Lecture 22 - Hearing Flashcards
Ear parts
outer, middle and inner part
Outer and middle part are
air filled
Inner part is
fluid filled
Outer -
Outer = external auditory canal, pinna (auricle), tympanic membrane (ear drum)
Pinna
Pinna - as sound waves come into the ear, the lumps and bumps of the pinna causes the reflection of sound waves into the external auditory canal
Tympanic membrane
Tympanic membrane - also called the ear drum, sound waves cause vibrations on the tympanic membrane
Middle -
Middle = Malleus and incus and stapes which make up the ossicles, middle ear cavity, auditory (eustachian) tube
Eustachian tube
Auditory (Eustachian) tube - air is maintained by this structure, opens intermittently to equalise the intratympanic air pressure with the pressure in the external auditory canal, goes down the back of our nose and nasopharynx which is how it gets air from the atmosphere up into the middle ear cavity, if you have a cold then the end of the tube can be blocked by snot and if you fly and change altitudes lots with a cold then it is very difficult to equalise this pressure with a cold and can be painful due to blockage
Ossicles include =
Malleus, incus and stapes
Ossicles
Malleus, incus and stapes together are called the ossicles (part of the middle ear). These are the smallest bones in the body, muscles and ligaments surround them and help them to fulfil their role, the role of these three bones is to transfer vibrations from sound waves from the tympanic membrane through into the middle ear, tympanic membrane vibrations causes the malleus to move which then causes the incus to move which then causes the incus to move which then causes the stapes to move
Stapes
Stapes = one of the 3 bones that is important for delivering vibrations through to the inner ear
Inner -
Inner = Semicircular canal, vestibulocochlear nerve (vestibular branch and cochlear branch), cochlea
Semicircular canal
Semicircular canal - responsible for our sensation of balance
Cochlea is made up of
Cochlea = lots of fluid filled tubes
Cochlea
Made up of a upper chamber, middle chamber and lower chamber
Upper chamber of cochlea
scala vestibuli
Middle chamber of the cochlea
Scala media
Bottom chamber/lower chamber of the cochlea
Scala tympani
Scala vestibuli
Upper chamber
Fluid filled with perilymph which is a fluid that closely resembles the make up of extracellular fluid (high sodium, low potassium)
Scala media
Middle chamber
Also called the cochlear duct
Different fluid to the other two
It is filled with endolymph which is a fluid that closely resembles ICF (low sodium, high potassium)
Scala tympani
Bottom chamber
Fluid filled
Filled with perilymph
Organ of corti
Sits on basilar membrane just underneath it, hair cells and projecting from the upper surface of these hair cells are little cilia (fine hairs) and on top of this is the tectorial membrane
Perilymph
Perilymph resembles extracellular fluid so high sodium and low potassium
for the cochlea it is in the scala vestibuli and scala tympani
Endolymph
endolymph which is a fluid that closely resembles ICF (low sodium, high potassium)
for the cochlea it is in the scala media
Sound waves causes
the tympanic membrane to vibrate
Vibrations are transmitted via the
ossicles
Steps of sound transduction
Steps (summarised)
Tympanic membrane deflects
Sound waves are waves of high pressure and low pressure which causes the tympani membrane to deflect in and out, high pressure causes the tympanic membrane to deflect back
Middle ear bones move
Membrane in oval window moves
Pushing on the stapes, is transmitted to the inner ear at the oval window
Basilar membrane moves
Sound transduction within the inner ear
When the stapes is pumping on the oval window ti creates some pressure in the Scala vestibuli (upper chamber) and this increase in pressure causes the reissnar’s membrane at the bottom of the chamber to bow which transfers the pressure from the Scala vestibuli to the Scala media, now increase in pressure in the Scala media which causes the basilar membrane to bow down
Hair cells are touching the tectorial membrane and when we are getting the organ of corti moving up or down it is causing a bending of these hair cells (cilia) and this is our signal transduction pathway
100 waves per second = 100 Hz = tympanic membrane is deflecting 100 times per second = ossicles are moving 100 times a second = deflections in the membrane are happening 100 times a second
Deflection of stereocilia
Displacement of the basilar membrane causes deflection of stereocilia
Sound transduction vs phototransduction
sound is faster because it is entirely mechanical
Deflection of stereocilia steps and what happens
Opening of mechanically gated ion channels
Opened by physical force, so when hairs on the hair cells bend it pulls on these ion channels and opens them
Potassium ions enter the cell and cause depolarisation
Endolymph
Wave of depolarisation down the cell causes the opening of voltage gated calcium channels and then have an influx of calcium which then causes vesicles containing neurotransmitter (probably glutamate) to be released on to the synapse of the cochlear afferent nerve fibres ( afferent nerve fibres of the vestibular cochlear nerve) and creates EPSPs and goes off to other parts of the brain
Tip-link in stereocilia
Hairs are different lengths, they are tapered which means that they gradually lessen in size, tectorial membrane doesn’t touch all of the hairs but all of the hairs bend when the tectorial membrane moves up and down due to tip-link (protein, thin strands that connect hairs) which allows for when one bends it pulls on all the others and causes all the other ones to bend as well and when they are bending they are pulling on the mechanically gated ion channels and causing them to open
Sound transduction summary from outer to inner ear
Sound waves cause tympanic membrane to vibrate
Vibrations are transmitted via the ossicles
Displacement of the basilar membrane causes deflection of sterocilia
Opening of mechanically gated ion channels
Potassium ions enter the cell and cause depolarisation
Central pathways
Auditory receptors in cochlea
Brain stem neurons
Medial geniculate nucleus
Auditory cortex (each side receives information from both ears)
My notes
PSPs generated in the cochlear afferent nerve fibres which project into somewhere called the cochlea nuclei in the brainstem and axons from the cochlea nuclei ascend through the lateral lemniscus up to the area called the medial geniculate body/nucleus and from here project out to the primary auditory cortex (2 of them, one on each side of the brain) and both sides of the brain receive information from both ears and the fact it does this allows us to distinguish certain characteristics of sound.
Pitch =
Frequency
Pitch = frequency …. describe this sound quality
Pitch (frequency) – discrimination is determined by activity in hair cells at specific points on basilar membrane
Able to detect different frequencies because our hair cells are activated by different frequencies at different points on the basilar membrane
Basilar membrane is the membrane that sits under the organ or court which is responsible for detecting different frequencies
Further more the auditory cortex itself is tonotopically mapped so that cells in different parts of the auditory cortex respond to different frequencies of mourned and these 2 things combined allow us to detect between different frequencies of sound
Apex = wide and floppy - detect low frequency here
Base = at foot of stapes, narrow and stiff - detect high frequency here
Intensity =
loudness
Intensity = loudness… describe this sound quality
Intensity (loudness) – is encoded in the number of impulses per second in auditory nerve fibres
Amplitude
Louder the noise, the bigger the vibrations, bigger the displacement in the basilar membrane, bigger the depolarisation, get more impulses firing in our auditory nerve fibres
….impulses on the cochlear afferent nerve fibres per second….
Duration - describe this sound quality
Duration – the sound is signalled by duration of the afferent discharge caused by the stimulus
Longer the sound, longer vibration is going on, the longer depolarisation is happening in hair cells and the long you have the impulses going on
Direction - describe this sound quality
Direction – the sound source is indicated by time difference in activation of receptors in each ear, and by intensity differences in each ear
Receptors in each ear will tell the brain which ear the sound is coming from
Both of these factors allow us to determine the direction of where the noise is coming from
Deafness
Raised threshold to sound stimuli
U-shaped curve has risen up
Deafness can be due to
Impaired sound transmission through outer or middle ear (conduction deafness)
Blockage or infection (otitis media)
Ear infection (otitis media)
Infection causes a build up of fluid (in the middle air and sound waves travel differently than air)
Prevents correct sound transmission
Damage to receptors or neural pathways (sensorineural deafness)
Exposure to loud noises, tumour, meningitis etc.
Exposure to loud sounds = auditory apparatus cannot recover therefore permanent damage, damage to hair cells leads to hearing damage
Can tumours lead to deafness?
yes
Acoustic neuroma (vestibular schwannoma)
Acoustic neuroma (vestibular schwannoma)
Benign brain tumour
Can progress really slowly for a number of years and normally arise through abnormal growth of schwann cells that line the vestibular cochlear nerve and increased growth puts pressure on this nerve and prevents correct sound transmission to the brain
Hearing loss in one ear
Tinnitus
Patients usually have the two things above
Detected early (usually)
Glomus tympanicum
Glomus Tympanicum
Usually grows behind the ear drum
Hearing loss and tinnitus (therefore usually detected early)
Surgery can restore hearing
Ear canal cancer
Ear Canal cancer
Very rare with 5-year survival rate of just 35% (very aggressive and metastatic)
Hearing loss
Surgery removes the auditory canal, ear drum and ossicles resulting in permanent hearing loss
Auditory tube =
Auditory tube = permits pressure equalisation between atmosphere and middle ear cavity
Number of impulses per second in auditory nerve fibres =
intensity
