lecture 23 - nervous system 3: hearing & the ear Flashcards
what is sound?
pressure waves travelling through air or another medium
defined by:
• frequency
• amplitude
what is frequency?
pitch of sound
measured in Hertz cycles per second
what is amplitude?
intensity - loudness
measured in dB
log scale
what happens when a sound creates a pressure wave?
it travels through a medium
air molecules are alternatively compressed and rarefied
the wave travels from source to ear through the air but the gas molecules themselves remain in their locations
what is the ear composed of?
outer ear
middle ear
inner ear
structures of the outer ear
pinna
auditory canal / auditory meatus
tympanic membrane
structures of the middle ear
air filled space
comprised of the malleus (malleus, incus and staples)
connected to the nasopharynx by the Eustachian tube
structures of the inner ear
cochlea
organs of balance
nerve fibres leaving the cochlea go though a hole in the skull into the brain
role if the pinna
directs sound to ear canal
externalisation of sounds
• listen without pinna (headphones)
• sound seems to be inside head
localisation of sounds in vertical plane
• above/below or in front/behind head
• interference of sound
role of the auditory canal/meatus
leads to ear drum
protects sensitive machinery of ear
resonance - amplifies sounds in frequency of range of speech
what are the 3 bones in the middle ear
malleus
incus
stapes
what is the tympanic membrane?
the ear drum
structure of the middle ear
malleus attached to tympanic membrane and allows sound to be conducted through small bones
footplate of stapes attached to window in cochlea called oval window which allows sound to be taken to cochlea
it is an air filled cavity - pressure needs to be the same as the ear canal
how do you adjust the pressure in the middle ear?
by swallowing
opens the Eustachian tube connected to the nasopharynx
allows pressure to equalise with the ear canal
why do we need a middle ear?
problem of transferring energy from 1 medium to another
inner ear is fluid filled
without middle ear there would be poor transfer of energy from air to cochlea fluids
function of the middle ear
overcomes air-fluid mismatch
helps transfer sound more efficiency
structure of the cochlea
consists of a spiral of 2 and a half turns
3 membranous tubes that separate large tube into 3 divisions
what are the 3 divisions of the cochlea?
top one - Scala vestibuli
lower one - Scala tympani
between them is the Scala media
at each level, cochlea nerve fibres come down
cell bodies of the fibres are in the spiral ganglion
what is the Scala media and the Scala tympani separated by?
basilar membrane
on the basilar membrane in the organ of corti - contains cellular machinery of the ear
what is the Scala media and the scala vestibuli separated by?
Reissner’s membrane
how is the fluid in the different cochlea compartments different?
in the vestibule and tympani there is a fluid called perilymph that has a high Na and low K concentration
in the media there is the endolymph which is produced by the stria vascularis which secrete K+ into the space
organ of corti
sits on the basilar membrane
complex structure
has hair cells - stereocillia
what are the 2 types of hair cells in the organ of corti?
inner and outer hair cells
only 1 row of inner hair cells
3 rows of outer hair cells
they have nerve fibres associated with them
what lies over the hair cells in the organ of corti?
the tectorial membrane - stereocillia of the outer hair cells are embedded in it
what is the fluid in the organ of corti?
fluid above the hair cells and below the tectorial membrane in endolymph
perilymph baths the area between the basilar membrane and the basolateral membranes
what does the stapes do in the separation of sound frequencies in the basilar membrane?
introduces sound vibrations into the cochlea
it moves forwards and backwards to introduce pressure into the scar vestibuli (outer chamber)
when you increase pressure, it has to be accommodated - round window bulges outwards
when you apply pressure it begins at the base and travels as a wave that gradually increases and then decreases to the apex
this is called the travelling wave
frequency analysis by the cochlea
low frequencies produce maximal displacement at the apex of the cochlea
high frequencies produce maximal displacement at the base
displacement of the basilar membrane activates the hair cells
what happens when hair cells are activated by displacement of the basilar membrane?
cell depolarise and K+ enters when bundle bends towards tallest stereocillia, Ca++ entry and neurotransmiite release
cell hyperpolarises when bundle moves towards shortest steroecillia
they only generate graded potentials not APs
what do inner hair cells do when activated?
release neurotransmitter glutamate
activate cochlea nerve fibres associated with inner hair ells in region where basilar membrane displacement is maximal
what do outer hair cells do?
shorten and lengthen is response to depolarisation and hyperpolarisation
brought about by motor proteins called prestin
increase the sensitivity of our hearing
