Sound Conduction and Transduction Flashcards by Owen Vineall

What scale is used to measure how loud a sound is?

Decibels (logarithmic scale)

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What is the name given to the wing shaped flap skin and cartilage that makes up the outer ear?

Pinna

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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

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Is the tympanic cavity fluid-filled or air-filled?

Air-filled

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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

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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

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What are the three compartments of the inner ear?

Scala Vestibuli
Scala Media
Scala Tympani

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Which types of fluid do each compartment contain?

Scala Vestibuli + Scala Tympani = perilymph

Scala Media = endolymph

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What structure connects the two perilymph compartments?

Helicotrema

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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.

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Describe the difference in sensitivity of different parts of the basilar membrane.

Higher frequency sounds = base

Lower frequency sounds = apex

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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

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Where is the Organ of Corti found?

It lies on top of the basilar membrane and beneath the tectorial membrane

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What are the two types of cell in the organ of corti?

Inner and outer hair cells

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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

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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

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Which compartment of the cochlea does the stereocilia of the hair cells project into?

Endolymph (base is in the perilymph)

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What internally generated sounds are the outer hair cells responsible for?

Otoacoustic emissions

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What are stereocilia connected by?

Tip links

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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

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Describe what happens when the basilar membrane is displaced downwards.

Hyperpolarisation
Stereocilia move towards the modiolus
K+ channels close

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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

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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

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Up to what point is the auditory pathway from one ear ipsilateral?

Cochlear nuclei

Beyond this point there is bilateral representation

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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