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 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
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
What are the values for normal, mild, moderate and severe 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 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
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 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 shown here?
perforation
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
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 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 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
