BMS248 Lecture 5 - The Auditory System

Question 1

What are the 4 features of sound that need encoding?

Answer 1

1. Frequency (Pitch) - Hz
2. Intensity (Loudness) - dB
3. Onset
4. Duration

Question 2

What is the relationship between frequency and wavelength/energy?

Answer 2

Inverse

Question 3

What are sound frequency, intensity, onset, and duration encoded by and where? (2)

Answer 3

. Hair cells
2. Nerve fibres
- found in the cochlea (Organ of Corti)

Question 4

What is sound frequency (pitch), and how can we hear a range of frequencies?

Answer 4

The number of cycles (waves) per second.
Achieved by mechanics of cochlea, and physiology of hair cells.

Question 5

What is sound intensity (loudness), and how can we hear a range of sound intensity?

Answer 5

The amplitude of the wave from peak-to-peak.
Achieved by the firing rate of many nerve fibres.

Question 6

Why is a rapid onset important?

Answer 6

For localising different sounds and creating a map of the auditory world around us

Question 7

What are the 3 chambers that form the cochlear spiral from top to bottom?

Answer 7

1. Scala Vestibuli (SV): 0mV
2. Scala Media (SM): +80mV
3. Scala Tympani (ST): 0 mV

Question 8

What two things are found in the Scala Media? What innervates these?

Answer 8

1. Organ of Corti (-60mV): innervated by the Auditory nerve
2. Basilar membrane: sits underneath organ of corti

Question 9

The SV and ST contain what? What are the compositions of K+, Ca2+, and Na+ ?

Answer 9

Perilymph:
- Low K+
- Normal Ca2+
- High Na+

Question 10

The SM contains what? What are the compositions of K+, Ca2+, and Na+ ?

Answer 10

Endolymph:
- High K+
- Low Ca2+
- Low Na+

Question 11

What creates the high K+ concentration in the SM - The Endocochlear Potential? What is the value of this?

Answer 11

Stria Vascularis creates Endocochlear potential of +80mV

Question 12

What is the Hair Cell resting potential in the Organ of Corti? Therefore what is the driving force into the Hair Cells from the SM?

Answer 12

-60mV
Driving force of 140mV - vital for function

Question 13

What is the tonotopic organisation of the cochlea? (Inner hair cells at apex vs base of cochlea)

Answer 13

Hair cells at the APEX respond to LOW frequency (pitch)
Hair cells at the BASE respond to HIGH frequency (pitch)

Question 14

How is cochlear tonotopicity established? (How does specific frequency activate a specific IHC)

Answer 14

By the basilar membrane travelling wave - from base to apex
Sound of one frequency causes maximal movement of the BM at one location (the characteristic frequency location) - CF
E.g. a LOW frequency sound will travel further along the BM and cause maximal movement near the apex - CF location closer to apex

Question 15

What are the characteristics of the apex vs base of the BM?

Answer 15

Apex: wide + floppy
Base: narrow + stiff

Question 16

What is the “place-frequency” code?

Answer 16

Sound frequency is encoded in the location of the active IHC, not the firing pattern - this positional information is preserved along the entire auditory pathway.

Question 17

What is the role of the inner hair cells of the cochlea?

Answer 17

Primary sensory receptors - they encode all of the auditory information and pass it onto the nerve fibres

Question 18

What channels are found on the tips of the shorter stereocilia on IHCs? What are these connected to?

Answer 18

Mechanoelectrical Transducer Channels (MET) generate an inward K+ current - these are connected to tip links which pull the channels open

Question 19

At rest what happens at IHCs?

Answer 19

1. Slight tension on tip links - some MET channels open
2. Resting inward MET current - K+ enters down an electrical gradient
3. Large gradient for K+ exit and small gradient for K+ entry
4. Depolarised resting potential (-55mV)
5. Resting activity in nerve fibres

Question 20

What happens during Excitatory stimulation of IHCs?

Answer 20

1. Large deflection of the hair bundle towards the taller stereocilia
2. Increased tension of tip links
3. Opening of MET channels - large MET current (K+)
4. Depolarisation of hair cell (-30mV) - activates Ca2+ channels
5. Increased activity of afferent nerve fibres
6. K+ channels repolarise the cell

Question 21

What happens during Inhibitory stimulation of the IHCs?

Answer 21

1. Large deflection of the hair bundle towards the shorter stereocilia
2. Tip links slacken
3. MET channels close - turns off MET current
4. Hyperpolarises cell below resting potential to -65mV
5. None or little neuronal activity
6. K+ channels open for longer to repolarise the cell - ready for the next cycle

Question 22

Why does K+ enter down an electrical gradient but leave down a chemical gradient?

Answer 22

Due to the separation between endo- and perilymph.
K+ both depolarises and repolarises the cell:
- Depolarisation by entry
- Repolarisation by exit

Question 23

What is the role of the outer hair cells of the cochlea?

Answer 23

Function as the cochlear amplifier

Question 24

OHCs have Prestin in their cell membrane - what does this molecule allow?

Answer 24

Allows the cell to shorten or elongate in response to changes in membrane potential - electromotility

Question 25

What is OHC resting potential?

Answer 25

-40mV

Question 26

What happens to OHC length when they DEPOLARISE? (Excitatory)

Answer 26

Shorten (-20mV)

Question 27

What happens to OHC length when they HYPERPOLARISE? (Inhibitory)

Answer 27

Lengthen (-50mV)

Question 28

How do OHCs function as the cochlear amplifier?

Answer 28

Combined movement of 3 rows of OHCs increase the movement of the basilar membrane - this increases the stimulation of the IHC bundles at the CF region