Physiology of hearing Flashcards
what is sound
vibration of a medium
spreads out as a wave of pressure
- any vibrating object creates pressure waves in the medium around it
pressure waves dissipate as you get further away
what are Hz
frequency in cycles/ sound
what is the range of human pitch hearing
20 - 20,000 Hz
how is intensity of sound measured
log ratio scale
dB (bels)
3dB = 2x sound energy
10 dB = ten times the sound energy/ twice the perceived loudnessw
what is threshold
the quietest sound that can be heard at each frequency
what is dB SPL
sound pressure log graph
sound pressure = 20 x log10 P/Pref
what is dB HL
dB SPL corrected by referencing actual sound pressure against hearing thresholds in YA with normal hearing
what are the components of the outer ear
pinna, ear canal
what are the components of the middle ear
tympanic membrane (TM), ossicles
what are the components of the inner ear
cochlea, vestibule
why do we have 2 ears
sound localisation in the horizontal plane
- inter-aural time difference
- difference in intensity
what is the function of the pinna
visible part of the ear
amplifies + filters incoming sounds
directional - dependant filtering at certain frequencies
what is the function of the ear canal
the resonance results in a 10dB (3x) increase in level for speech frequencies
what specialisations does the ear canal have
2.5 cm long
Hairs in the outer 1/3 to trap debris
earwax - antiseptic/ antifungal, traps debris
Skin - grows away from the drum and carries debris with it to the outside world
what is impedance mismatch
how do we get sound waves in the air to enter the fluid of the cochlear instead of bouncing off
- 99% of sound deflected by the surface of water
how do we overcome the impedance mismatch
collect sound energy over a large area -> TM
convert vibration of air (not very dense) int vibration of bone (dense)
concentrate all the energy onto a small area (oval window)
use the bone as a piston to transfer energy into the fluid
by how many times does the area ratio TM: oval window decrease and why
17 fold smaller
amplifies sound by concentrating energy
- 25 dB gain
where do the malleus and incus originate from (embryonically)
first brachial arch
where does the stapes originate (embryonically)
second brachial arch
what are the protective muscles in the ear
stapedius reflex
tensor tympani
what is the role of the stapedius reflex
involuntary contraction of the stapedius muscle in the middle ear to keep the ossicles steady, reducing the intensity of the sound
what is the role of the tensor tympani
located on the back of the eardrum to prevent it from breaking
protect against loud sounds
where is the Eustachian tube
connects the nasopharynx to the middle ear
what is the purpose of the Eustachian tube
allows are to enter ad leave the middle ear
closed at rest, opens during swallowing and Valsalva manoeuvre (ears popping/ forcing air into middle ear)
keeps the air pressure in the middle ear space the same as the ambient atmospheric pressure
why is important to keep air pressure inside the middle ear the same as atmospheric pressure
allows the TM to vibrate
dysfunction leads to blocked feeling and poor hearing (e.g. aeroplane flight)
what is conductive hearing loss and how does it occur
problem with outer/ middle ear
- inner ear works fine
ear canal - wax, foreign body, congenital atresia
TM - perforation
ossicles - congenital fusion, damage from infection
what is the treatment for conductive hearing loss
vibrate the skull - bone conduction
- hearing aids
what are the two sensory structures in the inner
vestibular apparatus
cochlea
what is the role of the vestibular apparatus
contains sensory structures for balance and head movements
what is the role of the cochlea
contains sensory epithelium from hearing -> organ of Corti
describe how vibrations get into the cochlea
1- oval window faces into vestibule
2- vestibule contains sensory epithelia for saccule and utricle
3- vestibule leads into scala vestibuli (upper cochlear duct)
4- pressure waves travel along scala vestibuli then back through scala tympani (lower cochlear duct)
5- waves terminate at the round window
what is the saccule
a bed of sensory cells in the inner ear
translates head movements into neural signals for the brain to interpret
what is the utricle
fluid-filled sac containing hair-like cells that translate head movements into neural signals for the brain to interpret
what is the organ of Corti
sensory epithelium containing auditory hair cells
what is the stria vascularis
regulates ionic and metabolic functions of the scala media
what fluid is located inside the scala media
endolymph
- high in K
- low in Na
what is the scala vestibuli/ tympani full of
perilymph
- ^ Na
- low K
more typical ECM
how do we detect frequency
the basilar membrane
- stiff and light at one end
- flexible and heavy at the other end
- its resonant frequency changes over its length
what is the structure of the hair cells in the ear
epithelial origin, resembles cells that line the stomach
stereocilia form bundle at apical pole of the hair cell
stereocilia arranged from shortest to tallest
what happens when the stereocilia are pushed towards the tallest end
depolarisation
K+ channels opened, K+ flows into cells from the endolymph
what happens when stereocilia are pushed towards the shortest end
hyperpolarisation
K+ channels opened
what is the role of inner hair cells
turn vibrations into neural signals
- sent down afferent and lateral efferent neurons
what is the role of outer hair cells
amplify vibrations
motile
1- when stimulated (by movement of basilar membrane) OHCs change shape + stiffness
2- feeds back energy into the basilar membrane (reverse transductions)
3- acts as an amplifier, increasing amount of vibration on basilar membrane (cochlear amplifier)
also improves frequency selectivity
transmits signals down medial afferent neuron
what are otoacoustic emissions and what are they used for
OHC move in response to noise
we can hear them moving
quick, objetive screening test for hearing loss in babies
what are hair cells vulnerable to
infections, ototoxins, noise
aging
sensorineural hearing loss
what happens with loss of IHCs
no signal to the brain
all dB loss in hearing
what happens with outer hairs cells
basilar vibration in insufficiently amplified
~50dB loss in hearing
loss of sound discrimination
how do hearing aids work
make everything louder
microphone - amplifier - speaker
in what situations are hearing aids helpful
conductive hearing loss
doesn’t solve sensorineural hearing loss -> loss of discrimination/ dynamic range/ central processing effects
what are the two ways that pitch is encoded
place code
temporal code
how does place code work
vibration of basilar membrane
accuracy limitation to place code
- wide region vibrates
how temporal code work
phase locking to stimulus
upper frequency limitation to temporal code - neurons can only fire so quickly
what is the clinical relevance of coding mechanisms in the ear
possible to put a hearing implant directly into the cochlea to stimulate the spiral ganglion of cells if there is a severe hearing loss due to loss of hair cells
cochlear implants have been revolutionary for congenital and acquired sensorineural loss
describe the sequence of sound processing
1- first order neurons in the spiral ganglion of the cochlea
2- travel in the CN VIII through internal acoustic meatus to cerebello-pontine angle
3- synapse on cochlear nuclei (junction of medulla and pons)
4- via olive and trapezoid body (pons) to inferior colliculus (midbrain)
5– medial geniculate body (thalamus)
6- superior temporal gyrus (cortex)
when would you put a hearing implant directly onto brainstem
cochlear nucleus in the brainstem if there is no auditory nerve (after resection of a tumour in the nerve) but results aren’t great
damage in the CNS (stoke) doesn’t tend to produce hearing loss because, above the cochlear nuclei representation is bilateral
