Sound conduction and transduction Flashcards
What is sound?
Transverse waves- consists of compressed and rarefied air, it is characterised based on frequency/pitch and loudness depends on amplitude
What is sound measured in?
Decibels- logarithimic scale
What pitch range can humans hear at?
20-20,000 Hz
What is speech frequency?
2-5 kHz
What is the external auditory meatus?
Cone at the start of the outer ear which focuses noise and increases pressure at the tympanic membrane
What is the pinna?
The wing of the ear
What does the shape of the pinna give you an idea about?
The elevation of the sound- whether it is at the floor level or the ceiling
Which bones in the ear are the smallest bones in the human body?
Ossicles
What is the tympanic cavity filled with?
Air
What area comprises the middle ear?
Between the cochlea and the tympanic membrane
What causes the tympanic membrane to vibrate
Air waves
What are the two ways in which the ossicles help to improve the signal?
They increase the pressure of the vibrations by:
Focusing vibrations from the large surface area of the tympanic membrane to the smaller surface area of the oval window
The incus has a flexible joint with the stapes such that the ossicles use leverage to increase the force on the oval window
What do the ossicles convert the movement of the tympanic membrane to?
The foot plate
Why can you not have a membrane that goes straight to the cochlea?
The cochlea is fluid filled so if you had a membrane that went straight from air to fluid, 99% of the energy would just bounce off the interface because of impedance
What is impedance?
Sound waves can travel easily through air (low impedance) but through fluid require much more energy
What are the two muscles that make sure that the ossicles don’t go out of control?
Tensor tympani and stapedius
What is the auditory reflex?
If you hear a loud noise, your tensor tympani and stapedius will contract and reduce movement of the ossicles so reduce the vibration. They will also work when you’re chewing and talking
What is the latency of the auditory reflex?
50-100ot ms
What is hyperacusis?
Painful sensitivity to low intensity sounds
Hyperacusity can occur in conditions that lead to flaccid paralysis of the auditory reflex muscles, give an example
Bell’s Palsy
How are patients with broken ossicles made able to hear again?
They are given a headphone like structure that sends vibrations through the bones in your skull straight to the inner ear
What connects the cochlea to the ossicles?
Stapes
What vibrates the oval window?
Stapes and footplate
What is the membrane below the oval window?
Round window- a pressure release window
What does the round window do when the stapes pushes the oval window in?
It moves outwards to equalise the pressure
How does the basilar membrane respond to pressure waves moving through the cochlea?
It vibrates
What are the three components of the inner ear?
Scala vestibuli, scala tympani and scala media
What is the difference between the scala media and the other two?
The scala media contains endolymph fluid and the other two contain perilymph fluid
How are the scala vestibuli and scala tympani connected?
By a small hole called the helicotrema which allows the fluid to mix
How does the inner ear work?
The stapes vibrates and generates a pressure wave in the perilymph fluid in the scala vestibuli, this makes the basilar membrane vibrate, the cochlea is made of bone and the fluid is incompressible so when the round window vibrates outwards, the oval window vibrates inwards
How does the cochlea identify specific tones with a tonotopic map?
Different parts of the basilar membrane are sensitive to different frequencies
Where do high frequency sounds make the basilar membrane vibrate the most?
Towards the base of the cochlea
What is the Organ of Corti?
The sense organ of the cochlea of the inner ear which converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve. It consists of inner and outer hair cells
Where is the Organ of Corti?
It lies on top of the basilar membrane and beneath the tectorial membrane
What are the 4 main differences between inner and outer hair cells?
Inner- found on their own, send connections to the brain (afferent), have stereocillia which move in response to endolymph and not in contact with tectorial membrane
Outer- found in groups of 3, receive efferent connections, in contact with tectorial membrane and are electromotile- can expand and contract
What is the basis of the cochlear amplifier
Outer hair cells can expand and contract which means that they can amplify the amount of vibration
How do inner and outer hair cells compare in terms of numbers?
There are around 3000 inner and 20000 outer
What is the auditory nerve formed by?
Fibres of inner and outer hair cells
What is found in the spiral ganglion?
The inner and outer fibres cell bodies
What do stereocillia cells need to be able to do?
Depolarise and repolarise rapidly
What are otoacoustic emissions and what cells are responsible for them?
The sounds that the ear makes itself and outer hair cells
What are tip links?
They connect sterocillia together
What happens when an inner hair cell moves due to the movement of endolymph?
It depolarises as potassium channels are opened
What happens whens the endolymph causes upward movement of the basilar membrane?
It displaces the stereocilia away from the modiolus, K+ channels open and K+ enters the hair cell from the endolymph so the hair cell depolarises
What is the modulus?
Bony conical structure at the centre of the cochlea
What happens when the endolymph causes downward movement of the basilar membrane?
It displaces the stereocilia towards the modiolus which causes K+ channels to close and the hair cell hyperpolarises
Why do these events occur?
Endolymph and perilymph have different K+ and Na+ concentrations and the stereocilia stick out into the endolymph but their base is in the perilymph
How does the potassium to sodium ratio compare between the scala media (endolymph) and the scala tympani (perilymph)?
Scala media- high K+/low Na+
Scala tympani- low K+/high Na+
What maintains this concentration gradient
Stria vascularis
What forms the auditory vestibular nerve?
Fibres from the organ of corti that project out of the cochlea via the spiral ganglion
At this point, where does it project to?
The ipsilateral cochlear nucleus
What happens after the cochlear nuclear?
Everything is bilateral
If you have a central hearing deficit and you only lose hearing in one ear, where is the problem
Cochlear nucleus or auditory nerve
What is the pathway after the cochlea nucleus?
Superior olive then inferior colliculus then medial geniculate nucleus then auditory cortex
What is the inferior colliculus involved with?
Reflex associations- turning your head towards a loud noise
Where do the collateral pathways of this pathway go to?
Cerebellum and reticular formation
Where do all the ascending auditory pathways converge?
Inferior colliculus
Where is the primary auditory cortex?
In the temporal lobe
What does the secondary auditory cortex respond to?
Sounds coming on and off, duration of sound
How do the axons of the medial geniculate nucleus project to the primary auditory cortex?
Acoustic radiations
What often happens to neurones in the primary auditory cortex?
They often start specialising so don’t just like any sound but have favourites
Why is lateral inhibition useful in the ear?
It is useful in the tonotopic system so that it can be used to tune a signal to peak frequency
How can you localise a short sound?
Interaural time delay- the time it takes fro the sound to travel from one ear to the other
How can you localise a continuous sound?
Interaural intensity difference- if you hear a sound from one side of room in one ear, it will cast a shadow which means that the sound reaching the other ear will have a reduced intensity
What is conductive hearing loss?
When diseases of the middle ear destroy or stiffen the ossicles (or joints), so the amplification system is eliminated resulting in conductive hearing loss. A heavily waxed ear can also block sound from reaching eardrum
What is sensorineural hearing loss?
When the cochlea or cochlear nerve is damaged, the signal transmitted to the auditory cortex is reduced or lost
When is sensorineural hearing loss commonly seen?
Acoustic schwannoma (tumour of the cochlear nerve) or cerebellar tumours
What is presbyacusis?
Hearing loss due to ageing
