateral representation
The inferior colliculus receives input from both cochlea. What is the inferior colliculus responsible for?
Reflex associations – turning your head towards loud noise
Describe a phenomenon that is involved in sharpening the signal coming from the cochlea.
Lateral inhibition in inferior colliculi (excited neurones inhibit negibouring neurones)
To which parts of the CNS do collaterals from the auditory pathway go?
Reticular formation.
Cerebellum
In which lobe is the primary auditory cortex?
Temporal
What is the secondary auditory cortex responsible for?
Responding to sounds coming off/on .
Responding to the duration of sound
What is the name given to the axons that project from the medial geniculate nucleus to the primary auditory cortex?
Acoustic radiations (they travel via the internal capsule)
How do you localise short sound burst?
Interaural time delay - difference in arrival of sound between two ears
How do you localise continuous sound?
Interaural intensity difference
Sound coming from one side of the head will have a higher intensity than in other ear
What is conductive hearing loss?
When diseases of the middle ear damage the ossicles or stiffen their joints so that the amplification system is eliminated – results in conductive hearing loss
What is sensorineural hearing loss and what can it be caused by?
When the cochlea or cochlear nerve get damaged, the signal transmitted to the primary auditory cortex is reduced or lost.
It can be caused by acoustic schwannoma (tumour of the cochlear nerve) or cerebellar tumours expanding and putting pressure on the cochlear nerve
What is the term used to describe loss of hearing due to the death of hair cells in normal ageing?
Presbyacusis
what bone is the ear housed?
Petrous part of temporal bone.
How is hearing organised?
tonotopically
- from cochlea: low frequency at apex, high frequency at base
- distribution of frequencies is represented in different nerves
- allows discrimination of pitch which is essential for speech processing
Describe measures of sound
Frequency/pitch (Hz): Cycles per second, perceived tone
Amplitude/loudness (dB): Sound pressure, subjective attribute correlated with physical strength
Human range of hearing:
- Frequency: 20-20,000Hz (due to OHC)
- Loudness: 0 dB to 120 dB sound pressure level (SPL)
How is sound amplitude measured?
decibel scale (a log scale) is useful because the range of sensitivity is very large
> allows us to compress the scale on a graph, and reflect the fact that many physiological processes are non-linear
Very high decibels can cause damage to ears.
How does hearing change over time?
- hearing acuity decreases with age, particularly higher frequencies
- medium and low frequencies could be affected with the progression of a hearing loss
What are the aims of the hearing assessment
answer these questions:
- Is there a hearing loss? If yes…
- Of what degree?
- Of what type?
What procedures are used in the hearing assessment?
- Tuning fork
- Audiometry
- Central processing assessment
- Tympanometry
- Otoacustic Emission
- Electrocochleography
- Evoked potentials
Why is a tuning fork (512Hz) used?
Explain the details of this test
- used to establish the probable presence or absence of a hearing loss with a significant conductive component
- used to provide early and general information, when audiometry is not available or possible
- determine where the issue is
- tests bone conduction.
There could be false negative on the Rinne test
What is pure tone audiometry?
most common test
- science of measuring hearing acuity for variations in sound intensity and frequency
- audiometer is the device used to produce sound of varying intensity and frequency
- air conduction
What is an audiogram?
A graph showing hearing thresholds at different frequencies in dB
- used to define if there is a hearing loss or not
- normal hearing threshold is located between 0 - 20dB
How does central processing assessment work?
- assessment of hearing abilities other than detection
- verbal and non verbal tests
- examples:
- > sound localization
- > filtered speech
- > speech in noise
How does an otoacoustic emissions probe work?
- normal cochlea produces low-intensity sounds called OAEs
- sounds are produced specifically by the outer hair cells as they expand and contract
- test is often part of the newborn hearing screening and hearing loss monitoring
How do auditory evoked potentials work?
what areas does it measure and what evokes it?
- Potentials generated by auditory stimuli
- Can record electrical changes on sound stimulus
Electrocochleography
- 0.2-4.0 ms, electrical activity from the cochlea and eighth nerve
- evoked by clicks or tone burst.
Auditory Brainstem Response (ABR)
- 1.5-10.0 ms, electrical activity from the eighth nerve and brainstem nuclei and tracts
- evoked by clicks
Late Responses (N1-P2, P300, MMN, and more)
- 80-500+ ms, electrical activity from the primary auditory and association cortex
- evoked by tone burst and oddball paradigm
How is an auditory brainstem response used? give relevant features
- Electrical responses from the auditory pathway
- ABR is more commonly used in clinic
- Does not require attention from the patient
- Alterations in the latency of waves can point to the location of the deficit
- objective measurement commonly used in babies and children
wave I = auditory nerve
wave III = cochlear nucleus
wave V = superior olive
When are cortical potentials used?
- could be affected on neurological conditions or processing problems (late phase of auditory evoked potentials)
- looks at how sound is processed
What are the different types of hearing loss?
- conductive hearing loss: problem is located in outer or middle ear
- sensorineural hearing loss: problem is located in the inner ear or the auditory nerve
- mixed hearing loss: conduction and transduction of sound are affected. Problem affects more than one part of the ear.
What do different types of hearing loss look like on audiometry?
draw
Conductive
> dissociation between air and bone conduction
-Sensorineural
> air and bone are the same
> information is not transduced to cortex
Mixed
> bone and air conduction affected
> transduction is affected
What are the severities of hearing loss?
depending on where the hearing thresholds are located, hearing loss can be classified as mild, moderate, severe or profound
What are the causes of hearing loss?
Conductive hearing loss
- Outer Ear
- > wax
- > foreign body
- Middle ear
- > otitis: bubbles seen through ear drums
- > otosclerosis
Sensorineural hearing loss
- Inner ear
- > presbycusis (loss with age of OHC)
- > ototoxicity (after exposure to drugs etc.)
- Nerve
- > VIII tumour
How is hearing loss treated?
- underlying cause
- cochlear implants
- hearing aids
- brainstem implants
How do hearing aids work?
- amplify the sound, does not replace structures
- only works if there is preservation of some hair cells
- depending on the type, degree and characteristics of the patient can tailor (eg frequencies)
How does a cochlear implant work? Give requirements for this type of treatment
- replaces the function of the hair cells
- by receiving sound, analysing it, transform it into electrical signals
- sends an electric impulse directly to the auditory nerve
Requirements:
> needs a functioning auditory nerve
> works when there is a hair cell problem
How does a brainstem implant work?
- when the auditory nerves are the affected structures, electrical signals can be sent to a set of electrodes placed directly into the brainstem
- very risky, so is advised for people with bilateral important auditory nerve damage
- not the best results
What does tympanometry entail
examination used to test:
> condition of the middle ear
> mobility of the eardrum (tympanic membrane)
> conduction bones by creating variations of air pressure in the ear canal
Below are the results of tympanometry graphs.
Tell me what they mean
Most common results:
A - normal
- > lower means resistant to sound
- > higher mean too much sound going in
C - negative middle ear pressure
B:
- Middle ear effusion
- Occluded ear canal
- Perforation of tympanic membrane
- Eustachian (pharyngotympanic) tube dysfunction
Describe the structure and function of the basilar membrane
- structure where the organ of Corti lies in is the basilar membrane
- arranged tonotopically, using the same principle as a xylophone (thicker at the bottom than the top giving it physical properties relating to sound)
- high frequency vibrates at high amplitude near the base
- low frequency vibrates at high amplitude near the apex
- sensitive to different frequencies at difference point along its length
Describe the hearing function of the inner ear
cochlea
- transduce vibration into nervous impulses
- produces a frequency (or pitch) and intensity (or loudness) analysis of the sound
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