Sound conduction and transduction

Question 1

What are some causes of hearing loss?

Answer 1

loud sounds, genetic conditions, drugs, ageing and infections

Question 2

What is he human hearing range?

Answer 2

20Hz to 20kHz

Question 3

What is the basis of the Bel scale?

Answer 3

We compare the logarithm of the intensity of a sound with the logarithm of the intensity of the faintest possible sound.
Log10(I) − Log10(I0)
The decibel scale is just this multiplied by 10.

Question 4

What is the role of the 3 ossicles?

Answer 4

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

Question 5

Define impedance and resonant frequency?

Answer 5

The impedance measures of
reluctance of a system in receiving
energy from a source.

The frequency at which the
impedance of the system is
minimal is called the
resonant frequency.

Question 6

What are the 3 ossicles?

Answer 6

Maleus, Incus and Stapes

Question 7

Define and give some causes of conductive hearing loss?

Answer 7

Conductive hearing loss is when the ear is not capable of transmitting the vibration of sound waves
onto the cochlea.

Can be due to fluid accumulation, perforated tympanic membrane and abnormal growth of bone in ear canal (osteosclerosis)

Question 8

What does movement of the Stapes result n?

Answer 8

Generates a difference in pressure between the two liquid-filled chambers of the cochlea,
which in turns causes the vibration of the basilar membrane

Question 9

What compromises the Organ of Corti?

Answer 9

The Organ of Corti includes the basilar and tectorial

membranes and the hair cells and supporting cells

Question 10

What is special about the properties of the basilar membrane?

Answer 10

It is an elastic structure of heterogenous mechanical properties that vibrates at different positions
along its length in response to different frequencies.

This allows it to break down complex sounds down by distributing the energy of each component frequency
along its length. Sensory neurones are needed along the length to detect thee frequencies.

Question 11

What are the sensory receptors of the ear?

Answer 11

Hair cells

Question 12

Explain how mechano-transduction occurs?

Answer 12

The motion of the basilar membrane deflects the hair bundles of the hair cells.
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.

Question 13

What is the importance of tip links?

Answer 13

Stereocilia are connected by filamentous linkages called tip links. They work as small springs stretched by the stereocilia’s sliding
Scientists observed:
• Tip links share their location with ion channels
• Their disruption abolishes mechanotransduction

Thus response currents are the result of the opening of ion channels activated by the stretching of the tip links.

Question 14

What points to the existence of an active process in hair cells?

Answer 14

Healthy hair bundle actively complies with
the direction of the stimulus: the measured stiffness
becomes negative…ie. they have the capacity to do work.

Question 15

Why is there a need for an active process?

Answer 15

Large portion of energy is lost in the viscous damping effects of the cochlear liquids.
The sensitivity and the sharp frequency selectivity of the cochlea cannot be explained solely by passive mechanical properties: basilar membrane (BM) impedance.

Question 16

What are the 4 aspects of active process?

Answer 16

Amplification: A particular segment of a living basilar
membrane (red curve) vibrates far more in
response to its resonant frequency,
Frequency tuning: A living BM selectively amplifies single frequencies
Compressive non linearity: amplification diminishes progressively with the increasing intensity of the stimulus.
Spontaneous otoacoustic emission?

Question 17

Compare inner hair cells and outer hair cells?

Answer 17

There are more outer hair cells. Afferent projections mainly come from IHCs and efferent mostly go to OHCs.

Question 18

What do outer hair cells do?

Answer 18

Electromobility: Their cell body shortens and elongates when their internal voltage is changed. Happens at a rate of 80KHz and due to the protein prestin.
Allows for enhancement of frequency selectivity and sensitivity

Question 19

Describe transmission from the hair cells to the brain?

Answer 19

Hair cells (mostly IHCs) form synapses with sensory neurons in the cochlear ganglion (spiral ganglion).
Ganglion cells in a particular area of the spiral ganglion respond best to the resonant frequency of the basilar membrane in that same area.
The axons in the cochlear nerve transmit the information to the cochlear nucleus.

Question 20

Why are many nerve fibres needed?

Answer 20

A population of nerve fibres can produce a phaselocked collective response even at frequencies that single nerve fibres could not manage individually.
It allows to encode middle and middle-high frequencies.

Question 21

Define sensorineural hearing loss and what are some causes?

Answer 21

This is when problem is rooted in the sensory apparatus of the Inner ear or in the vestibulocochlear nerve.
Can be due to loud noises, ageing,Menere’s disease, genetic defects and aminoglycoside antibiotics.

Question 22

Describe the basis of cochlear implants?

Answer 22

Hearing loss can e due to loss of hair cells so we can bypass them and stimulate the nerve directly.
They work by detecting sounds, breaking them down into their constituent frequencies and sending the signal directly to the auditory nerve via antennas.

Question 23

Describe organisation within the cochlear nucleus?

Answer 23

Low frequency neurones ventrally, high frequencies dorsally

Question 24

Describe 2 cell types in the cochlear nucleus?

Answer 24

T stellate cells: they encode sound frequency and
intensity of narrowband stimuli.
Bushy cells: Bushy cells produce more sharply but less temporally precise versions of the cochlear
nerve fibres. They provide the resolution
required to encode the relative time of
arrival of inputs to the two ears.

Question 25

What does the medial superior olive do?

Answer 25

Here the interaural time difference is computed: sounds are first detected at the nearest ear before they reach the other one. Bushy cells carry information about the timing of inputs at every cycle.

Question 26

What does the lateral superior olive do?

Answer 26

The LSO detects differences in intensity between the two ears. Neurons are excited by sounds arising
from the ear in the same side (ipsilaterally), while they are inhibited by opposite sounds (contralaterally).

Question 27

Describe the role the Superior olive has in feedback?

Answer 27

SOC neurons send feedback to the Hair Cells:
Neurons from the medial superior olive - IHCs bilaterally.
Neurons from the lateral superior olive - OHCs ipsilaterally.
The feedback is used to balance the responses from the two ears, but also to reduce the sensitivity of the cochlea

Question 28

Whats the role of the inferior colliculus?

Answer 28

Here responses from different frequencies merge. In the IC all ascending pathways converge.

Question 29

What does the superior colliculus do?

Answer 29

Here auditory and visual maps merge. Neurons 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.

Question 30

Where is the primary auditory cortex located?

Answer 30

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.