From sound to synapses: the mechanisms and basis of auditory function

Question 1

Describe the epidemiology of hearing loss

Answer 1

Hearing loss affects about 10% of the population in the UK.

Question 2

List the main causes of hearing loss

Answer 2

• Loud traumatic sounds: military, industrial, clubs
• 200 genetic conditions that cause hearing problems
• Infections like meningitis or congenital ones such as rubella or syphilis • Drugs: used for severe heart infections and chemotherapy, also antibiotics
• Ageing

Question 3

Describe the morbidity of hearing loss

Answer 3

Hearing loss can be extremely depressing: blindness deprives us of the contact with things, deafness deprives us of the contact with people (Helen Keller). Social interactions, sense of danger deeply rely on hearing: people deprived of hearing feel vulnerable.
hearing is auditory percept and is essential for sensing our surroundings.

Question 4

Summarise the complexity of the auditory system

Answer 4

Curiosity.The auditory system is an extraordinary machine.
WE CAN PERCEIVE A LARGE RANGE OF FREQUENCIES
Hearing range in humans: from 20 Hz to 20 kHz (upper limit drops off somewhat in adulthood) Compared to vision: static images changing at a rate of 20 times a second are perceived as continuous.The ear works at 20000 times per second.
These detectors can transduce vibrations as small as an atom, and they respond 1000 times faster than visual photoreceptors

Question 5

Define pitch

Answer 5

We define pitch the perception of frequency.

Question 6

Define frequency

Answer 6

Frequency: speed of the vibration as defined by number of wave cycles occurring per second - measured in Hertz (Hz)

Question 7

Define intensity

Answer 7

Intensity: power of the sound - as determined by the amount of energy delivered per second (which is equal to the number of joules per second passing through one square meter)

Question 8

Describe what is meant by timbre

Answer 8

The timbre is instead what distinguishes two sounds at the same frequency and intensity.

i.e to distinguish between a flute and a violin playing the same note
the first harmonic tells you what tone or note the instrument is playing

Question 9

When is our hearing most sensitive

Answer 9

most sensitive between 10003000Hz.

Question 10

What are the four major features of sound

Answer 10

Like all wave phenomena, sound waves have four major features: waveform, phase, amplitude (usually expressed in logarithmic units known as decibels, dB), and frequency (expressed in cycles per second or Hz).

Question 11

Compare what loudness and pitch mean for the human listener

Answer 11

For a human listener, the amplitude and frequency of sound pressure change at the ear roughly correspond to that listener’s experience of loudness and pitch, respectively.

Question 12

In physical terms, what does sound refer to

Answer 12

It refers to pressure waves generated by vibrating air molecules. Physical sound waves radiate in three dimensions, creating concentric spheres of alternating compression and rarefaction.

Question 13

What is meant by the waveform of a stimulus

Answer 13

The amplitude plotted against time

Question 14

Describe how sensitive our ears are

Answer 14

The internal ear can detect movements large as a fraction of a nanometer, roughly the size of a water molecule.

Question 15

Describe the dynamic range of intensities that the auditory system can detect

Answer 15

We can hear sounds with a power that ranges from 1 ⋅ 10−12 𝑚2 to 1 𝑚2 (from 0.000000000001 𝑚2 to 1 𝑚2 ) 1W = 1J so that 1 W = 1 J/s so that 1W/m2 = 1J/s.m2. This measures the sound intensity I of a sound.It is given by the amount of
energy delivered per second: how many Joules per second pass through one square meter.
The lowest sound intensity we can hear is very low: we could spread 1 Watt of sound energy over an area as large as three times as the UK and still be able to hear it. The loudest sound intensity we can hear (threshold of pain) is 12 orders of magnitude larger.

Question 16

Why do we have the decibel scale of sound and how is it derived

Answer 16

We want to compact a large range into a more manageable scale. Instead of measuring the intensity I with respect to the faintest perceivable intensity of sound 𝐼0, we compare their logarithms:
Log10(I) -Log10 (I0) = Log10 (I/I0)

I0 = 1 x 10^-12 W/m^2

This is the Bel scale. It defines the sound level.
If we multiply everything by 10 we get the deciBel scale:

Therefore 1 dB= 10Log10 (I/I0)
Plug value for I0 in

𝐿𝑜𝑔10(100000) = 5 
𝐿𝑜𝑔10(10) = 1

Thus, a 20 dB change is equal to a 10-fold increase (+20 dB) or decrease (–20 dB) in loudness.

Question 17

Compare the frequencies of human speech

Answer 17

Higher frequencies= constenants
Lower frequencies= vowels.

Speech is a complex cocktails of frequencies and intensities

Question 18

Essentially, how do we understand speech

Answer 18

The internal ear must detect the complex patterns of sound, break them down into their constituent frequencies and send a trustworthy signal to the brain- and it is able to do to this because of the way the inner ear is built.

Question 19

What does the auditory system consist of

Answer 19

The auditory system consists of the hearing apparatus (outer ear, middle ear and inner ear) and a pathway from the inner ear to the brainstem and auditory cortex.

Question 20

Summarise the external ear

Answer 20

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The first part is the external ear consisting of the part attached to the lateral aspect of the head and the canal leading inward.
The pinna and external ear canal form a tube closed at one end by the tympanic membrane. This tube has a resonant frequency of 3 kHz. The threshold for hearing in the frequency range 2.5–4 kHz is therefore decreased by –15 dB (i.e. these frequencies are easier to hear).

This tube is the external acoustic meatus

Question 21

Summarise the middle ear

Answer 21

The second part is the middle ear—a cavity in the petrous part of the temporal bone bounded laterally, and separated from the external canal, by a membrane (the tympanic membrane) and connected internally to the pharynx by a narrow tube (pharyngotympanic tube)

Question 22

Summarise the inner ear

Answer 22

The third part is the internal ear consisting of a series of cavities within the petrous part of the temporal bone between the middle ear laterally and the internal acoustic meatus medially.

Question 23

Give an overview of the passage of sound from air to the hair cells

Answer 23

Sound vibration penetrates inner ear and stimulates the tympanic membrane, and thanks to 3 small bones- the vibration is transmitted inside the cochlea (which is a snail-shaped organ filled with liquid).
Along the cochlea runs the basilar membrane, which is lined with hair cells (in 4 rows)- which are the sensory receptors of the inner ear.
these hair cells take their name from the hair bundle, which is a cluster of modified microvilli which protrudes from the cell body.

Evolutionary success shared by all vertebrates

Question 24

Summarise how the ear detects sound and describe the evolutionary success of the hair bundle

Answer 24

The ear detects sound waves in the air and, via a series of mechanical couplings, projects
the stimuli onto the hair cells, the sensory receptor of the internal ear.
The hair bundle is a cluster of modified microvilli called stereocilia.
The hair bundle is an evolutionary success shared by all vertebrates.

Question 25

Describe the role of the middle ear and describe how its anatomy allows it to perform this role

Answer 25

The middle ear communicates with the mastoid area posteriorly and the nasopharynx (via the pharyngotympanic tube) anteriorly. Its basic function is to transmit vibrations of the tympanic membrane across the cavity of the middle ear to the internal ear. It accomplishes this through three interconnected but movable bones that bridge the space between the tympanic membrane and the internal ear. These bones are the malleus (connected to the tympanic membrane), the incus (connected to the malleus by a synovial joint), and the stapes (connected to the incus by a synovial joint, and attached to the lateral wall of the internal ear at the oval window).

Question 26

describe the ossicles of the middle ear.

Answer 26

Alternating air pressure (the sound wave) makes the tympanic membrane vibrate. The ossicles vibrate along with it. The ossicles are:
•
The malleus (hammer) which is attached to the tympanic membrane itself
•
The incus (anvil) which provides a bridge across the middle ear
•
The stapes (stirrup) whose base plate sits in the oval window at the entrance to the cochlea.

Question 27

describe impedance matching of the middle ear

Answer 27

The three ossicles transmit the vibration of the tympanic membrane onto the cochlea, which is a snail-shaped organ filled with liquid.Their role is to match the impedance and reduce the loss in energy as the vibration goes from the air to the cochlea.
The impedance measures the reluctance of a system in receiving energy from a source ( when waves move from one medium to the other- some is transmitted and some is reflected, if the impedance of the two mediums is the same- then all the sound is transmitted).
The frequency at which the impedance of the system is minimal is called the
resonant frequency.
The transfer of energy is optimal when a system is stimulated at its resonant frequency and that frequency depends on its mechanical properties (local changes in mechanical properties- change impedance and their resonant qualities)

Question 28

Describe how the middle ear increases the pressure of vibration

Answer 28

 Middle-ear – increases pressure of vibration by:
o Focusing vibrations from the larger tympanic membrane to the smaller oval window.
o The incus has a flexible joint with the stapes so the ossicles can use leverage to increase force on the oval window.

Question 29

Describe the protective mechanisms of the middle ear

Answer 29

Vibrations of the ossicular chain are dampened down when they become extreme. Two muscles perform this function:
•
Tensor tympani muscle on the malleus (innervated by CNV)
•
Stapedius muscle on the stapes ( innervated by CNVII)
The reflex contraction of these muscles has a delay of 40–60 ms and cannot protect the cochlea from a sudden loud explosion. This reflex suppresses low frequencies more than high frequencies and may explain how we understand speech in a noisy environment

Question 30

How is maximal efficiency of sound transfer from the middle ear to the inner ear achieved

Answer 30

For maximum efficiency, the pressure on either side of the eardrum needs to be equal. The middle ear mucosa constantly absorbs air, and therefore the pressure in the middle ear gradually drops below atmospheric pressure. The pharyngotympanic tube connects the middle ear to the pharynx and allows the pressure to equilibrate when it is opened (by swallowing or yawning). Blockage of this tube leads to a relative hearing defect.

Question 31

What is meant by conductive hearing loss

Answer 31

We talk about conductive hearing loss when the ear is not capable of transmitting the vibration of sound waves onto the cochlea. Cerumen, infections such as otitis, tumors can all affect transmission.
Essentially, the failure of sound to reach the inner ear
Describes damage to the external or middle ear, which lowers the efficiency of which sound energy is transferred to the inner ear ad can be partially overcome by artificially boosting sound pressure levels with an external hearing aid.

Question 32

List the main causes of conductive hearing loss

Answer 32

In children, fluid accumulation in the inner ear is a common cause of conductive hearing loss (cold).
• A perforated tympanic membrane is a form of conductive hearing loss.
• An abnormal growth of bone (otosclerosis) can obstruct the ear canal.
• Barotrauma is a temporary form of conductive hearing loss (blocks the ear canal)

Question 33

When do the tensor tympani and stapedius muscles contract

Answer 33

Loud noises or during self-generated vocalisation, counteracts the movement of the ossicles and reduces the amount of sound energy transmitted to the cochlea, serving to protect the inner ear.

Question 34

Describe hyperacusis

Answer 34

Conditions that lead to flaccid paralysis of either these muscles, such as Bell’s palsy (nerve VII) can trigger a painful sensitivity to moderate or even low-intensity sounds.

Question 35

Describe the consequences of the motion of the stapes in the middle ear

Answer 35

The motion of the stapes generates a difference in pressure between the two liquid-filled chambers of the cochlea, which in turns causes the vibration of the basilar membrane
Together with the mechanical advantage of the lever system of the incus and malleus (at frequencies near 1000 Hz), this amplifies the pressure changes by 1.3 × 17 or 22-fold (+28 dB). This ensures that sound waves are transmitted efficiently from air to the fluid-filled cochlea.
Air is low impedance, whereas the liquid in the cochlea is high impedance, so without this step, all the sound would be reflected

Question 36

Summarise the organ of corti and what triggers the hair cells

Answer 36

The Organ of Corti includes the basilar (below)and tectorial (above) membranes, the hair cells
and supporting cells
The relative motions of the tectorial and basilar membranes as. result of sound vibration triggers the hair cells

Question 37

Describe how the basilar membrane works as a frequency analyser

Answer 37

The basilar membrane is an elastic structure of heterogenous mechanical properties that vibrates at different positions along its length in response to different frequencies (narrow and tough at the base, broad and floppy at the apex)
The impedance of the basilar membrane varies along its length, meaning that so does the local resonant frequency- due to changes in mechanical properties- therefore each point has its resonant frequency and responds to a stimulus vibration that matches this frequency
The basilar membrane breaks complex sounds down by distributing the energy of each component frequency
along its length (Tonotopic map). We need therefore sensory receptors along the whole length of the basilar
membrane in order to detect all frequencies: these receptors are the hair cells.

Question 38

Summarise tonotopy

Answer 38

The points responding to high frequencies are at the base of the basilar membrane.
The points responding to low frequencies are at the apex, giving rise to a topographical mapping of frequency.
This spectral decomposition appears to be an important strategy for detecting the various harmonic contributions that distinguish natural sounds that have a periodic character, such as minimal vocalisations, including vowels and some consonants in speech.

Question 39

Describe the hair cells of the inner ear

Answer 39

he hair cells are the sensory receptors of the inner ear
The motion of the basilar membrane deflects the hair bundles of the hair cells, that act as sensors.
The bending of stereocilia towards the tallest stereocilium changes the internal voltage of the cell, ultimately producing an electric signal that travels towards the brain.This is called Mechano-transduction (MT).
We have 3 rows of stereocilia
The hair cells convert the oscillating movements of stereocilia into neuronal signals.

Question 40

What causes the oscillations of the sterocilia

Answer 40

The travelling wave initiates sensory transduction by displacing the sensory hair cells that sit atop the basilar membrane.
Because the basilar membrane and the overlying tectorial membrane are anchored at different positions, the vertical component of the travelling wave is transferred into a shearing motion between these two membranes..
This motion bends the tiny processes, called stereo cilia, that protrude the apical ends of the hair cells, leading to voltage changes across the hair cell membrane, if triggered towards the tallest stereo cilia (staircase arrangement).
Step deflections of increasing magnitude- current develops.

Question 41

Describe the tip links

Answer 41

Fine filamentous structures which run in parallel to the plane of bilateral symmetry, connecting the tips of adjacent cilia.
The tip links which consist of cell adhesion molecules cadherin 23 and protocadherin 15, provide the means fro rapidly translating hair bundle movement into a receptor potential.

Question 42

Describe the role of the tip links

Answer 42

The tip links project the force of the stimulus onto ion channels
Stereocilia are connected by filamentous linkages called tip links.
 They work as small springs stretched by the stereocilia’s sliding.
Historically, scientists observed:
• Tip links share their location with ion channels
• Their disruption abolishes mechanotransduction
They concluded that the response currents are the result of the opening of ion channels activated by the stretching of the tip links.

Question 43

Describe what happens as a result of the opening of MT ion channels

Answer 43

The opening of MT ion channels in response to an external stimulus, relaxes the tip link and, in turn, the whole hair bundle.
A healthy hair bundle actively complies with the direction of the stimulus: the measured stiffness becomes negative when channels open!
The hair bundle has the capacity to do work.This points to the existence of an active process in hair cells.
Analagous to opening a door with an elastic band- if the door was not able to open- it would get stiffer- but as the door opens- the band gets softer- negative stiffness

Question 44

Describe the modulation of the flow of ions

Answer 44

Displacement of the hair bundle parallel to the plane of bilateral symmetry in the direction of the tallest stereo cilia, stretches the tip links, directly opening the cation-selective mechanoelectrical transduction channels located at the end of the link and depolarising the hair cell. Movement in. the opposite direction compresses the tip links, closing the MT channels and hyper polarising the hair cell. As the linked sterocilia move back and forth, the tension on the tip link varies, modulating the ion flow and resulting in a graded receptor potential that follows the movements of the stereo cilia.

Question 45

How do we know that mechanoelectrical transduction is an active process

Answer 45

The sensitivity and the sharp frequency selectivity of the cochlea cannot be explained solely by passive mechanical properties: basilar membrane (BM) impedance. 4 aspects of the active process:
Amplification, frequency tuning, compressive nonlinearity and spontaneous otoacoustic emission
a large portion of energy is lost in damping effects of cochlear liquids, so an active process exists to amplify sounds;

Question 46

Describe the active process of amplification

Answer 46

A particular segment of a living basilar membrane (red curve) vibrates far more in response to its resonant frequency, than a dead BM. This instead behaves as a passive system (blue curve).

Question 47

Describe the active process of frequency tuning

Answer 47

A dead basilar membrane produces a broad response and it is not tuned for a specific frequency (blue curve). A living BM instead selectively amplifies single frequencies.

Question 48

Describe the active process of compressive nonlinearity

Answer 48

The motion of the BM is augmented 100-fold during low-intensity stimulation, but amplification diminishes progressively with the increasing intensity of the stimulus.
Necessary as we need to compress the vast range of intensities that we can identify into the range of movement possible by the basilar membrane, which is limited.

Question 49

Describe the active process of spontaneous otoacoustiic emission

Answer 49

70% of normal humans ears emit one or more pure tones when in a quiet environment. This is only possible in healthy cochleas.This is due to the work performed by the ear in normal conditions to counteract the viscous drag in the cochlea.
Used to assess whether the hearing aid is working properly.
Hair cells of the cochlea are performing active work to amplify sounds, in a quiet environment, this work is wasted and so can leave your ears in the form of sounds.

Question 50

how many inner hair cells and outer hair cells do we have

Answer 50

Inner hair cells (IHCs): ~3500 per human cochlea Outer hair cells (OHCs): ~110000 per human cochlea

Question 51

Describe the role of the inner hair cells

Answer 51

95% of afferent projections (sensory axons that carry signals from the cochlea towards the brain) project from IHCs. IHCs provide sensory transduction.

Question 52

Describe the role of the outer hair cells

Answer 52

Most of the efferent projections (from the brain to the cochlea) connect to OHCs.
The origin of the cochlear amplification and otoacoustic emissions might be the OHCs.Their cell body shortens and elongates when their internal voltage is changed.This is called electromotility. It is due to the reorientation of the protein prestin.

Question 53

Summarise the features and functions of the inner hair cells

Answer 53

Found on their own

Not in contact with the tectorial membrane

Send impulses to the brain

They have stereocilia that move in response to the movement of endolymph in the scala media

Roughly 3500 in the body

Question 54

Summarise the features and functions of the outer hair cells

Answer 54

Found in groups of three
They are in contact with the tectorial membrane
They receive input from the brain
Electromotile so can expand and contract to amplify the amount of vibration (this is the basis of the cochlear amplifier)
Damage can result in sensorineural hearing loss
Roughly 20,000 in the body

Question 55

What is important to remember about the hair cells

Answer 55

Vibrations of the basilar membrane result in oscillating movement of the hair cells (Fig. 9.4). The stereocilia projecting from the upper surface of the hair cells are fixed at their extracellular end to the immobile tectorial membrane. They sway with the same frequency as the part of the basilar membrane that the hair cells rest upon. This results in oscillating changes in the physical arrangement of the hair cell membrane and, consequently, changes in the structure of membrane ion channels.