Sound Conduction and Transduction Flashcards
What scale is used to measure how loud a sound is?
Decibels (logarithmic scale)
What is the name given to the wing shaped flap skin and cartilage that makes up the outer ear?
Pinna
Describe the shape of the outer ear and its importance.
It is conical – starts off wide at the external auditory meatus and narrows to the tympanic membrane
This focuses the noise and increases the pressure on the tympanic membrane
Is the tympanic cavity fluid-filled or air-filled?
Air-filled
State 2 ways in which the ossicles increase the pressure of vibration of the tympanic membrane.
Focussing the vibrations from the large surface area of the tympanic membrane to the small surface area of the oval window – this decrease in surface area means that the pressure is increased
The incus has a flexible joint with the stapes, such that the ossicles use leverage to increase the force on the oval window
This amplifies the sound by 30 dB
What is the point of the middle ear? Why isn’t the tympanic membrane continuous with the cochlea?
The cochlea contains fluid, in which you are trying to induce a pressurewave
If the tympanic membrane was continuous with the cochlea, you would go straight from air to fluid and 99% of the energy will bounce back due to impedance
Sound waves require more energy to travel through fluid than air so the increase in pressure of vibration allowed by the ossicles is crucial for this conduction
What are the three compartments of the inner ear?
Scala Vestibuli
Scala Media
Scala Tympani
Which types of fluid do each compartment contain?
Scala Vestibuli + Scala Tympani = perilymph
Scala Media = endolymph
What structure connects the two perilymph compartments?
Helicotrema
Describe how the cochlea functions.
The vibration of the tympanic membrane is conducted and amplified to a vibration of the oval window by the footplate of the stapes.
This vibration induces a pressure wave in the perilymph in the scala vestibuli.
This vibrates the scala media leading to vibration of the basilar membrane.
The round window vibrates as well to equalise the pressure in the cochlea.
Describe the difference in sensitivity of different parts of the basilar membrane.
Higher frequency sounds = base
Lower frequency sounds = apex
What is the Organ of Corti?
The sense organ of the cochlea, which converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve
Where is the Organ of Corti found?
It lies on top of the basilar membrane and beneath the tectorial membrane
What are the two types of cell in the organ of corti?
Inner and outer hair cells
Describe the features and function of inner hair cells.
Found on their own
Not in contact with the tectorial membrane
Send impulses to the brain
They have stereocilia that move in response to the movement of endolymph in the scala media
Roughly 3500 in the body
Describe the features and function of outer hair cells.
Found in groups of three
They are in contact with the tectorial membrane
They receive input from the brain
Electromotile so can expand and contract to amplify the amount of vibration (this is the basis of the cochlear amplifier)
Damage can result in sensorineural hearing loss
Roughly 20,000 in the body
Which compartment of the cochlea does the stereocilia of the hair cells project into?
Endolymph (base is in the perilymph)
What internally generated sounds are the outer hair cells responsible for?
Otoacoustic emissions
What are stereocilia connected by?
Tip links
Describe what happens when the basilar membrane is displaced upwards.
Depolarisation
Stereocilia move away from the modiolus
K+ channels open
K+ enters from the endolymph
Describe what happens when the basilar membrane is displaced downwards.
Hyperpolarisation
Stereocilia move towards the modiolus
K+ channels close
Describe the difference in K+ and Na+ concentration in the different compartments of the cochlea.
Scala Media = High K+ and Low Na+
Scala Tympani = High Na+ and Low K+
NOTE: stria vascularis maintains this concentration
Describe the auditory pathway from the cochlea to the primary auditory cortex.
Spiral ganglion -> cochlear nuclei -> superior olive -> inferior colliculus -> medial geniculate nucleus -> primary auditory cortex
Up to what point is the auditory pathway from one ear ipsilateral?
Cochlear nuclei
Beyond this point there is bilateral representation
The inferior colliculus receives input from both cochlea. What is the inferior colliculus responsible for?
Reflex associations – turning your head towards loud noise
Describe a phenomenon that is involved in sharpening the signal coming from the cochlea.
Lateral inhibition
To which parts of the CNS do collaterals from the auditory pathway go?
Reticular formation
Cerebellum
In which lobe is the primary auditory cortex?
Temporal
What is the secondary auditory cortex responsible for?
Responding to sounds coming off/on
Responding to the duration of sound
What is the name given to the axons that project from the medial geniculate nucleus to the primary auditory cortex?
Acoustic radiations (they travel via the internal capsule)
How do you localise short sound burst?
Interaural time delay
How do you localise continuous sound?
Interaural intensity difference
What is conductive hearing loss?
When diseases of the middle ear damage the ossicles or stiffen theirjoints so that the amplification system is eliminated – results in conductive hearing loss
What is sensorineural hearing loss and what can it be caused by?
When the cochlea or cochlear nerve get damaged, the signal transmitted to the primary auditory cortex is reduced or lost
It can be caused by acoustic schwannoma (tumour of the cochlear nerve) or cerebellar tumours expanding and putting pressure on thecochlear nerve
What is the term used to describe loss of hearing due to the death of hair cells in normal ageing?
Presbyacusis
Overview of how sound travels through ear
Sound enters inner ear and stimulates tympanic membrane. Sound vibrates through 3 small bones to the cochlea
What membrane runs along cochlea
Basilar membrane where hair cells sit in 4 rows
Define conductive hearing loss
Hearing loss due to issues transmitting vibration to cochlea
Causes of conductive hearing loss
Fluid accumulation in cold
Perforated tympanic membrane
Otosclerosis of bones
Inside of cochlea diagram
Sound lecture 12th minute
Mechanical properties of basilar membrane
An elastic structure that is narrow and tough near ossicles but broad and floppy near end
Significance of basilar membrane properties
The impedance changes along the membrane meaning that it vibrates at different positions along the length in response to different frequencies
How do hair cells detect sound
Motion of the basilar membrane deflects the hair bundles of a hair cell at a certain point along the membrane. The bending of stereocilia towards the tallest stereocilia (kinocilia) changes the internal voltage of the cell
Mechano transduction
Changing of hair cell internal voltage sends electrical impusle to brain
What are tip links
Filamentous linkages between stereocilia. Associated with ion channels on the stereocilia
What happens when an tip links is stretched
Ion channels open which relaxes the tip link and hair bundle
Existence of an active process in hair cells
Once a stereocilia is pulled and it’s ion channel opens it become relaxed and complies with direction of the stimulus. This means that the hair bundle must have been doing work to maintain it in its position
2 types of hair cells
Outer and inner
Difference in number between outer and inner hair cells
3 rows of outer cells
1 of inner
%of afferent projections to brain from hair cells
95 from inner cells
Where do most efferent projections to ear go
Outer hair cells
What is role of outer hair cells
Cochlear amplification and otoacoustic emissions
What do OHCs do when their internal voltage is changed
Cell body shortens then elongates in process called electromotility
What causes electromotility in OHCs
Change in orientation of membrane protein prestin
Where do hair cells synapse with sensory neurones
Spiral ganglion cells then cochlear nucleus
Tonotopic map of spiral ganglion
The cells of spiral ganglion respond best to frequency of basilar membrane in that area
Sensorineural hearing loss
Hearing loss due to issues with sensory aspect of inner ear
Retrocochlear hearing loss
Due to issues with vestibulocochlear nerve
Most common cause of hearing loss
Sesnorineural
Where do nerve fibres from organ of corti go
Cochlear nucleus in medulla
Tonotopic arrangement of cochlear nucleus
Low frequencies are more ventral
High frequencies more dorsal
Role of superior olivary complex
Compares bilateral activity of ear in that you will hear the same sound in both ears but with a time delay and at same intensity
How does medial superior olivary complex deal with interneural time differences
Delay lines
Difference between medial and lateral superior olive
Medial deals with time difference
Lateral deals with intensity difference
What do neurones do to lateral superior olive ipsilaterally
Excite
What do neuorones do to lateral superior olive contralaterally
Inhibit
How is it ensured excitation and inhibition from different sides reach lateral superior olive at same time
Contralateral inhibitory axons are much larger and synapses are larger so conduct quicker
Name of synapses for inhibitory neurones going to SOC
Calyces of held
Feedback mechanism to hair cells from lateral superior olive complex
SOC sends efferent fibres with feedback to OHCs and afferent fibres of IHCs
Purpose of feedback from SOC
Increases representation of signals and protects from damage in loud noises.
Balances the sound from both ears and reduces sensitivity of cochlea
Cause of hearing loss due to malfunctioning auditory pathway
Demyelination due to inflammation or virus
Very common in MS
How do blast injuries cause sensorineural hearing loss
Cause disruption in balance between inhibition and excitation
Where is superior olivary nuclei
Pons
Where is inferior colliculus
Midbrain
Where do all ascending auditory pathways converge
Inferior colliculus
Important feature of inferior colliculus
No localisation of sound there
Role of superior colliculus
All the information from inferior colliculus is used here to make a map of the sounds
How is superior colliculus fundamental for reflexes
Site where auditory and visual maps merge so head and eye can be orientated to sounds
Role of primary auditory complex
Central area of auditory complex where loudness rate and frequency modulation are mapped
Location of primary auditory complex
Superior bank of temporal lobe
Is primary auditory cortex tonotopically mapped
Yes
