The Auditory System Flashcards
What is the definition of sound?
sounds are audible variations in air pressure (compression and rarefaction)
the sequence of alternating pressure waves propagate through compressible media such as air or water, as a result of vibrating objects (e.g. tuning fork)
the pressure waves cause local regions of compression and rarefaction
What frequencies of sound can humans detect?
What frequencies are they most sensitive to?
humans can detect sound waves with frequencies from 20 - 20,000 Hz (cycles per second)
the corresponding wavelengths of sound waves range from 17m to 17mm
most sensitive to frequencies between 1,000 - 4,000 Hz (cycles per second)
What is the speed of sound?
343 m/sec
At what frequency does the human ear hear sounds the best?
because of the shape of the pinna and outer ear, the human ear heards sounds at frequencies of around 3kHz best
this is the frequency at which most human sounds are made
Wha scale is used to measure sound?
What type of scale is this and how can it vary between individuals?
sound is measured in decibels (dB)
this is a logarithmic scale
there is subject variabilty meaning some people’s threshold for hearing can be in the minus e.g. -10 dB
young children’s threshold of discomfort/pain can be lower than adults
How does the tympanic membrane coordinate with the ossicles to transmit sound?
What amplification is produced?
the tympanic membrane oscillates in response to sound
the inward movement of the membrane causes the ossciles to move
the tympanic membrane is 20x larger in diameter than the oval window, therefore there is a 20 fold increase in force
together with the lever mechanism of the ossicles, this produces a 20-30dB amplification
Label the components of the outer, middle and inner ear
Outer ear:
- pinna
- auditory canal
- tympanic membrane
Middle ear:
- ossicles
Inner ear:
- semicircular canals
- oval window
- cochlea
- pharyngotympanic (auditory) tube
Label the inner ear
What is the structure of the cochlea like?
it is a spiral-shaped organ of 2 3/4 turns
it consists of bony and membranous labyrinths
Where does the vestibuli meet the scala tympani?
at an apex known as helicotrema
What is the composition of perilymph and endolymph found in the inner ear like?
perilymph:
- closely resembles CSF
endolymph:
- secreted by stria vascularis and drained via endolymphatic duct
- high concentration of K+ - 145mM
- low concentration of Na+ - 2mM
- intracellular fluid has a positive potential of 80 mV
- large potential gradient of 140mV across membranes of hair cells
What cells are found in the organ of corti?
What is their role?
the organ of corti contains hair cells
they have stereocilia projecting into the endolymph and are embedded into the tectorial membrane
when they bend towards the tallest cilium, the hair cells depolarise
when they bend in the opposite direction, the hair cells hyperpolarise
What is the chain of events that occurs after pressure changes move the oval window?
- pressure changes move oval window
- moves perilymph in scala vestibuli
- moves vestibular membrane
- moves endolymph
- moves basilar membrane, which sets up a travelling wave
What happens when the basilar and tectorial membranes are moved?
when the basilar membrane is displaced, so is the organ of Corti
the round window bulges in response to releive the pressure
the shear forces set up by the relative displacements of basilar and tectorial membranes cause the stereocilia to bend
in response, the hair cells depolarise and hyperpolarise causing the primary afferent nerves to discharge
What is the difference in the ways high and low frequency sounds travel along the basilar membrane?
different frequency sound waves travel different distances along the basilar membrane
high frequency vibrates the stiff base, loses energy and doesn’t travel far
low frequency waves travel further as energy does not dissipate
What is meant by “place theory”?
the basilar membrane acts as a frequency analyser
it distributes the stimulus along the organ of Corti so that different hair cells respond to different frequencies
there are maximum displacement areas for different frequencies
What is meant by “frequency theory”?
phase locking for sounds up to 400Hz - afferent fibre discharge in phase with the stimulus
if frequency is greater than 500Hz, fibre cannot discharge during each cycle
however, frequency information is detected by the activity of a population of afferent fibres that fire in sequence
What is meant by “duplex theory”?
for high frequencies, the place theory applies and activate afferent fibres that supply hair cells near the base of the cochlea
place and frequency theories are required to explain the frequency coding of sound - this is duplex theory
What are the 2 different types of hair cells in the organ of Corti and what are their roles?
inner hair cells:
- transduce sound waves into electrical signals which get sent to the brain
outer hair cells:
- dynamically adjust stiffness of the tectorial membrane
- this amplifies quiet sounds and suppresses loud sounds
What is meant by “dancing hair cells”?
What is the purpose of this type of movement?
outer hair cells contract like muscles when they are depolarised by sound stimuli
this contraction amplifies the movement of the basilar membrane, making low intensity sound louder
the amount of amplification (contraction) performed by the outer hair cells is regulated by feedback from the auditory brainstem
it is turned down when there is a high level of background sound
What is a way of testing the health of the cochlea that is used in newborns?
the movement of the basilar membrane caused by outer hair cells evokes a sound which is detectable with microphones placed into the ear
this is performed when measuring autoacoustic emissions
this is a way of testing the health of the cochlea that does not involve perceptual decisions and is used in newborns to test their hearing
What is meant by air and bone conduction?
air conduction:
- air conduction uses the apparatus of the middle ear (pinna, eardrum & ossicles) to amplify and direct the sound to the cochlea
bone conduction:
- bypasses some or all of these and allows sound to be transmitted directly to the inner ear (at a reduced volume)
- or via the bones o the skull to the opposite ear
How can the difference between bone and air conduction be used in practice?
sound travels through bone, therefore we can use bone conduction to bypass the middle ear
this determines whether deafness is due to a problem with the middle or inner ear
What is conductive deafness?
What can cause this?
impairment of sound transmission in the external or middle ear
- middle ear infections - otitis media
- damage to ossicles - necrosis, dislocation
- thickening of tympanic membrane
- perforation of tympanic membrane (loud sounds)
- otosclerosis
- mechanical obstruction or fluid/ear wax build up
What is otosclerosis?
the ossicles become immobile because of growth of surrounding bone
What is sensorineural deafness?
damage to the pathway from hair cells of the cochlea to the auditory nerve and the brain
What are the causes of sensorineural deafness?
- acoustic neuroma - benign tumour of the auditory nerve
- toxic degeneration of the auditory nerve - e.g. streptomycin
- acoustic trauma - continued exposure to loud noise
- viral infections of inner ear or auditory nerve - e.g. mumps, rubella
- Meniere’s disease, presbycusis, meningitis
How can meningitis lead to deafness?
What is the site of permanent hearing loss?
damage can result from both the direct effect of the infection on the brain and also the body’s response to it
strong reactions likely to cause permanent damage are more commonly associated with pneumococcal meningitis
the site of permanent hearing loss is almost always the cochlea
When does damage to the cochlea following meningitis usually happen?
What determines whether someone can have a cochlear implant?
damage usually happens within the first couple of days of illness
cochlear implant centres determine implant candidacy on an individual basis and take into account:
- hearing history
- cause of hearing loss
- amount of residual hearing
- speech recognition ability
- health status
- family commitment to aural habilitation/rehabilitation
What would a prime candidate for a cochlear implant look like?
- having severe to profound sensorineural hearing impairment in both ears
- have a functioning auditory nerve
- having lived at least a short amount of time without hearing (approx 70+ dB hearing loss)
What is presbycusis?
age related hearing loss
What is streptomycin?
an antibacterial used for tuberculosis and plague
it is associated with toxic degeneration of the auditory nerve, leading to sensorineural deafness
What is the background behind the Weber Test?
- place a tuning fork on the midline of the head
- sound travels through the bone and sets up vibrations directly in the cochlea
- organ of Corti is activated without the use of the tympanic membrane and ossicles
If a subject has conductive hearing loss in one ear, in which ear will the sound be the loudest when the Weber test is performed?
Why?
the damaged ear
sound is heard more loudly in the damaged ear because hearing in the normal ear is inhibited by ambient sound (auditory masking)
How can Weber’s Test be interpreted?
unilateral conductive hearing loss:
- patient hears the tone louder in the damaged ear
unilateral sensorineural hearing loss:
- patient hears the tone louder in the normal ear
What are the stages involved in Rinne’s Test?
- tap tuning fork and hold against the pinna
- wait for the tone to fade
- place tuning fork against the mastoid process
- repeat with ear plug
- is the tone heard when placed on the mastoid process?
What can be concluded if a tone is heard on the mastoid process in Rinne’s Test?
bone conduction has a higher threshold than air conduction
if you have gone below air conduction threshold, you should have also gone below bone conduction
if you hear the tone again it is an indication of middle ear deafness
Why are Weber’s and Rinne’s tests qualitative?
What is a quantitative test for hearing loss?
you do not know the degree of hearing loss that is present
an audiogram is a quantitative test for frequencies within the hearing range
if the hearing is reduced, the threshold is elevated
What is meant by audiometry?
it allows hearing loss to be quantified
hearing loss is measured in decibels hearing level (dBHL)
What are the values for normal, mild, moderate and severe hearing loss?
What is the purpose of audiometry?
What does it show if air and bone conduction are elevated?
it allows comparison of air and bone conduction
if air conduction thresholds are higher than bone conduction then patient has a conductive hearing loss
if both air and bone conduction are equally elevated then patient has sensorineural hearing loss
WHat is shown here?
this is an audiogram
dBHL = decibel hearing level
0dBHL = average threshold at any given frequency
What does a normal tympanic membrane look like when using an otoscope?
the malleus can be seen against the membrane
the umbo is the most depressed part of the tympanic membrane in the centre
a shaft of light below the umbo is the light reflected from the otoscope
What is otitis media?
Where does it occur?
inflammation of the middle ear, or middle ear infection
it occurs in the area between the tympanic membrane and the inner ear, including the pharyngotympanic tube
What are the two categories of ear inflammation that can underlie an earache?
otitis media and otitis externa
What are other diseases, other than ear infections, that can also cause ear pain?
cancers of any structure that shares nerve supply with the ear
shingles can lead to herpes zoster oticus
How long does otitis media take to heal?
it is painful but it is not threatening and usually heals on its own within 2-6 weeks
What is serous otitis media?
What are other names for it?
otitis media with effusion (OME) / serous otitis media / secretory otitis media (SOM)
it is a collection of fluid that occurs within the middle ear space as a result of the negative pressure produced by altered Eustachian tube function
this can occur from a viral URI, with no pain or bacterial infection, or it can precede and/or follow acute bacterial otitis media
How can otitis media with effusion cause conductive hearing impairment?
fluid in the middle ear sometimes causes conductive hearing impairment, but only when it interferes with the normal vibration of the ear drum by sound waves
over weeks and months, middle ear fluid can become thick and glue-like, increasing the likelihood of causing conductive hearing impairment
What is early-onset OME associated with?
- feeding while lying down
- early entry into group childcare
increased duration of OME in first 2 years of life:
- parental smoking
- too short a period of breast feeding
- greater amounts of time spent in group childcare
What is meant by retraction pocket?
when part of the tympanic membrane becomes weakened and is sucked inwards by a negative pressure within the middle ear
this permits it to drape itself over the small bones which conduct sound (ossicles) as well as the innermost or medial wall of the middle ear
What may happen if someone with a retraction pocket has repeated infections?
erosion of the ossicles may occur
this leads to significant deafness or a cholesteatoma (skin growing within the middle ear)
What are the three grades of a retraction pocket?
grade I:
- early retraction not touching anything within the middle ear cleft
grade II:
- draped over the small bones of the ears
grade III:
- retracted to the medial wall of the middle ear, either with the ability to elevate it without difficulty or in
- grade III (b) when it is stuck to the medial wall of the middle ear
What is tympanosclerosis?
calcification of tissues in the middle ear
What is cholesteatoma?
What may be seen on clinical examination?
a destructive and expanding growth consisting of keratinizing squamous epithelium in the middle ear and/or mastoid process
the patient may have recurrent ear discharge
granulation tissue and a discharge (through a marginal perforation of the ear drum) may be seen on examination
What is a cholesteatoma cyst?
What is it usually infected with?
a cholesteatoma cyst consists of desquamating (peeling) layers of scaly or keratinised (horny) layers of epithelium, which may also contain cholesterol crystals
often the debris is infected with Pseudomonas aeruginosa or other bacteria
What can happen if a cholesteatoma is untreated?
it can eat into the three small bones located in the middle ear
(malleus, incus, stapes - the ossicles)
this results in nerve deterioration, deafness, imbalance and vertigo
it can also affect and erode, through enzymes it produces, the thin bone structure that isoaltes the top of the ear from the brain
this lays the covering of the brain open to infection with serious complications
What nerve is affected by congenital and acquired types of cholesteatoma?
the facial nerve
this extends from the brain to the face and passes through the inner and middle ear and leaves at the anterior tip of the mastoid bone
it then rises to the front of the ear and extends into the upper and lower face
What is shown here?
perforation
What is meant by encoding sound localisation?
there are different mechanisms for:
horizontal localization - binaural (both ears)
vertical localization - monaural
for horizontal localization, there is a need to compare sounds at the two ears
What are the 2 mechanisms for encoding sound localisation?
- interaural time difference (ITD)
- interaural intensity difference (IID)
What is interaural time difference (ITD)?
What is it used to detect?
the time difference between sounds occurring at the left and right ears
there is a direct relationship between sound location and delay
used for low frequency sounds
Why can ITD not be used for high frequency sounds?
many peaks of a high frequency wave will fit between the ears
you cannot detect a specific one arriving first at one ear
What is the difference between monaural and binaural processing and where it occurs?
monaural processing:
- occurs in cochlear nucleus
- inputs from ipsilateral ear only
binaural processing:
- occurs in medial superior olive (MSO)
- inputs from both ipsilateral and contralateral cochlear nuclei
What is interaural time difference (ITD) used to measure?
What cells does it involve and when are they maximally activated?
measures binaural inputs - excitatory
- “coincidence detector” cells
- medial superior olive (MSO)
- maximally activated when impulses reach the cell together
- impulses reach ears at different times but the length of axons compensates
What is important in ITD?
What frequencies does it apply to?
phase locking and axonal delay are important in ITD
ITD applies for frequencies 20 Hz - 2 kHz
some neurones react more strongly to inputs from both ears than just one, and vice versa
What is shown in this image?
binaural cross correlation model
- sound source from the midline of the head
- right and left cochlear nucleus discharge at the same time
- t1 = t2
What happens if a sound source is located on the right side according to the binaural cross correlation model?
if the sound source is located on the right side, the right cochlear nucleus discharges first
Why is interaural intensity difference (IID) used for higher frequency sounds rather than lower frequency sounds?
- lower frequency sounds diffract around the head so that their intensity at both ears is similar
- higher frequencies have less energy which is easily dissipated through objects
- the intensity will be lessened when reaching the ear further from the sound source compared to the ear where the sound is not blocked by the head
- these differences in intensities are interpreted in the lateral superior olive as a particular direction of the sound source
What is meant by the “sound shadow” when explaining interaural intensity difference?
- the head casts a “sound shadow”
- lower intensity received at ear “in shadow”
- comparison of intensity in each ear is localisation
What frequencies does IID apply to?
applies to frequencies of 2kHz - 20 kHz
What type of localisation is detected in IID and ITD?
horizontal localisation
What is the background behind how vertical localisation works?
- use the shape of the pinna to compare actual and reflected sounds
- change in elevation - changes the reflections on the pinna and hence incoming signals
What happens as sound moves vertically?
What happens when the sound wave reaches a person’s body?
as sound moves vertically, the delays between the direct pathway and the reflected pathway change
when a sound wave reaches a person’s body, it reflects off of the person’s head and shoulders and the curved surface of their outer ear
each reflection makes subtle changes in the sound wave
What is a head-related transfer function (HRTF)?
when sound waves are reflected off of the pinna, the reflecting waves interfere with one another
this causes parts of the wave to get bigger or smaller, changing the sound’s volume or quality
these changes are head-related transfer functions
What is the difference between HRFTs and ILDs/ITDs?
unlike with ILDs and ITDs, the sound’s elevation, or the angle at which it hits your ears from above or below, affects its reflections of the surfaces of the body
the reflections are also different depending on whether the sound comes from in front of or behind your body
How are HRTFs interpreted by the brain?
HRTFs have a subtle but complex effect on the shape of the wave
the brain interprets differences in the wave’s shape, using them to find the sound’s origin
