Sensory Three - Hearing part one Flashcards by Calum Macindoe

What range of sound can humans detect?

20 - 20,000hz

otherwise infra or ultrasound

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What are the external structures of the ear?

Pinna
Concha
External Auditory meatus
Tympanic membrane (ear drum)

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What is the function of the pinna?

Diffracts and focuses sound waves into the concha

Essential for localisation in the vertical plane.

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Whats the function of the concha?

Resonator

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Whats the function of the tympanic membrane?

Resonates with sound waves to transfer this energy to the middle ear

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Whats the maximum time difference between the ears hearing a sound?

760ms

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What is threshold?

The lowest intensity (dB) that can be heard of a sound at a particular frequency

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What is the perception limit?

The highest intensity that can be heard of a particular frequency.

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List the structure of the middle ear:

Middle ear cavity
Eustachian tube (connects to pharynx)
3 Ossicles:
- Malleus
- Incus
- Stapes
2 muscles
- Tensor tympanic muscle
- Stapedius muscle

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What is the function of the middle ear?

Conducts sound from the tympanic membrane to the oval window of the cochlea.

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What are the functions of the muscles in the middle ear?

The Attenuation reflex.

They contract in response to loud sounds making the ossicular tree very rigid to protect the inner ear and prevent conduction.

Takes 50-100ms to sttart so doesnt always work

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When else does the attenuation relfex occur?

When we speak to prevent us from hearing our own voices

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Describe the structure of the inner ear:

The cochlea and vestibule+semicircular canal= organ of balance.

While the cochlea is allows mechanical to electrical transduction of sound.

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Which bone houses the cochlea?

The temporal bone

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Describe the structure of the cochlea

Two parts:

3 fluid filled compartments
Bony mateous, central structure containing auditory nerves and blood vessels

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What are the three fluid compartments of the cochlea called?

Scala

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What are two external structures of the cochlea?

Oval window (where the stapes inserts)
Round window ( covered by a membrane)

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Describe the scala of the cochlea:

Scala vestibuli
- Seperated by membrane
Scala media
- Seperate by the organ of corti
Scala tympani

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Describe the fluids of the scala

Scala vestibuli and tympani are perilymphatic and contain fluid similar to ECF

Scala media has high K concentration and is considered similar to ICF and is endolympahtic

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Why does the scala media contain high concentrations of K?

Its endolymphatic fluid generates the endocochlear potential

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What innervates the sensory cells in the cochlea?

They receive innervation from the spiral ganglion neurons.

What forms the lateral wall of the cochlea?

The stria vascularis and spiral ligament

What is the function of the stria vascularis?

It is an endothelial tissue that secretes K into the scala media to generate the endochochlear potential.

Whats unique about the spiral ligament?

Contains fibroblasts which provide structural and metabolic function.

What is the function of the organ of corti?

Contains sensory cells for sensory transduction

Describe the structure of the organ of corti:

Contains inner and outer hair cells. IHC - Supported by pillar cells OHC- Supported by dieters cells These structures are all connected to the basilar membrane and innervated afferently by the spiral ganglion. The opposing membrane is the tectorial membrane

How does sensory transduction work?

The sound is conducted and causes the basilar membrane to move. It moves up towards the tectorial membrane and if the OHC contact, friction causes them to switch into the on position. Presetin protein becomes activated, they contract into an off position. BUt by doing so the INC come into contact are switched into the on position - opening up K channels and sensory transduction occurs.

What is the role of OHC and INC?

OHC- amplifies sound, selects intensity and Hz (as depending on which part of the basilar membranes OHC come into contact with the tectorial membrane) IHC- Sensory cells

What do INC and OHC both have?.

A plate of 100 sterocilia INC have linear lined stereocili and OHC are v shaped.

What is special about stereocilia length?

Length corresponds to Hz of sound.

What sort of gating to the IHC and OHC have on their ion channels?

Mechanically gated

What are the three position the organ of corti / basilar membrane can take and what do they represent?

``` Up = excitatory middle = rest down = inhibitory ```

When the OHC depolarize, what is the name of this process?

Reverse transduction

What is unique about the stereocilia of an individual hair cell?

They are interconnected so it is an all or none response.

What are the two transduction processes in hearing?

Forwards transduction and reverse transduction

Describe forwards transduction

Movement of stereocilia to the excitatory position, leading to the activation of mechnically gated K channels and the depolarisation of IHC leading to neurotransmission.

Describe reverse transduction

- Also known as cochlear amplification Movement of stereocilia to the excitatory position, leading to the activation of mechnically gated K channels in OHC, this energy influx causes the protein prestin to contract, pulling the stereocilia into the unexcited position, but allowing IHC to become actiavted.

What innervates the organ of corti?

The spiral ganglion fibres, Either: - Afferent fibres - Efferent fibres

What do spiral ganglion afferent fibers innervating the organ of corti do?

Innervate mainly the IHC Varying in response and spontaneous firing rate Provides the Hz and intensity encoding

What do spiral ganglion efferent fibers innervating the organ of corti do?

Innervate mainly the OHC and regulate excitability depending on the feedback from brainstem auditory pathways. This contributes to dynamic changes in hearing sensitivity as ambient sound levels change.

What are the types of spiral ganglion AFFERENT neurons?

Type one fibres - Large, myelonated, inneravted IHC (10-30 per IHC) Type Two fibres - (5%) Small, unmyeolonated, 10 OHC per 1 fibre.

Describe the synaptic connections of the afferent type one fibres

Afferently terminate on the cochlear nuclei | Efferently innervated from Lateral olive sup nuclei

Describe the synaptic connections of the afferent type two fibres

Afferently terminate on the cochlear nuclei | Efferently termiante from medail sup olive nuclei

What is the neurotransmitter on IHC?

Glutamate

Seeing as the neurotransmitter is glutamate what can loud sound lead to?

NMDA activation

What is the neurotransmitter on OHC/

ACh

How is frequency discrimination achieved at a anatomical level?

- Sound vibrations are filtered along the cochlear partition to produce focused displacement of the basilar membrane at frequency specific regions (high Hz base and low Hz at apex)

What is passive tonotopy and how does it relate to frequency discrimination?

Each wave reaches its maximal amplitude at the position appropriate for the Hz of stimulation , then rapidly declines in size as it advances toward the cochlear (movement of the basilar membrane)

How is the basilar membrane the mechanical analyzer of sound?

The basilar membrane is narrow at the base and wide at the apex.

How is complex sound analyzed on the basilar membrane?

There will be multiple peaks at different locations therefore it processes each peak in the brain.