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Neuroscience and Mental Health > 10. Sound conduction and transduction > Flashcards

10. Sound conduction and transduction Flashcards

1
Q

What is the external auditory meatus?

A

Ear canal

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2
Q

What is the ear drum also known as?

A

Tympanic membrane

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3
Q

What does the middle ear comprise?

A
  • Tympanic membrane
  • Malleus
  • Incus
  • Stapes
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4
Q

Signals are transferred from the cochlear to the central pathways via which nerve?

A

Vestibulocochlear nerve (VIII)

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5
Q

What is sound?

A
  • Sound causes a periodic change in air pressure, thus consists of compressed and rarefied air
  • Occur at 343m/sec
  • Frequency: number of compressed or rarefied patches of air that pass our ears
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6
Q

What frequencies is the human ear sensitive to?

A

20 - 20,000 Hz

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7
Q

What is the intensity of sound?

A
  • The difference in pressure between the compressed and rarefied air regions
  • Determines the loudness of sound that we perceive
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8
Q

Describe the passage of sound from the outside to the cochlea?

A
  • Pinna (outer ear) collects sound and channels it down the external auditory meatus
  • Entrance to ear - 2.5cm inside the skul
  • Tympanic membrane vibrates
  • The 3 bones (ossicles) transfer the movement of the ear drum to the fluid filled cochlea
  • Hair cells in the cochlea can depolarise and hyperpolarise to transfer frequency as a neural signal
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9
Q

What is the eustachian tube?

A
  • A tube that links the nasopharynx to the middle ear
  • It is part of the middle ear
  • Equalises pressure between middle ear and nasal cavity
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10
Q

What is the oval window?

A
  • Membrane-covered opening between the middle ear and the vestibule of the inner ear
  • Behind the stapes bone
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11
Q

What does the inner ear comprise?

A 
• Cochlea
• 3 fluid-filled chambes
- scala vestibule
- scala media
- scala tympani
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12
Q

What is the function of the ossicles?

A
  • Amplify the sound pressure
  • Important as the fluid in the inner ear resists movement
  • Makes the pressure bigger at the oval window compared to the tympanic membrane (small SA of OW also helps)
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13
Q

What membrane separates the scala vestibule and the scala media?

A
  • The Reissner’s membrane

* Sound causes pressure difference either side of this membrane, separating the 2 fluids

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14
Q

What membrane separates the scala media and the scala tympani?

A

The Basilar membrane

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15
Q

What is the fluid filling the chambers of the inner ear called?

A
  • Perilymph (CSF like - low k, high Na)

* Endolymph (high K, low Na)

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16
Q

Describe the Basilar membrane and how it carries out its function

A
  • Wider at the apex than the base by x5
  • More flexible at the apex and stiffer at the base
  • Movement of the stapes causes the endolymph to flow in the cochlea => travelling wave in the membrane
  • Distance of the wave depends on the frequency
  • Different locations of the membrane are maximally deformed at different frequencies
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17
Q

What is the function round window?

A
  • Window with a membrane between the middle and inner ear
  • When there is pressure at the OW, perilymph is pushed into the scala vestibule
  • Pressure travels to the scala vestibule, through the helicotrema and back down the scala tympani
  • Fluid pressure has nowhere to go - RW bulges to allow for pressure
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18
Q

What stereocilia?

A

Inner and outer sensory hair cells on top of the basilar membrane

19
Q

What is the function of the stereocilia?

A
  • Amplify and improve the clarity of sound

* Extent of movement depends on frequency

20
Q

What is the difference between the inner and outer hair cells?

A 
Inner
• 3,500
• Primary sensory cells
• Generate APs in the auditory nerves
• Stimulated by the fluid movements
• 95% of afferent projections from here

Outer
• 20,000
• Become short on depolarisation
• Become long on hyperpolarisation
• Increased the amplitude and clarity of sounds
• More efferent projections connected here

21
Q

Describe the pathway of a signal from the cochlea to the auditory cortex?

A
  • E - Eighth nerve (vestibulocochlear)
  • C - Cochlear nuclei
  • O - superior Olivary nucleus
  • L - Lateral Leminiscus
  • I - Inferior Colliculus
  • M - Medial geniculate body
  • A - Auditory Cortex
22
Q

What is tonotopy?

A
  • The spatial arrangement of where sounds of different frequency are processed in the brain
  • Different regions of the basilar membrane vibrate at different frequencies due to variations in thickness and width
  • Nerves that transmit information from different regions of the basilar membrane therefore encode frequency tonotopically
  • This tonotopy then projects through the vestibulocochlear nerve and is present throughout the auditory nuclei
  • Low frequencies transmitted ventrally, high frequencies dorsally
23
Q

How does neural firing compare at low, mid and high frequencies?

A

• Low frequency - phase locking: action potentials firing at times corresponding to a peak in the sound pressure waveform
• Mid frequency - phase locking and tonotopy
• High frequency - tonotopy
- different neurones fire on successive cycles

24
Q

What is the interaural time difference?

A
  • The difference in arrival time of a sound between two ears
  • Important in the localisation of sounds
  • Detected by neurones in the brainstem
25
Q

How can some people with hearing loss still detect sound?

A
  • Direct transmission through the cochlea
  • Bone conduction
  • Clinically important - allows us to detect where a problem is
26
Q

Where is there a problem if bone conduction is better than air conduction?

A

Anywhere between the pinna and the cochlea

27
Q

What are the main causes of hearing loss?

A
  • Loud traumatic sounds
  • 200 genetic conditions
  • Infections e.g. meningitis
  • Drugs
  • Ageing
28
Q

What is the decibel scale?

A
  • Measures the intensity of sound

* Logarithmic scale using base 10

29
Q

What are the possible causes of conductive hearing loss?

A
  • Cerumen (ear wax)
  • Infections (otitis)
  • Tumours
  • Fluid accumulation in the middle ear
  • Perforated tympanic membrane
  • Otosclerosis - abnormal bone growth
  • Barotrauma
30
Q

What does the Organ of Corti include?

A

Basilar and tectorial membranes, hair cells and supporting cells

31
Q

Describe the stereocilia

A

• Bend towards the tallest stereocilia towards the tallest stereocilium changes the internal voltage of the cell
- mechano-transduction
• Connected by filamentous linkages - tip links
• Tip links share their location with ion channels
• Disruption of tip links abolishes mechano-transduction - loud noises, takes 12 hours to recover

32
Q

What happens when efferent fibres are activated to outer hair cells?

A
  • Frequency selectivity and sensitivity is enhanced
  • Bodies shorten and elongate when internal voltage is changed - electromobility
  • Happens at rate of 80 kHz
  • Due to reorientation of the protein ‘prestin’
33
Q

What happens at the spiral ganglion?

A
  • Hair cells (mostly inner) form synapses with sensory neurones here (aka cochlear ganglion)
  • NTs constantly released at rest, but the rate changes in response to a change of the presynaptic voltage, due to MT ion channel gating
  • Each ganglion cell responds best to stimulation at a particular frequency
34
Q

What is sensorineural hearing loss?

A

Problem with sensory apparatus of the inner ear or the vestibulocochlear nerve (retrocochlear hearing loss)

35
Q

What are the causes of sensorineural hearing loss?

A
  • Loud noises
  • Ménière’s disease - excess fluid in the cochlea
  • Many genetic mutations affect the Organ of Corti
  • Aminoglycoside antibiotics are toxic for hair cells - Streptomycin
  • Congenital diseases
  • Ageing (presbycusis)
36
Q

How do cochlear implants restore hearing?

A
  • Hearing loss is primarily due to the loss of hair cells
  • These do not regenerate in mammals
  • A cochlear implant involves an elongated coil inserted into the cochlea with pairs of electrodes
  • This bypasses the dead cells and stimulate the nerve fibres directly
  • They detect sound, break it down into constituent frequencies, then send signals directly to the auditory nerve via antennas
  • The pairs of electrodes correspond to single frequencies
37
Q

What does the dorsal cochlear nucleus do?

A
  • Locates sounds in the vertical plane
  • Detect and differently affect sounds coming from different directions due to their asymmetrical shape - spectral cues
  • Only animals with the dorsal cochlear nucleus can do this
38
Q

What are T-stellate cells?

A
  • Encode sound frequency and intensity of narrowband stimuli
  • Their tonotopic array represents sounds’ sprectra
  • Part of cochlear nucleus
39
Q

What are Bushy cells?

A
  • Produce more sharply but less temporally precise versions of the cochlear nerve fibres
  • Provide the resolution required to encode the relative time of arrival of inputs to the two ears
  • Part of the cochlear nucleus
40
Q

What does the superior olivary complex (SOC) do?

A

• Compares the bilateral activity of the cochlear nuclei
• The medial superior olive computes the interaural time difference
• The lateral superior olive detects differences in intensities between the two ears
- interaural level difference is computed to localise sounds in the horizontal plane
- neurone inhibited by opposite sounds

41
Q

Which hair cells do Superior Olivary Complex neurones feed back to?

A
  • Neurones from the Medial SO => inner hair cells bilaterally
  • Neurones from the Lateral SO => outer hair cells ipsilaterally
42
Q

What happens at the inferior colliculus?

A
  • Responses from different frequencies merge here
  • The more we ascend towards the cortex, the more neurones become responsive to complex sounds
  • Information about sound location in the IC - precedence effect
43
Q

What happens at the superior colliculus?

A
  • Auditory and visual maps merge
  • Neurones are tuned to respond to stimuli with specific sound directions
  • The auditory map here created is fundamental for reflexes in orienting the head and eyes to acoustic stimuli
44
Q

What happens in the auditory cortex?

A
  • Neurones respond to complex sounds
  • The primary auditory cortex is located in the superior bank of the temporal lobe
  • This is the central area of the AC and it is tonotopically mapped

Neuroscience and Mental Health (17 decks)

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