Auditory System

Question 1

How does the sound look like?

Answer 1

concentric waves

sinusoidal wave

Question 2

Amplitude

Answer 2

Volume

Question 3

Frequency

Answer 3

Pitch

Question 4

External ear

Answer 4

Collects sounds and boosts frequencies around 3 kHz (human speech 2-5 kHz)

Question 5

Middle ear

Answer 5

• 200-fold sound energy amplification
• Pressure focus from large tympanic membrane to small oval window
• ossicles

Question 6

Inner ear

Answer 6

Cochlea: sound transduction to neural signals

Question 7

How does Cochlea detect sound?

Answer 7

The cochlea contains sound detectors: hair cells

Question 8

What is the importance of the basilar membrane?

Answer 8

Movement of the basilar membrane bends the stereocilia of the hair cells.
• Basilar membrane vibrates with the frequency of sound
•Tectorial membrane sits just above stereocilia of the hair cells, forming a cover
•Inner and outer hair cells: transduce/convert mechanical motion of basilar membrane into neural signals

Question 9

The cochlea: two compartments with different cation concentrations

Answer 9

Scala media

Scale Tympani

Question 10

Scala media

Answer 10

Scala media contains endolymph that has high K+ and low Na+concentration, this drives K+ into the cells

Question 11

Scala tympani

Answer 11

Scala tympani contains perilymph (high Na+, low K+)

Question 12

Organ of Corti

Answer 12

potential difference:125mV
Large driving force for K+ to enter the cell
Hair cells have high K+concentration
K+ can both hyperpolarize and depolarize

Question 13

What happens during stereocilia movement?

Answer 13

Tip-links open and close K+ ion channels

Question 14

Ribbon synapses

Answer 14

enable neurons to transmit signals over a dynamic range of several orders of magnitude in intensity.
The hair bundle on a cochlear hair cell senses variations in sound-induced pressure in the cochlea. The resulting, voltage-dependent signal is transduced and passed via the afferent nerve fibre and the auditory nerve to the brain, where it is perceived as sound.

Question 15

Changes in air pressure are transmitted to

Answer 15

changes in fluid pressure in inner ear

Question 16

Sound energy is filtered and amplified in all 3 parts:

Answer 16

in external ear (shape), middle (ossicles) and inner ear

Question 17

Sound transduction

Answer 17

These changes induce movement of basilar membrane and stereocilia of hair cells
Hair cells contain mechanoreceptors that open and permit K+ flow into the cell.
This causes depolarization because of differences in K+ concentration in endolymph and perilymph.
The changes in membrane potential lead to neurotransmitter release and APs in afferent fibers

Question 18

Outer haircells

Answer 18

Signal amplification

Question 19

Inner hair cells

Answer 19

detect movements of atomic dimensions (0.3 nm) and millisecond precision and comprise 95% of fibers projecting to the brain

Question 20

Signal amplification

Answer 20

Basilar membrane vibrates 100-fold more than predicted 
Outer cells (3 rows) receive projections from superior olive, adjust the basilar membrane motion and act as an amplifier

Question 21

Sound amplification

Answer 21

Hair cells change shape in response to voltage change

Question 22

What senses voltage changes and contracts in response?

Answer 22

Prestin is the motor protein that senses voltage changes and contracts in response

Question 23

In which 3 parts is sound energy filtered and amplified?

Answer 23

In external ear by the shape of pinna and meatus.
In middle ear by the ossicles.
In inner ear the vibration of the basilar membrane is amplified by outer hair cells and motor protein prestin

Question 24

Tonotopy

Answer 24

Tuning of basilar membrane or tonotopy: cochlea as frequency analyzer.
Hair cells at the base of cochlea are sensitive to high-frequency sounds,hair cells at the apical end to low-frequency sounds.

Question 25

What does tonotopy allow?

Answer 25

Tonotopy allows for conducting higher frequency sounds

Question 26

Auditory pathway: Why do fibers cross?

Answer 26

Significant number of nerve fibers cross the brain and make connections with neurons on the side opposite from the side of the ear in which they begin. This happens very early on in the auditory system. Inter-aural comparisons are an important source of information for the auditory system about where a sound came from

Question 27

How do we locate sound? (1)

Answer 27

Medial Superior Olive computes the location of a sound by interaural time differences. Inputs from the two ears into MSO are delayed by various amounts relative to one another by the length of the axons

Question 28

How do we locate sound? (2)

Answer 28

Lateral superior olive encodes sound location through interaural intensity differences

Question 29

interaural intensity differences

Answer 29

The projections from one ear will excite the ipsilateral (same side) LSO and at the same time inhibit contralateral LSO (opposite side)This arrangement results in a good detection of sound location when it is directly lateral to the listenerIt takes two LSOs to represent horizontal positions

Question 30

What about high frequencies?

Answer 30

Time differences are very difficult to detect •There is a different mechanism – detection of interaural intensitydifferences •Head acts as an acoustical obstacle, “shadow” of lower intensity

Question 31

Sound location detection by 2 mechanisms:

Answer 31

* Medial superior olive detects interaural time differences: it contains neuronal map where each neuron responds most strongly when APs from two ears arrive simultaneously * Lateral superior olive detects interaural intensity differences of sounds: ipsilateral excitation and at the same time contralateral inhibition

Question 32

How is tonotopical representation achieved by the auditory system?

Answer 32

* Different parts of cochlea respond to different frequencies of sound: high frequencies at the base and low frequencies at the apex * Both medial and lateral superior olives share this feature, afferent fibers from high and low frequency areas in cochlea project selectively to high and low frequency parts in superior olives

Question 33

Inferior Colliculus

Answer 33

Inferior Colliculus integrates the cues for localizing sounds in space and contains topographical representation of the audible space

Question 34

Auditory thalamus

Answer 34

Auditory thalamus detects specific combinations of spectral and temporal combinations of sounds (important for speech recognition)

Question 35

How is the primary auditory cortex organized and what is its function?

Answer 35

Auditory cortex is tonotopically organized and is essential for frequency discrimination and sound localization, also plays an important role in processing of communication sounds

Question 36

What are the belt areas?

Answer 36

The belt areas of the auditory cortex have a less strict tonotopic organization and also process complex sounds, such as those that mediate communication

Question 37

What does the primary auditory cortex contain?

Answer 37

* Contains patches of EE cells (excited by inputs from 2 ears) and EI cells (excited by input from one ear and inhibited by input from the other ear) * Remind the ocular dominance columns in visual cortex * Combination-sensitive neurons – neurons that respond to specific sound combinations. (Remind simple cells in visual cortex) * Cells that respond to specific temporal sequences of sounds (as in communication or music)

Question 38

Just like in visual system, auditory system contains 2 streams from primary auditory cortex:

Answer 38

* ventral (object processing) and | * dorsal (space and motion)

Question 39

How are Increasingly complex combinations of sound processed?

Answer 39

increasingly complex combinations of sounds are processed from cochlea to auditory cortex