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Neurophysiology & Human Behaviour > Hearing > Flashcards

Hearing Flashcards

1
Q

What are the properties of sound?

A
  • Amplitude (intensity): How loud the sound is.
  • Frequency: Pitch of the sound.
  • Timbre: ‘Texture of sound’.
  • Location: Source of sound.
  • Spectral flux: Changes in sound over time.
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2
Q

What is the intensity equation for sounds?

A

P = √(pressure)

P0 = 20 μPa (minimum sound pressure detectable by human ears)

Units = dB SPL (decibels sound pressure level)

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3
Q

What information can be obtained from a sound spectrum?

A
  • Pitch: Fundamental frequency
  • Timbre: Shape of the sound spectrum
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4
Q

What is the range of audible frequencies in human hearing?

A

20 Hz - 20 kHz

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5
Q

What is the range of audible frequencies defined by?

A
  • Range lies within the frequencies whose threshold of detection lies below the pain threshold, which is fairly constant.
  • Sounds beyond thresholds are not un-necessarily inaudible, but cause substantial pain at volumes required for them to be audible.
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6
Q

What is the function of the pinna?

A

Colourises sound coming from different directions in distinct ways to aid in monoaural sound localisation.

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7
Q

What is the function of the external auditory meatus?

A

Acts as a resonator and amplifies sounds up to 4 kHz

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8
Q

Why is a large amount of sound reflected at the air-water interface between middle ear and cochlea?

A
  • Density difference
  • Compressibility difference
  • Creates an impedence difference (most sound takes path of least resistance in air)
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9
Q

How can the middle ear structures amplify vibrations from air into perilymph?

A
  • Tympanic membrane 14x area of footplate of stapes, so pressure at oval window 14x air pressure.
  • Pivot action between ossicles allow for amplification of vibrations via moments.
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10
Q

What proportion of sound energy is transmitted from external ear to the perilymph of the cochlea?

A

~50%

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11
Q

What are the functions of the muscles associated with ossicles?

A
  • Stapedius: Changes angle of stapes and decreases efficiency of transfer of vibrations into oval window.
  • Tensor tympani: Contraction increases tension in tympanic membrane and thus impedence, causing more vibration energy to be absorbed.
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12
Q

What loss of sensitivity is caused by activation of the middle ear muscles?

A

30-40dB SPL

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13
Q

At what sound intensity level do the ossicle-associated muscles contract?

A

~80 dB

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14
Q

How is resonance minimised in the middle ears?

A

Numerous air cells and cavities in petrous temporal bone communicate with the middle ear, allowing sound waves to travel down them and become attenuated.

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15
Q

What is the function of the Eustachian tube?

A

Equalises pressure between atmosphere and middle ear.

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16
Q

What are the types of hearing loss?

A
  • Conductive hearing loss: Caused by failure of sound reaching the inner ear efficiently. This is usually due to earwax in the external auditory meatus, but can also by a result if otitis media (inflammation of middle ear). Although gain in sound is reduced, the hearing loss can be overcome by amplifying incoming sounds.
  • Sensori-neural hearing loss: Caused by damage to the hair cells or the neurones in the auditory pathway. E.g. presbycusis (age-related hearing loss).
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17
Q

Why are Eustachian tube infections more common in infants?

A

Their Eustachian tubes are shorter and wider than adults.

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18
Q

What is the significance of the spiral shape of the cochlea?

A
  • Spiral shpae has no functional significance.
  • Spatially, it allows a fairly long structure of the cochlea to fit in quite a tight inner ear space.
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19
Q

What is the structure of the cochlea?

A
  • 3 compartments separated by 2 membranes:
    1. Scala vestibuli
    2. Reissner’s membrane
    3. Scala media
    4. Basilar membrane
    5. Scala tympani
  • 3 openings:
    1. Oval window
    2. Round window
    3. Helicotrema
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20
Q

What are the dimensions of the cochlea?

A

Length: 35mm

Width: 100μm (base) - 500μm (apex)

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21
Q

What is the path taken by sound as it travels through the ear?

A
  1. Changes in air pressure caused by sound in the external auditory meatus causes movement of the tympanic membrane.
  2. Movements of the tympanic membrane moves the ossicles, including the stapes which is pushed in/out of the scala vestibuli through the oval window.
  3. Perilymph (like all fluids) is incompressible, so displacement of stapes into perilymph displaces the flexible cochlear partition into the scala tympani.
  4. Compression of the partition into the scala tympani displaces perilymph through the round window.
  5. Ultimately, round window displaces into the middle ear which is filled with air and is compressible and connected to atmosphere through Eustachian tube, dissipating the pressure change.
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22
Q

What is the function of the helicotrema?

A

Vibrations below 20Hz displaces perilymph out of the helicotrema, preventing them from moving the cochlear partition and being transduced as sound. This is important for countering transient fluctuations in atmospheric pressure and is active all them time (in contrast to Eustachian tube which is only active transiently).

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23
Q

What is the tonotopic map of the cochlea?

A
  • Different frequencies of sound vibrate maximally at different parts of the cochlea, allowing different frequency components of sound to be separated out.
  • Linear increments in cochlea distance represent logarithmic increments in frequency.
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24
Q

What are the principles behind tonotopic map of the cochlea?

A
  1. Sounds of higher frequencies prefer to traverse near the base. This is because their higher frequencies mean a lot of inertial force would be required to move perilymph if they travel to the apex.
  2. Sounds of lower frequencies prefer to traverse near the apex. This is because their lower frequencies mean that they are unable to effectively traverse the stiff basilar membrane at the basal aspects.
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25
Q

What is the nature of waves travelling along the cochlear partition?

A

As waves move from the base to the apex, they are continuous (as opposed to solely vibrating the parts that are associated with frequency of sound). Instead, when the wave reaches these parts, the amplitude reaches a maximum.

26
Q

What is the composition of endolymph?

A
  • High K+ and low Na+, similar to ICF.
  • Endolymphatic potential = +100mV.
  • Ionic gradients determined by the Na+/K+-ATPase.
27
Q

What is the arrangement of hair cells in the Organ of Corti?

A
  • Contains 3 rows of outer hair cells and 1 row of inner hair cells.
  • Hair cells lie on the basilar membrane and are covered by the a gelatinous mass (Tectorial membrane).
  • All hair cells have apical membranes covered by stereocilia arranged in graded V or W fashion.
28
Q

What is the significance of the graded nature of the stereocilia?

A
  • There is substantial amplification of the movements of the cilia so that there is a disproportional relationship between displacement of the cilia compared to opening of ion channels.
  • 1μm displacement of cilia results in opening of all channels in hair cells.
29
Q

What is the transduction mechanism in the cochlear partition?

A
  • Vertical movements in the basilar membrane caused by vibrations in the cochlear partition due to sound are translated into horizontal movements of the tectorial membrane.
  • Shearing between tectorial membrane and stereocilia causes ion channels on adjacent hair cells to be pulled open by tip-links between hair cells.
  • Opening of ion channels causes inward K+ current driven purely by electrical potential gradient.
  • Movement of cilia in opposite direction results in closing of ion channels.
30
Q

What is the function of outer hair cells?

A
  • They are very sensitive and are able to detect amplitudes of sounds much more accurately compared to inner hair cells.
  • They shorten on depolarisation and lengthen on hyperpolarisation.
  • They alter the sensitivity of the cochlear partition in accordance to the sounds being detected.
31
Q

What is the function of the inner hair cells?

A
  • They detect and relay sound information to the brain.
  • They synapse with fibres of the cochlear nerve via excitatory Glu synapses.
  • There is massive divergence between IHCs and cochlear nerve fibres, with one IHC synpasing with ~20 nerve fibres.
32
Q

What is the difference in connectivity between auditory fibres and OHCs/IHCs?

A

OHCs:

  • Synapse with both afferents and efferents
  • Synapse with 1 auditory fibre only, but 1 auditory fibre synapses with multipe OHCs (convergence)

IHCs:

  • Synpase with afferents only
  • Synapse with multiple auditory fibres (divergence)
33
Q

Why do OHCs have higher sensitivity compared to IHCs?

A
  • Their stereocilia are embedded within tectorial membrane, so there are greater shearing forces.
  • There is great convergence between OHCs and cochlear nerve fibres, resulting in pooling of information.
34
Q

Where are the cell bodies of auditory afferent fibres located?

A

Spiral ganglion

35
Q

What are the relative proportions of afferent fibres dedicated to IHC/OHC outputs?

A

IHC: 95%

OHC: 5%

36
Q

How is intensity encoded?

A
  • Frequency of firing (higher frequencies)
  • Recruitment of fibres
37
Q

How is frequency coded?

A
  • <~4KHz: Periodicity code (pitch)
  • >~4KHz: Place code (timbre)
38
Q

Why are place codes not used at low frequencies?

A

Frequency falls of the end if the cochlea in terms of the tonotopic map.

39
Q

Why are periodicity codes not used at high frequencies?

A
  • Frequency exceeds the maximum firing freuqncy of the neurone imposed by the refractory period.
  • Periodicity becomes obscured by the time constant of IHCs.
40
Q

What is the two-tone suppression?

A

This phenomenon is whereby if a second frequency differing from the characteristic frequency (in neighbouring regions) of an auditory fibre is added to the characteristic frequency, the responses of the fibres to frequencies flanking the characteristic frequency are reduced, sharpening the response of the auditory fibre to its characteristic frequency and minimising response to second frequency.

41
Q

What are the types of efferent pathways present?

A
  • Crossed pathway: Ends exclusively at the OHCs.
  • Uncrossed pathway: Ends at afferent terminals of cochlear nerve fibres where they synapse with IHCs.
42
Q

What are the effects of the crossed pathway?

A
  • Stimulation substantially decreases sensitivity of particular fibres towards their characteristic frequency but does little to accessory frequencies.
  • Stimulation also decreases selectivity of fibres towards frequencies (detuning).
  • These are reflex responses the background noise and tones of specific frequencies.
43
Q

What are the functions of the crossed pathway?

A

The purpose of these reflexes is thought to be to reduce the response of the cochlea to constant background noises in order to detect more significant noises. For example, continuous stimulation of one type of fibre by a noise of constant frequency would reduce the sensitivity of that particular fibre to that noise, thus reducing the cochlear response to it.

44
Q

What is the central auditory pathway and nature of connectivity?

A
45
Q

In which nerve tract do fibres from DCO to IC travel?

A

Trapezoid bodies

46
Q

In which nerve tract do fibres from VCN to SOC travel?

A

Lateral lemniscus

47
Q

How many axons are contained within each cochlear nerve?

A

~30000

48
Q

What is the anatomy of the cochlear nuclear complex?

A
  • The cochlear nuclear complex is located in the brainstem.
  • It is the first structure fibres from the cochlear nerve synapses with.
  • It is made up of 3 nuclei:
    1. Dorsal cochlear nucleus (DCN)
    2. Anteroventral cochlear nucleus (AVCN)
    3. Posteroventral cochlear nucleus (PVCN)
49
Q

What are the functions of the nuclei found in the cochlear nuclear complex?

A
  • DCN: Cells within are complex and show periodic bursting to sustained stimulation. Involved in determining nature of sound (e.g. frequency, timbre…).
  • AVCN: Simple cells that have responses analogous to the auditory fibres. They show frequency specificity and phase-locking for lower frequency sounds.
  • PVCN: Contains mixture of cells displaying AVCN and DCN properties.
50
Q

What is the nature of connectivity for nuclei in CNC?

A

DCN: Crosses dorsally and ascends to IC with output from SOC in lateral meniscus.

VCNs: Cross ventrally to SOC in trapezoid body.

51
Q

What is the anatomy of the superior olivary complex?

A
  • Extends from rostral medulla to mid pons.
  • Separated into 2 main subdivision:
    1. Medial superior olive (MSO)
    2. Lateral superior olive (LSO)
  • It is the first part of the auditory pathway to receive bilateral inputs from fibres on both sides, and so is involved in localisation of sound.
52
Q

What are the functions of the SOC subdivisions?

A
  • MSO: Responds to time differences in stimuli from both ears (phase difference detection).
  • LSO: Excited by ipsilateral stimulation and inhibited by contralateral stimulation (intensity difference detection).
53
Q

What is the anatomy and functions of the inferior colliculus?

A
  • Found in the midbrain and receives inputs from both DCN and SOC.
  • Integrates information regarding nature of sound (from DCN) and localisation of sound (from SOC).
  • May have projections to superior colliculus (involved in visual motor responses) and so are involved in reflex direction of vison towards sounds and orienting pinna towards sources of sounds.
54
Q

What is the anatomy of the medial geniculate body?

A
  • Found in the thalamus and receives inputs from bilateral ICs.
  • Divided into 2 regions:
    1. Lateral: Neurones display similar properties to ICs and project into the primary auditory cortex (A1).
    2. Medial: Neurones respond to more complex sounds and carry information to secondary auditory cortex (A2).
55
Q

What is the location of the primary auditory cortex?

A

Upper banks of superior tempral gyrus

56
Q

What is the structure of the primary auditory cortex?

A
  • Tonotopically organised and preserves tonotopic map of cochlea.
  • Neurones responding to particular frequencies are organised into columns.
  • Neurones within each frequency column respond to sum and differences in intensities between both ears and so are involved in sound localisation.
57
Q

What are the angles associated with sound localsation?

A
  • Angle of azimuth: Angle of incoming sound relative to the horizontal plane of the head.
  • Angle of elevation: Angle of incoming sound relative to the vertical plane of the head.
58
Q

What are biaural cues for sound localisation?

A
  • Interaural phase differences: Low frequencies (1-2 kHz)
  • Interaural intensity differences: High frequencies (>2 kHz)
59
Q

Why do cones of confusion not really exist?

A
  • The asymmetry of pinna means that even sounds from corresponding loci on the cone of confusion are slightly different from each other in terms of colouration.
  • The ability to move our heads means that we are able to gain further information regarding to sound source.
60
Q

Where are nonoaural colourations through to be decoded and why?

A
  • Before midbrain.
  • Signals arriving at midbrain are very direction-specific.
61
Q

What is the accuracy of human sound localisation?

A

Angle of azimuth: 1-2o

Angle of elevation: ~10o

Neurophysiology & Human Behaviour (20 decks)

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