4 - hearing — how we do it and how it goes wrong lecture Flashcards
at what dB roughly is conversation?
60/70dB
below what dB is hearing damage negligible?
75dB
what is the measurement for frequency?
Hz
what is the normal hearing range?
20-20000Hz
what is the hearing range in Hz for speech?
200-500 Hz
noise damage is cumulative — explain
- longer exposure + louder noise = more damage
- hearing deteriorates as we get older
- happens preferentially in the high frequencies
it is possible to guess someone’s age depending on what Hz they can hear
by how much does the external ear (pinna) magnify sound by?
about 15dB
what is an advantage of the asymmetric shape of the pinna?
the curls of the pinna modulate sound to allow you to tell if someone is behind or in front of you
how can the external ear reduce hearing?
can collect debris and moisture — encourage bacterial growth — reduces hearing
describe the ear canal skin
- migrates
- grows in one direction without flaking off
- debris slowly moves outwards
what are the 2 types of wax and in who are they most common?
- “honey wax” — africans and caucasians
- “flaky wax” — asians
what is the universal quality of wax?
WATERPROOF — wax stops water from hanging around and reducing our hearing
why does the eardrum require air on both sides?
to be able to vibrate with the sound pressure waves
what is the function of the eustachian tube?
lets air in in emergencies to prevent eardrum from bursting due to abnormal pressures
decibel increments?
logarithmic
in water, there is a lot more energy, therefore _____ displacement of the _____ window occurs to transmit the sound
lower displacement
oval window
what part of the ossicles is embedded in the tympanic membrane? what is the function?
handle of malleus — starts vibration of the ossicles
where is the vibration of the ossicles transferred to?
oval window
what is the transmission peak across the middle ear?
1kHz = in range of our speech reception
what is the centre of speech frequency we use to communicate with each other?
peak frequency = 2 kHz
what are the 2 muscles in the middle ear?
stapedius and tensor tympani
what is the function of stapedius and tensor tympani?
stiffen the ossicles — make us deafer — protect inner ear from noise damage
how long can the 2 muscels of the middle ear work before needing time to relax?
40 mins
functions of the middle ear?
- tympanic membrane/stimulus coupling — large TM to catch energy — focus on to small oval window — use ossicles to transmit vibration energy
- impedance transformer
- differential application of pressure to round and oval windows
= amplify sound in air to bring it more like sound in water (in water there is a lot more energy therefore lower displacement of the oval window occurs to transmit the sound)
what does the round window do?
- prevents echoing
- acts as a beach for fluid
what happens when the oval window vibrates?
sets up a wave in the perilymph fluid which travels through the cochlea and ends at the round window
what hair cells are embedded in Reissner’s (vestibular) membrane?
outer hair cells
the trained human ear can detect what frequency of change?
0.75Hz
outer vs inner hair cells
outer — motor nerve fibres, embedded into Reissner’s membrane
inner — sensory nerve fibres
two angstroms (0.1 nanometre) deflection fo sterocilia auses depolarisation via movement of _____?
potassium
what is the function of outer hair cells?
= muscular (not for hearing)
- tense tectorial membrane (when basilar membrane vibrated with sound) —> focuses the resonant frequency of the basilar membrane so you can hear the difference between smaller changes in frequency
what do inner hair cells do?
- they are the original sensors of sound
- depolarise when the hair moves 2 angstroms in fluid movement in organ of corti
which fluid is rich in K+?
endolymph
what contracts to stiffen the organ of corti and fine tune the frequency of hearing?
OHCs
what are the 2 types of hearing impairment?
- conductive
- sensori-neural
what parts of the ear do the 2 types of hearing impairment involve?
- conductive — the external and middle ear
- sensori-neural — the sensation of hearing sound in the cochlea and associated nerve pathway
can wax cause a conductive hearing impairment?
only when the wax completely occluded the external ear canal
what is perichondritis? what kind of hearing impairment does it cause?
= an infection of the tissue lining your ear cartilage — normally due to pseudomonas aeruginosa
- swelling in external ear canal —> conductive hearing impairment
in sensorineural deafness, what is the typical impact on hearing?
= a patient may have complete inability to hear tone at all frequencies
= a patient may have complete inability to hear tone at a particular frequency
in conductive deafness, what is the typical impact on hearing?
= a patient can hear tones, but only at a higher amplitude (dB)
sensorineural hearing loss is related to problems in the inner ear. what are some examples of causes of sensorineural deafness?
- exposure to loud noises
- infection/inflammation
- head trauma
- ageing (presbycusis)
conductive hearing loss is related to problems in the outer or middle ear. what are some examples of causes of conductive deafness?
- infection in ear canal
- foreign object in ear
- impacted earwax
- perforated tympanic membrane
- fluid in the middle ear
- ear infection for example otitis media
what is the cause of visible bubbles behind the eardrum seen with an otoscope?
- middle ear effusion
- occur in upper resp infection when mucus fills the middle ear and cannot drain down the eustachian tube
what is the name for fluid behind the ear?
otitis media with effusion / glue ear
what % of children have glue ear at some point?
80%
what is otosclerosis?
disease of ossicles — stapes footplate fuses to skull of oval window — in 1% of population — conductive hearing impairment
what is the name of a benign tumour of the cochlear nerve that can cause a sensori-neural deafness?
acoustic neuroma
which type of hair cell is more sensitive to loud noise? what is the effect of this?
outer — more damaged as we age — therefore sound differentiation worsens
what is considered profound hearing loss? treatment?
85+ dB — hard to treat with a hearing aid — cochlear implant possibly needed
what is normal hearing? (dB)
up to 25dB
what can be useful for spotting fluid behind ear?
type B tympanogrm — detection of tympanic membrane permeability
what range of frequencies is the normal ear most sensitive?
1000-4000 Hz
where is the cochlear nucleus?
medulla
where is medial geniculate body?
thalamus
where is inferior colliculus?
midbrain
what are the 5 different areas up to the cortex in the auditory pathway>
- vestibulocochlear nerve
- cochlear nucleus
- superior olivary nucleus
- lateral lemniscus
- inferior colliculus
what tests can be used to separate between conductive and sensori-neural hearing loss?
webbers and rinne’s tests
describe webbers test
- activate tuning fork then put base of tuning fork against the midline of skull
- listening for lateralisation of sound
- no hearing impairment — sound same on both sides
- conductive — sound louder in affected ear
- sensori-neural — sound louder in normal ear
describe rinne’s test
- activate tuning fork then put base on mastoid process behind ear
- when sound stops bring tuning fork to the outside of ear and should still hear sound
- carried out on both ears separately
—> In a normal ear, air conduction is greater than bone conduction, that is to say sound is more efficiently transmitted to the cochlea via air than via bone. So, a patient who hears the tuning fork loudest when it is placed 1cm from the external auditory meatus has a positive Rinne’s test and a person who hears loudest when placed over the mastoid process has a negative Rinne’s test
—> conductive hearing loss = -ve result (bone louder)
is a normal ear, is bone or air conduction greater?
air
