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Y2 LCRS2 Neuroscience and Mental Health > Sound conduction and transduction > Flashcards

Sound conduction and transduction Flashcards

1
Q

What is the range of frequencies heard by humans?

A

20Hz to 20kHz

The ear works at 2000 times per second

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2
Q

What is the difference between pitch and timbre.

A

Pitch = perception of frequency

Timbre = what distinguishes two sounds at the same frequency and intensity

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3
Q

What are the units used to measure soud intensity? What intensity of sounds can we hear?

A

Sound intensity = loudness = how much energy/how many Joules are delivered per second through one square meter. Therefore units: Watts m-2 or Joules/s.m2

Humans can hear from 1x10-12Watts/m2 to 1Watt/m2

Loudest sound intensity heard(threshold before it is too painful) is ~12 orders of magnitude larger (120 decibels)

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4
Q

What is the deciBel scale and why is it used?

A

deciBel scale = defines sound level (10xBel scale).

This is a more manageable scale than Watts/m2 as it is LOGARITHMIC. Instead of measuring intenisty with respect to the faintest perceivable intensity of sounf we compare logarithms.

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5
Q

Define sound.

A

Changes in air pressure

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6
Q

Define frequency of sound.

A

Frequency of sound = periodic changes in pressure, consisting of compressed and rarefied air – frequency = no. of compressed or rarefied patches of air passing ears / sec.

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7
Q

What are Hertz?

A

Cycle of sound = distance between successive patches of compressed air - Hz

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8
Q

Summarise how sound reaches the brain.

A
  • Mechanisms of compression and rarefraction cause sound to travel through air
  • It bounces off the PINNA and CONCHA of the exterior ear
  • It enters the ear canal
  • Sound vibrates the TYMPANIC MEMBRANE causing the three bones of the middle ear to vibrate: 3 ossicles= malleus, incus, and stapes.
  • This sends energy through the oval window and into cochlea
  • In the cochlea it is changes into chemical signal by hair cells in he organ of corti
  • Hair cells synapse onto the spiral ganglion fibres
  • These travel through the cochlear nerve into the brain
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9
Q

What is the function of the ossicles of the middle ear?

A

3 osscicles - malleus, incus, stapes

Function:

  • Transmit vibration of the tympanic membrane onto cochlea
  • Match impedance to reduce loss in energy as the vibration goes from air to cochlea
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10
Q

Define impedance.

Define resonant frequency.

A

Impedance - reluctance of a system in receiving energy from a source.

Resonant frequancy - frequency at which impedance of the system is minimal

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11
Q

How is the tension of the tympanic membrane adjusted?

A

Malleus and incus are repositioned by the tensor tympanic muscle and the stapedius muscles controlling the tension.

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12
Q

What is conductive hearing loss?

A
  • when the ear is not capable of transmitting the vibration of sound waves onto the cochlea
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13
Q

List some causes of conductive hearing loss.

A
  • Cerumen (ear wax)
  • Infections such as otitis
  • Tumours
  • Fluid accummulation (in children) during a cold
  • Perforated tympanic membrane
  • Otosclerosis - abnormal growth of bone in the ear canal
  • Barotrauma (temporary and can be finxed by the Valsalva maneouvre to reopen the Eustachian tubes)
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14
Q

What causes the vibration of the basiclarr membrane?

A

(Motion of stapes –> )difference in pressure between the two liquid filled chambers of the coclea –> vibration of the basilar membrane

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15
Q

What does the Organ of Corti consist of?

A
  • Basilar membrane
  • Tectorial membrane
  • Hair cells
  • Supporting cells
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16
Q

What is the function of the basilar membrane?

A

FREQUENCY ANALYSER

An elastic structure of heterogenous mechanical properties that vibrates at different positions along its length in response to different frequencies to break down complex sounds(by distributing the energy of each component frequency along its length)

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17
Q

Describe the characteristics of the basilar membrane which allow it to act as a frequncy analyser for complex sounds.

A
  • ~30nm
  • Narrow, tough (proximal to tympanic membrane) and broad, floppy distally.
  • Elastic
  • Hair cells (sensory receptors) present along the whole length to detect all frequencies
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18
Q

What is the function of hair cells in the inner ear?

A

Sensory receptors of the inner ear - motion of the basilar membrane deflects the hair bundles of hair cells which sense this and produce a response.

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19
Q

Summarise the process of mechano-transduction by hair cells.

A

Bending of the stereocilia towards the tallest stereocilim changes the internal voltage of the cell –> electrical signal to the brain

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20
Q

What is the function of tip links?

A
  • project the force of the stimulus onto ion channels - stretching of tip links–> opening of ion channels –> response currents
  • connect stereocilia to each other and stretch when stereocilia slide
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21
Q

What two observation made scientists believe that tip links were important in mechanotransduction?

A
  • Tip links share their location with ion channels
  • Disruption of tip links abolishes mechanotransduction (e.g. loud noises–> 12 hours to recover)
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22
Q

Why does the stiffness of the hair bundle appear to be negative in response to a stimulus? What is the importance of this?

A

The hair bundle is not a passive system - it ACTIVELY complies with the direction of the stimulus so that measured stiffness becomes negative.

Need for active amplification: a large portion of the energy is lost in viscous dampening effects of cochlear liquids. The sensitivity and sharp frequency selectivity of the cochlea would not be possible without active processes (/with the basilar membrance impodance alone)

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23
Q

What happens to the hair bundle and tip link once the MT ion channel opens?

A

Tip link becomes relaxed and in turn so does the whole hair bundle

24
Q

Describe the 4 aspects of the active process in the inner ear hair bundles.

A

Sensitivity and sharp frequency selectivity of cochlea can be explained by these 4 aspects of the active process:

  1. Amplification - helps hear weak sounds (blue = passive, red=active)
  2. Frequency tuning - a passive response would not be able to amplify specific frequencies and would produce a broad response
  3. Compressive nonlinearity - amplification diminishes progressively with increasing intensity of stimulus
  4. Spontanous otoacoustic emission - 70% of normal ears emit one or more pure tones when in a quiet environment - this is due to work done to counteract the viscous drag in the cochlea
25
Q

How many inner and outer hair cells are present per cochlea?

A

Inner - ~3500 per human cochlea

Outer - ~110000 per human cochlea

26
Q

Where do inner and outer hair cells project?

A

Inner hair cells - 95% of AFFERENT projections - sensory axons that carry signals from cochlea towards brain

Outer hair cells - EFFERENT projections that carry signals from brain to cochlea

27
Q

Which structures help cochlear ampllification and otoacoustic emissions(active processes)?

A

Outer hair cell by electromobility (spontaneous back and forth movement) - because when efferent fibres are activated, frequency selectivity and sensitivity are enhanced.

BUT animals without OHC can also produce otoacoustic emissions so this might be wrong.

28
Q

Describe the electromobility of outer hair cells.

A

Cell body of OHC shortens and elongates when their internal voltage is changed = electomobility

Can happen at a rate of 80kHz

DUE TO reorientation of the protein PRESTIN

29
Q

Where do inner hair cells transmit information to?

A

Inner hair cells –> cochlear ganglion(spiral ganglion) –> cochlear nucleus via axons in the cochlear nerve

30
Q

What is tonotopy?

A

The spacial arrangement of where sounds of different frequencies are processed in the brain.

31
Q

Where does the tonotopic map begin?

A

At the cochlear ganglion(spiral ganglion) - each ganglion cell responds best to stimulations at a particular frequency i.e. responds best to resonant frequency of the basilar membrane in that same area.

32
Q

Describe how sensory information travels from the inner hair cells to the spiral ganglion.

A
  • Via neurotransmitter release (mechanical–>chemical–>electrical signal)
  • NTs are continuously released at rest but the rate changes in reponse to change in presynaptoc voltage (as a result of MT ion channel gating).
  • Each IHC transmits information to multiple spiral ganglion cells

  • Wikipedia:*
  • IHC are displaced by the vibrations in the fluid, and depolarise by an influx of K+ via their tip-link-connected channels, and send their signals via neurotransmitter to the primary auditory neurons of the spiral ganglion.*
33
Q

How is information transmitted from the cochlear ganglion to the cochlear nucleus?

A

Via the axons of the cochlear nerve - each axon is responsive to a single frequency

34
Q

What is the volley theory?

A

Volley theory states that:

  • groups of neurons of the auditory system respond to a sound
  • by firing action potentials slightly out of phase with one another
  • so that when combined, a greater frequency of sound can be encoded
  • and sent to the brain to be analyzed.
35
Q

Describe the use of population phase-locking in nerve fibres.

A
  • Phase-locked = occurs at same point of cycle, every cycle.
  • Nerve fibres produce a phase-locked collective response at frequencies that single nerve fibres could not manage individually.
  • This allows encoding of middle-high frequencies.
36
Q

What is sensorineural hearing loss?

A

Problem is rooted in the sensory apparatus of the inner ear or in the vestibulocohclear nerve (retrocochlear hearing loss)

This is the most widespread type of hearing loss.

37
Q

What are the causes of sensorineural hearing loss?

A
  • Loud noises - headphones at high volume can cause temporary or permanent hearing loss (Club: ~100 dB, Rock concert: ~120 dB)
  • Ménière’s disease: excess of fluid in the cochlea
  • Many genetics mutations affect the Organ of Corti
  • Aminoglycoside antibiotics are toxic for hair cells
  • Congenital diseases (rubella, toxoplasmosis)
  • Ageing (presbycusis)
38
Q

Describe how cochlear implants work.

A
  • Hearing loss is due to los of hair cells which cannot regenerate
  • We can bypass the dead cells and stimulate the nerve fibres directly, detect sounds, break them down into constituent frequencies and send signal to the auditory nerve via antennas
  • Elongated coil in inserted into cochlea with a pair of electrodes corresponding to single frequencies
  • Early models had 4 channels but you need 20 to be able to understand speech well
39
Q

Where is the cochlear nucleus? How are its neurons arranged?

A

Brainstem

Neurons are arranged tonotopically - low frequencies ventrally and high frequencies dorsally.

40
Q

What structure helps localise sounds in the vertical plane?

A

Pinna - shape of the outer ear- provides a monaural cue to localisation/spectral cue.

This is because our ears detect and affect sounds coming from different directions differently because of their asymmetrical shape. So vertical localisation is impaired in the convolusions of the pinnae are covered.

41
Q

What do high frequency sounds cause between the two ears?

A

High frequency sounds –> intensity differences between the two ears –> constructive and destructive interference

42
Q

Describe the function of the pinna in sound localisation.

A
  • The pinna has various ridges and folds that act to reflect and absorb certain frequency components of the sound wave .
  • Because the pinna is not symmetric, sounds which come from different directions will have different spectral characteristics. (i.e. certain frequencies will be louder or softer depending on the direction they enter)
  • So sounds which come from above our heads seem slightly different than sounds coming from below.
  • This allows us to localize (pinpoint the direction of) a sound source. We therefore immediately look up when someone calls us from an upper story window.
43
Q

Name a type of cell in the dorsal cochlear nucleus.

A

Fusiform cell

44
Q

Name and describe two types of cells found in the cochlear nucleus and superior olive which are heterogenous and specialised for auditory processing.

A

T-stellate cells = encode sound frequency and intensity of narrowband stimuli. Their tonotopic array represents the sepcra of sounds.

Bushy cells = produce more sharply but less temporally precise versions of the cochlear nerve fibres. Provide resolution required to encode relative time of interval of inputs to the two ears.

45
Q

What is the function of the superior olivary complex? What are its two parts?

A

SOC compares the bilateral activity of the cochlear nuclei.

It consists of:

  • Lateral superior olive
  • Medial superior olive
46
Q

Compare the function of the medial and lateral superior olive.

A

MEDIAL SUPERIOR OLIVE

  • Interaural time difference is computed: sounds are first detected at the nearest ear before they reach the other one.
  • Bushy cells carry information about the timing of inputs at every cycle.
  • A map of interaural delay can be formed due to delay lines (birds).

LATERAL SUPERIOR OLIVE

  • Detects differences in intensity between the two ears (>2 kHz in humans due to head size).
  • Neurons are excited by sounds arising from the ear in the same side (ipsilaterally), while they are inhibited by opposite sounds (contralaterally).
  • Interaural level difference is computed to localise sounds in the horizontal plane
47
Q

Where do the SOC neurons feedback to? Why is this necessary?

A

SOC neurons feedback to hair cells:

  • Neurons from the medial superior olive - IHCs bilaterally.
  • Neurons from the lateral superior olive - OHCs ipsilaterall

This feedback is used to balance responses from the two ears, but also to reduce the sensitivity of the cochlea.

48
Q

What happens in the inferior colliculus? What are its three subdivisions?

A
  • Responses from different frequencies merge - all ascending pathways converge.
  • In the IC many carry information about sound location e.g. precedence effect (when two identical sounds are presented in close succession they will be heard as a single fused sound)
  • Subdivisions: central nucleus, dorsal cortex and external cortex. Only central nucleus is tonotopically organised.
49
Q

How does sensitivity of neurons to complex sounds change as you go up the cortex?

A

The more we ascend towards the cortex the more neurons are responsive to complex sounds.

50
Q

What is the function of the superior colliculus?

A
  • Here auditory and visual maps merge.
  • Neurons are tuned to respond to stimuli with specific sound directions.
  • The auditory map created here is fundamental for reflexes in orienting the head and eyes to acoustic stimuli.
51
Q

Which neurons are important in the reflexes to turn your head towards sounds?

A

The neurons in the superior colliculus.

52
Q

Where is the primary auditory cortex A1 located?

A

In the superior bank of the temporal lobe - central aread of the AC

53
Q

How is the primary auditory cortex mapped? What is mapped within it?

A

Tonotopically mapped

Loudness, rate, frequency modulation are mapped in A1.

54
Q

What two streams are present in the superior auditory cortex?

A

“What” and “where” stream

55
Q

What is the Jeffress Model for ITD (iinteraural time differences)?

A

Medial superior olive is involved in making a neural map of the auditory space

Delay lines are involved - tehy act as coincidence detectors by firing maximally when receiving simultaneous inputs from both ears.

https://en.wikipedia.org/wiki/Coincidence_detection_in_neurobiology

Y2 LCRS2 Neuroscience and Mental Health (17 decks)

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