10. Sound conduction and transduction Flashcards
1
Q
What is the external auditory meatus?
A
Ear canal
2
Q
What is the ear drum also known as?
A
Tympanic membrane
3
Q
What does the middle ear comprise?
A
- Tympanic membrane
- Malleus
- Incus
- Stapes
4
Q
Signals are transferred from the cochlear to the central pathways via which nerve?
A
Vestibulocochlear nerve (VIII)
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
6
Q
What frequencies is the human ear sensitive to?
A
20 - 20,000 Hz
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
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
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
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
11
Q
What does the inner ear comprise?
A
• Cochlea • 3 fluid-filled chambes - scala vestibule - scala media - scala tympani
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)
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
14
Q
What membrane separates the scala media and the scala tympani?
A
The Basilar membrane
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)
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
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
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
How can some people with hearing loss still detect sound?
* Direct transmission through the cochlea
* Bone conduction
* Clinically important - allows us to detect where a problem is
26
Where is there a problem if bone conduction is better than air conduction?
Anywhere between the pinna and the cochlea
27
What are the main causes of hearing loss?
* Loud traumatic sounds
* 200 genetic conditions
* Infections e.g. meningitis
* Drugs
* Ageing
28
What is the decibel scale?
* Measures the intensity of sound
| * Logarithmic scale using base 10
29
What are the possible causes of conductive hearing loss?
* Cerumen (ear wax)
* Infections (otitis)
* Tumours
* Fluid accumulation in the middle ear
* Perforated tympanic membrane
* Otosclerosis - abnormal bone growth
* Barotrauma
30
What does the Organ of Corti include?
Basilar and tectorial membranes, hair cells and supporting cells
31
Describe the stereocilia
• 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
What happens when efferent fibres are activated to outer hair cells?
* 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
What happens at the spiral ganglion?
* 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
What is sensorineural hearing loss?
Problem with sensory apparatus of the inner ear or the vestibulocochlear nerve (retrocochlear hearing loss)
35
What are the causes of sensorineural hearing loss?
* 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
How do cochlear implants restore hearing?
* 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
What does the dorsal cochlear nucleus do?
* 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
What are T-stellate cells?
* Encode sound frequency and intensity of narrowband stimuli
* Their tonotopic array represents sounds' sprectra
* Part of cochlear nucleus
39
What are Bushy cells?
* 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
What does the superior olivary complex (SOC) do?
• 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
Which hair cells do Superior Olivary Complex neurones feed back to?
* Neurones from the Medial SO => inner hair cells bilaterally
* Neurones from the Lateral SO => outer hair cells ipsilaterally
42
What happens at the inferior colliculus?
* 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
What happens at the superior colliculus?
* 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
What happens in the auditory cortex?
* 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
