Auditory system

Question 1

What is sound?

Answer 1

The movement of air

Question 2

Sound also has wave properties that are important for hearing sound, what are these?

Answer 2

• Amplitude (volume)
• frequency (pitch)
• Phase
• Waveform

Question 3

Which sound is loud and has a high pitch?

Answer 3

Number 4, it has the highest amplitude and frequency

Question 4

The human ear can be divided into the outer, middle and inner ear. Explain what structures can be found in the outer, middle and inner ear.

Answer 4

• Outer ear → pinna and external auditory meatus (ear canal).
• Middle ear → ear drum (tympanic membrane), ossicles and eustachian tube.
• Inner ear → cochlea with endolymph fluid

Question 5

Each anatomical structure in the ear has its own function that helps with sound transduction. Describe the functions of the anatomical structures in the outer ear.

Answer 5

The outer ear collects sounds and boosts frequencies around 3 kHz.

• Pinna → protects inner parts of the ear and detects where sound is coming from (due to the shape of the pinna).
• External auditory meatus → ear canal that collects sound and directs it towards the ear drum (tympanic membrane).

Question 6

Each anatomical structure in the ear has its own function that helps with sound transduction. Describe the functions of the anatomical structures in the middle ear.

Answer 6

The middle ear is responsible for the amplification of sound energy about 200-fold.

• Ear drum (tympanic membrane) → When sound is directed to the eardrum by the ear canal, the sound wave triggers movement of the eardrum, which pushes sound forward. Here, sound is amplificated about 200x due to the pressure focus from the large eardrum to the small oval windows.
• Ossicles → are three small bones connected to the eardrum. When the ear drum vibrates in response to sound, the ossicles are also moved. Energy is then transfered into the endolymph fluid in the cochlea.

Question 7

Each anatomical structure in the ear has its own function that helps with sound transduction. Describe the functions of the anatomical structures in the inner ear.

Answer 7

The inner ear is composed of the cochlea that contains endolymph fluid → here sound is transducted to neural signals.

Question 8

What structures can be found inside the cochlea? (Name them in the order of sound transduction).

Answer 8

1. Flexible membrane (round window)
2. Scala vestibuli
3. Reissner’s membrane (membrane between scala vestibuli and cochlear duct)
4. Cochlear duct/scala media (filled with endolymph fluid)
5. Basilar membrane (membrane between cochlear duct and scala tympani).
6. Organ of corti (with hair cells and tectorial membrane).
7. Scala tympani

Question 9

Describe how sound is moved inside the cochlea.

Answer 9

• Sound is directed to the cochlea with the help of the ossicles that are connected to both the eardrum as the round window. As the ossicles move due to the vibrations of the eardrum, the round window will also vibrate.
• Vibrations are then sent into the perilymph fluid inside the scala vestibuli, where these vibrations ascend to the apex of the cochlea. The cochlear duct filled with endolymph fluid is located between the scala of vestibuli and tympani. The membranes of the cochlear duct are flexible and move in response to the vibrations traveling up to the apex of the cochlea. The vibrating membranes then send sound back to the scala of tympani.
• The organ of Corti is located around the basilar membrane of the cochlear duct. As the basilar membrane moves, this organ is stimulated and sends nerve impulses to the brain via the cochlear nerve.
• These nerve impulses are generated by hair cells that reside around the basilar membrane inside the organ of Corti and are covered by the tectorial membrane. As the basilar membrane vibrates, the hair cells are also moved and push against the tectorial membrane, triggering the hair cells to fire a nerve impulse.

This question is very elaborate, but couldn’t think of another way to describe/summarize this. If it’s not clear yet, watch this youtube video again: https://www.youtube.com/watch?v=PeTriGTENoc&ab_channel=BrandonPletsch

Question 10

What are the sound detectors of the cochlea?

Answer 10

The hair cells are sound receptors in the inner compartment, they catch the sound wave/vibrations of fluid.

Question 11

There are three compartments inside the cochlea: the scala vestibuli, scala media and scale tympani.

These compartments have different ion concentrations. Describe these differences.

Answer 11

• Scala vestibuli and tympani contain perilymph fluid → high in Na⁺ and low in K⁺.
• Scala media contains endolymph fluid → low in Na⁺ and high in K⁺ → normally the extracellular environment is high in Na⁺ and low in K⁺.

Question 12

Why is this difference between the scale media and tympani in ion concentrations important?

Answer 12

The upper part of hair cells is in endolymph and lower part in perilymph. So endolymph is high in K⁺ and has a potential of +80 mV, perilymph is low in K⁺ and has a potential of 0 mV.

• The inside of hair cells is depolarized (-45 to -60 mV). Thus the potential difference between the endolymph and the inside of the hair cells is about 125 mV.

This large difference means that there’s influx of ions, especially the driving force for K+ to enter the cell is large. At the same time, the driving force for K⁺ to leave the hair cells is also large. Therefore K⁺ is able to hyper- and depolarize hair cells very efficiently → special feature of the auditory system.

Question 13

What happens when due to the movement of the basilar membrane, the stereocilia of the hair cells interact with the tectorial membrane?

Answer 13

For this it’s important to remember that the apical part of hair cells is bathed in a high K⁺ solution, while the basal part of hair cells is bathed in a low K⁺ solution.

• The hair cells have mechanosensitive receptors for K+. When the stereocilia are stimulated by the tectorial membrane, the mechanosensitive channels for K⁺ open. K⁺ enters the cells and depolarizes it. This also results in the opening of Ca²⁺ channels. The influx of Ca²⁺ results in the release of neurotransmitters, which activate axons of efferent nerves.
• Influx of Ca²⁺ also causes the opening of somatic- and Ca²⁺ dependent K⁺ channels. Due to the fact that the extracellular environment of the basal part of hair cells has a low K⁺ concentration, K⁺ will move out of the cell → repolarization.

Here, hair cells exploit their different environments to provide extremely fast and energy-efficient repolarization.

Question 14

So for this very fast and energy-efficient repolarization, you also need a mechanism to release neurotransmitters very quickly.

Besides the fact that there are vesicles docked and ready to be released into the synaptic cleft, hair cells have a specialized system to facilitate neurotransmitter release.

What is this specialized system?

Answer 14

Ribbon synapses → large structures where there are a lot of vesicles that are ready to fuse and secrete neurotransmitters.

Question 15

Summary of what has been discussed so far.

Answer 15

Ok

Question 16

What’s the differecne between inner and outer hair cells?

Answer 16

• Inner hair cells → detect extremely fast movements of atomic dimensions and millisecond precision and comprise 95% of fibers projecting to the brain.
• Outer hair cells → three rows of outer hair cells do not transmit sound information but help to amplify the sound information. → they receive projections from superior olive, adjust the basilar membrane motion and act as an amplifier.

Question 17

How do outer hair cells amplify sound?

Answer 17

Special kind of proteins in the membrane → prestin. Prestin is related to muscles → can contract and relax.

Question 18

What is the function of the protein prestin?

Answer 18

Prestin lines the membrane of hair cells. It can sense voltage changes and due to its muscle properties, it can contract (and relax).

• When it detects voltage changes, the cells contract and becomes shorter. This will push the basilar membrane and amplify the movement of the membrane.

Question 19

How large is the signal amplification of the basilar membrane by the outer hair cells?

Answer 19

100x

Question 20

These outer hair cells are also implicated in certain conditions of the ear. What conditions?

Answer 20

• Cochlear microphonics → hearing sounds that aren’t there.
• Tinnitus → hearing high pitched sounds.

Question 21

Summary of sound amplification

Answer 21

Ok

Question 22

Depicted in the picture are the receptor potentials in hair cells that are generated in response to pure tones. To what frequency can hair cells do this reliably and what process is important in this?

Answer 22

It can do this reliably up to 1 kHz, above 1 kHz depolarization is not as fast anymore. Hair cells can still react to frequencies above 1 kHz (up to 3 kHz).

• Tonotopy (tuning of basilar membrane) is responsible for this.

Question 23

Describe the process of tonotopy.

Answer 23

The basilar membrane has a different shape towards the base of the cochlea compared to the apex of the cochlea. So the basilar membrane also responds differently to different sound frequencies, based on whether the sound is processed more in the base or apex of the cochlea.

• At the base, high-frequency sounds are much better detected.
• At the apex, low-frequency sounds are much better detected.

Question 24

Does tonotopy also apply to hair cells?

Answer 24

Yes.

• Hair cells at the base of the cochlea are sensitive to high-frequency sounds.
• Hair cells at the apex of the cochlea are sensitive to low-frequency sounds.

Question 25

Tonotypy allows for conducting higher frequency sounds. See picture.

Answer 25

Ok

Question 26

Describe the auditory pathway

Answer 26

• Cochlea
• Cochlear nucleus
• Superior olivary nucleus
• Crossing over of nerves
• Inferior colliculus
• Medial geniculate nucleus (thalamus)
• Auditory cortex

Question 27

Why are not all the fibers crossing?

Answer 27

The ears have to detect differences in locations of sounds → inter-aural comparisons are an important source of information for the auditory system about where a sound came from.

Question 28

What is the function of the (medial) superior olive nucleus?

Answer 28

The (medial) superior olive nucleus computes the position of sounds by inter-aural time differences (the difference of how quick the sound arrives in two ears).

Question 29

What happens in the medial superior olive (MSO) when the sound of a speaker is closer to the left ear compared to the right ear (so how does it detect that the sound is most close to the left ear)?

Answer 29

The sound of the speaker will arrive in the left ear first and some time later in the right ear. The MSO contains multiple neurons with axons that are connected to both ears.

So when the sound is closer to the left ear, the neurons of the MSO that connect to the left ear, are the ones that are stimulated first. The difference in activation of these neurons due to the sound arriving first in one of the ears, is the way the inter-aural difference is computed.

→ Closely study the picture.

Question 30

What confirms the theory of the function of the medial superior olive nucleus?

Answer 30

Anatomy → axonal projections from the cochlea to the MSO have different lengths.

Question 31

For higher frequencies, there’s a different mechanism. Why is that?

Answer 31

The reason for this, is that high frequencies travel very fast and therefore the inter-aural time differences of sound will become too small to detect.

Question 32

For higher frequencies, there’s a different mechanism. What mechanism is this and where does this occur?

Answer 32

For high-frequency sounds, there’s the detection of inter-aural intensity differences. This occurs in the lateral superior olive (LSO). Here, the head acts as an acoustical obstacle.

There is a left and right LSO. When a high-pitched sound is closer to the left of the head, it will cause excitatory input on the left side and less excitatory input on the right side. At the same time, the high excitatory input on the left side, also causes an inhibitory effect on the contralateral superior olive. This also occurs on the right side, where the excitatory input on the right will cause an inhibitory effect on the left LSO. But since the excitatory input is lower on the right side, the inhibitory effect on the right side is also lower.

Question 33

Summary superior olivary complex

Answer 33

Ok

Question 34

So there’s tonotopic representation in the cochlea. Is this also the case for other parts of the auditory parts?

Answer 34

Both medial and lateral superior olives share this feature, afferent fibers from high and low frequency areas in the cochlea project selectively to high- and low-frequency parts in superior olives.

Question 35

What is the function of the inferior colliculus?

Answer 35

The inferior colliculus integrates the cues for localizing sounds in space and contains topographical representation of the audible space.

Question 36

To what structure is the information of sound send after it has reached the inferior colliculus?

Answer 36

To the medial geniculate nucleus (close to the lateral geniculate nucleus) in the thalamus.

Question 37

The geniculate nucleus can be divided into the lateral, dorsal, ventral and medial nucleus. The ventral and dorsal geniculate nucleus are part of a certain pathway, as is the medial geniculate nucleus.

What are these pathways?

Answer 37

• Dorsal and lateral geniculate nucleus → parvocellular pathway
• Medial geniculate nucleus → magnocellular pathway.

Question 38

Next, the information of sounds is directed to the auditory cortex (located on temporal lobe, close to the parietal lobe). The auditory cortex can be divided into the core and belt.

What is the function of the core and belt?

Answer 38

• Core → information arrives here in tonotopical representation.
• Belt → processing of (more) complex information. Here, there’s no tonotopical representation anymore. But cells are organized to how they process complex sounds (speech, music, etc.)

Question 39

The primary auditory cortex several types of cells and neurons. Describe these cells and neurons.

Answer 39

• Patches of EE cells → excited by input that comes from two ears.
• Patches of EI cells → excited by input from one ear and inhibited by input from the other ear.
• Combination-sensitive neurons → neurons that respond to specific sound combinations.
• Cells that respond to specific temporal sequences of sounds (as in communication or music).

Question 40

The EI cells and combination-sensitive neurons are similar to certain cells in the visual cortex. To what cells of the visual cortex are they similar?

Answer 40

• EI cells → are a bit like the ocular dominance columns in the visual cortex.
• Combination-sensitive neurons → are a bit like the simple cells in the visual cortex.

Question 41

What’s also important in the auditory cortex?

Answer 41

Left and right hemisphere asymmetry.

• The left hemisphere is important for language processing, like speech.
• The right hemisphere is important for e.g. tonation, environmental sounds or music.

Question 42

Just like in the visual system, the auditory system also contains two streams that project from the primary auditory cortex: the ventral and dorsal stream.

For what are they important and to what do they project?

Answer 42

• Dorsal stream → important for processing of objects. It goes from the primary auditory cortex to the intraparietal lobule (IPL) and premotor cortex.
• Ventral stream → important for processing space and motion. It goes from the primary auditory cortex to the temporal and frontal lobe (Broca’s area).

Question 43

Where are Broca’s and Wernicke’s area located and what is their function?

Answer 43

• Broca’s area is located in the left hemisphere and is associated with speech production and articulation.
• Wernicke’s area is a critical language area and is located in the posterior superior temporal love

Question 44

In the lecture about the visual system, concept cells in the temporal lobe of the entorhinal cortex have been discussed.

How are these cells involved in the auditory system?

Answer 44

Concept cells combine auditory and visual information. These cells are not only able to react to certain pictures, but also are able to react to reading or hearing the name of the picture that is depicted.

Question 45

Summary of what has been discussed.

Answer 45

Ok