Lecture 22: Hearing Flashcards
What is the stimulus of hearing?
sound
What is sound?
high and low pressure waves in the air
What happens as the sound loudness increases?
you can damage your ear
What are the three sections of the ear called?
outer, inner and middle ear
What three structures make up the outer ear?
the pinna, the external auditory canal and the tympanic membrane
What is another name for the tympanic membrane
the ear drum
Where is the pinna and what is its purpose?
It is the outside of the ear and as the sound waves come into the ear, the pinna reflect the sound waves down the external auditory canal
Where is the external auditory canal located?
it travels to the tympanic membrane
What section of the ear is behind the tympanic membrane?
the middle ear
Is the middle ear fluid filled or air filled?
air filled
How is the air maintained in the middle ear?
by the auditory (eustachian) tube
What happens when there is an increase in pressure in the middle ear?
it puts pressure on the tympanic membrane and causes it to bow out (painful)
How can we relieve the pain from the increase in pressure in the middle ear?
this is done by the auditory (eustachian tube) because it goes down to the back of the nose (nasopharynx) to get air from the atmosphere into the middle ear cavity
When we hear the squeal/pop of our ears, what is happening?
this is the pressure equalising - the air is escaping down the eustachian tube and being replaced
When can the eustachian tube be blocked?
during a cold
What are the bones in the middle ear called?
malleus
incus
stapes
What is the collective name for the bones in the middle ear?
the ossicles
What is the role of the ossicles?
to transfer vibrations from sound waves from the tympanic membrane to the inner ear
Describe what happens to the sound waves that enter the ear
they come down the external auditory canal and cause vibrations of the tympanic membrane and this is transferred to the malleus which moves which causes the incus to move and this causes the stapes to move and this delivers the vibrations into the inner ear
What are the 3 components of the inner ear?
- the semicircular canal
- the cochlea
- the vestibulocochlear nerve
What is the semicircular canal for?
balance
Describe the cochlea
it is fluid filled tubes
Is the inner ear filled with fluid or air?
fluid
What are the 3 components of the external ear?
- pinna
- external auditory canal
- tympanic membrane
What are the 2 components of the middle ear?
- ossicles
- eustachian tube
How many chambers of the cochlea are there?
3
What are the three names of the chambers of the cochlea in order from top to bottom?
Scala vestibuli
Scala media
Scala tympani
Are the chambers of the cochlea fluid filled or air filled?
fluid filled
What is the name of the fluid that fills the scala vestibuli and scala tympani?
perilymph
What is the name of the fluid that fills the scala media?
endolymph
The scala vestibuli and scala tympani are filled with fluid called perilymph. What other body fluid does this resemble? What is its composition?
it resembles extra cellular fluid which has high Na+ and low K+
The scala media is filled with fluid called endolymph. What other body fluid does this resemble? What is its composition?
it resembles the intracellular fluid which has low Na+ and high K+
What is the main feature of the scala media?
the organ of corti
What does the organ of corti sit on on in the scala media?
the basilar membrane
The basilar membrane in the scala media has what kind of cells?
hair cells with cilia on top of them
What sits on top of the hair cells of the organ of corti?
the tectorial membrane
Describe the path of sound transduction
The sound waves come in, reflect off the pinna and travel down the external auditory canal until it reaches the eardrum.
The sound waves that come down the external auditory canal are high and low pressure. What does this mean for the tympanic membrane?
the high and low pressure causes the tympanic membrane to move in and out
What effect does a high pressure sound wave have on the tympanic membrane? What does this mean for the rest of the sound transduction pathway?
It causes it to deflect back which causes the middle ear bones to move (the malleus moves the incus which moves the stapes). This causes the pressure to be moved to the inner ear at the oval window. This deflects because of the high and low pressure and it delivers the pressure to the inner ear.
What happens to the cochlea when the stapes is pumping?
There is an increase in pressure in the scala vestibuli which causes the Reissner’s membrane to bow downwards to transfer the pressure to the scala media. There is therefore an increase in pressure in the scala media which causes the basilar membrane to bow downwards too. This means that the organ of corti is moving up and down so the hair cells are bending and this is the start of the signal transduction pathway
Where is the Reissner’s membrane?
the membrane at the bottom of the scala vestibuli
Hair cells have mechanically gates ion channels which open by what?
physical force
What happens when the hair on the hair cells bends?
the ion channels on the cilia are pulled open
The hair cells are different lengths. What is the significance of this?
They decrease in size so the tectorial membrane doesn’t touch all of them.
Why do all the hair cells move even though the tectorial membrane doesn’t touch them all?
because they are linked by thin strands of protein so that when one bends, they all bend
Because the scala media is filled with endolymph, what happens when the mechanically gated ion channels on the hair cells open?
The endolymph resembles the ICF which means it has a high K+ concentration. This means that the opening of the mechanically gated ion channels cause an influx of K+ which causes a depolarisation.
What happens when the hair cell is depolarised?
There is a wave through the cell which causes an opening of voltage gated Ca2+ channels and there is an influx of Ca2+ ions into the cell.
What is the effect of the influx of Ca2+ into the hair cell?
The influx causes glutamate to be released from vesicles onto the synapse of the cochlear afferent nerve fibres (of the vestibular cochlear nerve) to create an EPSP to go to the different parts of the brain
Summarise the sound transduction pathway in 5 steps
- sound waves cause the tympanic membrane to vibrate
- vibrations are transmitted via the ossicles
- displacement of the basilar membrane causes deflection of the stereo cilia
- opening of mechanically-gated ion channels
- K+ ions enter the hair cell and cause a depolarisation
Which of the structures doesn’t fit in an air filled chamber?
- Pinna
- Scala vestibuli
- stapes
- tympanic membrane
scala vestibuli
Describe the central pathway taking information to the brain
The _____ afferent nerve fibres project to the ______ nuclei in the _________. The axons from the _________ nuclei ascend through the __________ lemiscus to the medial _________ body. From here, they project to the primary _________ complex
The cochlear afferent nerve fibres then project to the cochlear nuclei in the brainstem. The axons from the cochlear nuclei ascend through the lateral lemiscus to the medial geniculate body. From here, they project to the primary auditory complex
How many primary auditory cortices are there are where are they located? Why is this?
2
one on each side of the brain
both sides receive information from both ears to distinguish characteristics of sound
What are some characteristics of sound that we need to distinguish? (4)
- pitch
- intensity
- duration
- direction
What is the pitch?
the frequency
What is responsible for us being able to detect different frequencies?
The basilar membrane is responsible for detecting different frequencies.
How can the basilar membrane detect between different frequencies?
At the start of the basilar membrane (in front of the stapes) the membrane is very stiff so the hair cells here detect high frequencies but further along, it becomes floppy until it reaches the apex to detect low frequency.
How does the auditory complex contribute to our perception of frequency?
The auditory complex is tonotopically mapped which means the different cells in the different parts of the auditory cortex respond to different frequencies of sound
What is the amplitude/intensity/loudness encoded by?
What happens when there is a louder noise?
The number of impulses on the afferent nerve fibres per second.
The louder the noise, the bigger the vibrations, the bigger the displacement in the basilar membrane the bigger the depolarisation and the more impulses are firing in the auditory nerve fibres
How is the duration of sound encoded?
the sound is signalled by the duration of the afferent discharge caused by the stimulus (a longer talk, the longer the depolarisation in the hair cells and the longer the impulses)
How is the direction of sound determined?
the sound source is indicated by the time difference in the activation of the receptors in each ear and by the intensities in each ear
What is one hearing defect?
deafness
Define deafness
there is a raised threshold to sound stimuli
What can cause deafness?
- impaired sound transmission through the outer or middle ear (conduction deafness)
- blockage or infection
- damage to receptors or neural pathways (sensorineural deafness)
- exposure to loud noise, tumor, meningitis
One example of deafness comes due to an ear infection. Describe this
This is called otitis media and it is an infection in the middle ear. This causes a build up of fluid in the middle ear and the sound waves travel differently n fluid (fluid dampens the amplitude) so sound transmission is dampened in the middle ear
How does exposure to loud sounds cause deafness?
because it damaged the hair cells in the organ of corti
How can a tumour lead to deafness? Give an example
eg. acoustic neuroma (vestibular schwannoma)
this progresses slowly and is caused by an overgrowth of Schwann cells that line the cochlear nerve which puts pressure on the nerve and prevents correct sound transmission to the brain. It can cause tinnitis
What is tinnitis?
loud ringing in the ears - you can hear your body functions
As well as an acoustic neuroma, other types of tumours are glomus tympanicum and ear canal cancer. What are these?
This grows behind the ear drum. It causes hearing loss and tinnitis and can be removed via surgery.
Ear canal cancer spreads easily to other organs and you need surgery to remove the auditory canal, ear drum and ossicles resulting in permanent hearing loss
