Sound transduction

Question 1

What is a caveat in the hypothesis regarding the active work of the outer hair cells

Answer 1

Caveat: otoacoustic emissions in animals without OHCs.

Question 2

Describe the transmission of nerve fibres to the cochlear nucleus

Answer 2

Hair cells form synapses with sensory neurons in the cochlear ganglion (spiral ganglion). Each ganglion cell responds best to stimulations at a particular frequency. The tonotopic (sound-location) map continues.
Ganglion cells in a particular area of the spiral ganglion respond best to the resonant frequency of the basilar membrane in that same area.
Parallel nerve fibres from each inner hair cell going to a ganglion- one damaged- can still detect frequency through other fibres

Question 3

Summarise what is meant by sensorineural deafness

Answer 3

We talk about sensorineural hearing loss when the problem is rooted in the sensory apparatus of the Inner ear or in the vestibulocochlear nerve (retrocochlear hearing loss).This is the most widespread type of hearing loss by a large margin.
Sensorineural – caused by a failure at the level of the cochlea or more centrally.

Question 4

List the causes of sensorineural deafness

Answer 4

• Loud noises, headphones at high volume can cause temporary or permanent hearing loss (Club: ~100 dB, Rock concert: ~120 dB)
• Many genetics mutations affect the Organ of Corti
• Aminoglycoside antibiotics are toxic for hair cells
• Congenital diseases (rubella, toxoplasmosis)
• Acoustic neuroma (tumor on the cochlear nerve)
• Ageing (presbycusis)- death of hair cells in normal ageing

Question 5

Where can lesions occur in sensorineural deafness

Answer 5

Lesions within the cochlea itself
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Lesions within the petrous temporal bone (trauma, complications of middle ear infection, tumours)
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Lesions at the cerebellopontine angle (particularly acoustic neuromas, but also meningiomas and inflammatory damage such as meningitis)
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Cortical or pontine lesions causing deafness are rare.

Question 6

Describe the role of antibiotics in causing sensorineural deafness

Answer 6

Excessive exposure to some ototoxic drugs, e.g. streptomycin and aminoglycosides (especially gentamicin) can cause sensorineural deafness. These antibiotics exclusively damage the outer hair cells in the cochlea.

Question 7

Describe Menier’s disease

Answer 7

This is a disease of uncertain aetiology – probably due to a build-up of endolymph. Characteristic symptoms are:
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Sensorineural deafness
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Ringing in the ears (tinnitus)
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Vertigo with vomiting, balance disturbance and nystagmus.
It tends to be a recurrent disease and the vertigo, in particular, may be disabling.
Management includes rest, antipsychotic drugs for the acute attack (prochlorperazine) and histamine analogues for prophylaxis (e.g. betahistine). Ultimately, surgical drainage of endolymph, destruction of the labyrinth or section of the vestibular nerve may be required.

Question 8

What is sensorineural deafness and what is it caused by

Answer 8

When the cochlea or cochlear nerve get damaged, the signal transmitted to the primary auditory cortex is reduced or lost
It can be caused by acoustic schwannoma (tumour of the cochlear nerve) or cerebellar tumours expanding and putting pressure on the cochlear nerve

Question 9

Describe the role of cochlear implants

Answer 9

Hearing loss is primarily due to the loss of hair cells. These do not regenerate in mammals. One solution is to bypass the dead cells and stimulate the nerve fibres directly: detect sounds, break them down into their constituent frequencies and send the signal directly to the auditory nerve via antennas.
An elongated coil is inserted into the cochlea with pairs of electrodes corresponding to single frequencies.
Early models: 4 channels.You need 20 channels to understand speech well.
We want to stimulate the cochlear nerve directly
obviously cannot replace all the hair cells- so won’t pick up every frequency

Question 10

31. Describe the auditory pathway from the cochlea to the primary auditory cortex.

Answer 10

Spiral ganglion  cochlear nuclei  superior olive  inferior colliculus  medial geniculate nucleus(in thalamus)  primary auditory cortex

Question 11

How is information. from the cochlear (spinal) ganglion transmitted to the cochlear nuclei

Answer 11

Cochlear nerve: axons transmit information to cochlear nucleus (with each axon responsive to a single frequency)

Question 12

Describe the cochlear ganglion

Answer 12

Cochlear ganglion: IHCs synapse with sensory neurones in cochlear ganglion, with constant NT release at rest but rate adjusted in response to change in presynaptic voltage (due to MT gating)

Question 13

Describe what happens to the cholera nerve fibre within the choclear nucleus

Answer 13

Each auditory nerve fibre branches, sending an ascending branch to the anteroventral nucleus and a descending branch to the posteroventral nucleus and the dorsal choclear nucleus.

Nerve fibres convey information to the cochlear nucleus where different kinds of neurons are arranged tonotopically (low frequencies ventrally, high frequencies dorsally).

Cochlear nucleus is found in the rostral medulla

Question 14

Where do fibres from the dorsal cholear nucleus pass to

Answer 14

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Fibres from the dorsal cochlear nucleus pass in the dorsal acoustic stria, then cross to the opposite side to join the lateral lemniscus and terminate in the contralateral inferior colliculus.
Cross at the level of the pons

Question 15

Where do fibres from the ventral cochlear nucleus pass to

Answer 15

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Most fibres from the ventral cochlear nucleus pass ventrally and cross to the opposite side in the trapezoid body. Some fibres end in the superior olivary complex on both sides. Others continue upwards in the lateral lemniscus to the contralateral inferior colliculus. The medial part of the superior olive receives information from both ears. This is believed to be important for sound localization. Fibres from the superior olive project to the inferior colliculi, on both sides, via the lateral lemnisci.

Question 16

Where do fibres from the inferior colliculus (in the midbrain pass to)

Answer 16

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Fibres from the inferior colliculus project, bilaterally, to the medial geniculate nuclei of the thalamus and, from there, to the ipsilateral primary auditory cortex, on the superomedial aspect of the temporal lobe.

Question 17

32. Up to what point is the auditory pathway from one ear ipsilateral?

Answer 17

Cochlear nuclei

Beyond this point there is bilateral representation

Question 18

Describe the clinical relevance of the bilateral connectivity of the ascending pathways

Answer 18

Damage to one side of the central auditory pathway at any level (other than the cochlear nerve) will not result in deafness in one ear. This is because of the bilateral projections to the auditory cortex, both directly and by communication between pathways.
Indeed, a monaural hearing loss strongly implicates unilateral peripheral damage, either to the middle or inner ear or to the auditory nerve itself.

Question 19

Describe spectral cues

Answer 19

The dorsal cochlear nucleus locates sounds in the vertical plane
Sounds of high frequencies produce intensity differences between the two ears. High-frequency sounds (wavelengths ~ size of the head and ears) produce interference with the body.The ears detect and affect differently sounds coming from different directions due to their asymmetrical shape.We call these spectral cues.
whereby high frequency sounds can produce constructive or destructive interference depending on site of origin, allowing ears to detect sound location via asymmetry of waves

Question 20

Describe the role of stellate cells and bushy cells

Answer 20

T-Stellate cells: encode sound frequency and intensity of narrowband stimuli
Bushy cells: produce more sharp but less temporally precise versions of cochlear nerve fibres to allow encoding of relative time of arrival of inputs to two ears

Question 21

Summarise the role of the superior oilvary complex in the pons

Answer 21

The SOC compares the bilateral activity of the cochlear nuclei.
Compares interaural time differences (below 3kHz)
Compares interaural intensity differences (above 2kHz) to localise sound

Question 22

Summarise the role of the medial superior olive

Answer 22

Here the interaural time difference is computed: sounds are first detected at the nearest ear before they reach the other one. A map of interaural delay can be formed due to delay lines.

Question 23

Describe how the interaural time difference is computed

Answer 23

Biaural inputs to the medial superior olive
The neurones of the MSO work as conincidence detectors, responding when both excitatory signals arrive at the same time.
For a coincidence mechanism to be useful in localising sound, different neurones must be maximally sensitive to different interaural time delays. The axons that project from the anteroventral cochlear nucleus evidently vary systematically in length to create delay lines.
These anatomical differences compensate for sounds arriving at slightly different times in the two ears, so that the resultant neural impulses arrive at a particular MSO neurones silmutaneously, making each cell sensitive to sound sources in a different place.

Question 24

What is a typical time delay between the two ears

Answer 24

0.5ms
Head is 18cm
Sound travels at 330m.s

Question 25

Summarise the role of the lateral superior olive

Answer 25

The LSO detects differences in intensity between the two ears (>2 kHz in humans due to head size). Interaural level difference is computed to localise sounds in the horizontal plane.
The human head begins to act as an acoustic obstacle, because the wavelengths of sound are too short to bend around it
As a result, when high-frequency sounds are directed towards one side of the head, an acoustical ‘shadow’ of lower intensity is created in the far ear.
These intensity differences provide a second cue about the location of sound

Question 26

Describe how the lateral superior olive computes interaural intensity differences

Answer 26

Excitatory axons project directly from the ipsilateral anteroventral cochlear nucleus to the LSO. Note that the LSO also recieves inhibitory input from the contralateral ear via an inhibitory neuron in the medial nucleus of the trapezoid body.
This excitatory-inhibitory interaction results in net excitation of the LSO on the same side of the head as there sound source.
For sounds arising in the midline or the opposite side, inhibiton from neurones from the medial nucleus of the trapezoid body is enough to completely silence the excitatory ipsilateral neurones to the LSO

Question 27

What is important to remember about the excitatory and inhibitory neurones

Answer 27

Excitation that arrives ipsilaterally must arrive at the same time as inhibition from the contralateral side.
The contralateral inhibitory signal is carried out via large axons with large synapses (the large calyces of Held). The axons that carry ipsilateral excitations are smaller and conduct more slowly.

Question 28

Describe the feedback from the superior olivary complex (particularly the lateral superior olive)

Answer 28

SOC neurons send feedback to the hair cells. Activity in these efferent fibres increases the representation of signals in noise and protects it from damage by loud sounds.
he feedback is used to balance the responses from the two ears, but also to reduce the sensitivity of the cochlea.
Feedback is to inner hair cells and afferent fibres to spinal ganglia
MSO: to inner hair cells bilaterally
LSO: to outer hair cells ipsilaterally

Question 29

What else can sensorineural hearing loss be due to

Answer 29

Hearing loss can be due to the malfunctioning of the auditory pathway in the brain.
• Demyelination = loss of myelin (can be due to inflammation or viral). Most common in multiple sclerosis MS.- inhibits feedback and control of amplification in the cochlea.
• Blast injuries can cause disruption in the balance between inhibition and excitation (leading to tinitus and inability to localise sound- can’t detect intensity differences)

Myelinated (from the SOC bilaterally) Unmyelinated (from the SOC ipsilaterally)

Question 30

Summarise the inferior colliculus in the caudal midbrain

Answer 30

In mammals: central nucleus, dorsal cortex and external cortex. Only central nucleus is tonotopically organised. 

In the IC many carry information about sound localisation. Precedence effect.
Here the neurones are hard to identify- do a mixture of roles- integrate interaural time and level differences as well as vertical cues to localise the location of sound- essentially an integration centre
All ascending auditory pathways in vertebrate converge here.

Question 31

Describe the precedence effect

Answer 31

Unless the sound is in the echo zone (30-50ms after the original sound)- the inferior colliculus will filter the sound out- considered to be useless
When a sound is followed by another sound separated by a sufficiently short time delay (below the listener’s echo threshold), listeners perceive a single auditory event; its perceived spatial location is dominated by the location of the first-arriving sound (the first wave front).

Question 32

Summarise the superior colliculus

Answer 32

Information from the IC forms a map of sounds in the superior colliculus.
SUPERIOR COLLICULUS
Here auditory and visual maps merge. Neurons are tuned to respond to stimuli with specific sound directions ( different neurones in different relative angles to identify the location of sound in the 3D world)
The auditory map here created is fundamental for reflexes in orienting the head and eyes to acoustic stimuli.
This is where localisation really happens

Question 33

Summarise cochlear nuclei in the medulla

Answer 33

Cochlear nuclei in Medulla (VERTICAL PLANE LOCATION): receives glossopharyngeal input from ears and interpret spectral cues - whereby high frequency sounds can produce constructive or destructive interference depending on site of origin, allowing ears to detect sound location via asymmetry of waves

Question 34

Summarise the primary auditory cortex

Answer 34

In the auditory cortex neurons respond to complex sounds.
PRIMARY AUDITORY CORTEX A1
figure: The Principles of Neural Science, Mc Graw Hill
The primary auditory cortex A1 is located in the superior bank of the temporal lobe. This is the central area of the AC and it is tonotopically mapped. Loudness, rate and frequency modulation also seem to be mapped in A1.
Related to gaze control in response to complex tasks.
 A1 can be trained.

Tonotopically mapped as it is in the basillar membrane.

Question 35

Describe the superior auditory cortex

Answer 35

In the visual system the outputs of the primary visual cortex are segregated. We can identify a “What” and “Where” stream in the auditory system (primates). In the visual pathway this is clearly defined.

Ventral- what
dorsal- where