Auditory System

Question 1

What is the importance of sound?

Answer 1

Allows for communication

Important for emotion

Helps us to map out the world

Helps with survival

Question 2

How is sound information relayed from the ear to the brain? And how are complex sounds processed?

Answer 2

Highly specialised structures. And the cochlear.

Complex sounds which travel into the ear create one directional movements in the ear drums. These vibrations allow us to process and recognise different sounds and pinpoint where sounds come from.

Question 3

What features of sound need to be encoded?

Answer 3

Frequency and intensity

The frequency is related to pitch and is in Hertz
- this is detected by the cochlear mechanicals and hair cells

Sound intensity:

Is loudness - measured in decibels
We hear a huge range of sound intensity
This detection is achieved via the firing of many nerve fibres

Note the rapid onset of sound is also encoded for to create a map of the auditory space.

Question 4

What is important for the ears?

Answer 4

It has to remain sensitive for long periods of time and its important they don’t fatigue.

Question 5

How is sound transmitted in the ear?

Answer 5

The cochlea has sensory hair cells and nerve fibres

These transmit sound to the brain as neuronal signals.

Question 6

What are the three chambers of the cochlear? What is located at the bottom of the scala media? What does the organ of corti receive innervation from?

Answer 6

The scala vestibuli, scala tympani and scala media

At the bottom of the scala media is the organ of corti - this is the sensory organ of the cochlear that contains the sensory hair cells.

The organ of corti receives innervation from the auditory nerve which spirals up towards the brain.

Question 7

Where is the scala media in reference to the scala tympani and the scala vestibuli?

Answer 7

Its in the middle of the other two. Just remember that.

The tympani is above it

and the VESTIBULI is below it.

Question 8

What is perilymph? Where is it found?

Answer 8

Perilymph is a fluid

Its fluid is similar to extracellular fluid

It has a LOW amount of potassium ions (remember the extracellular fluid has less potassium ions than the inside of the cell would, than the intracellular) (5mM)

It has a normal levels of calcium (1.3mM)

Where is it found?
In the scala tympani and scala vestibuli

Question 9

What is endolymph ? and where is it found?

Answer 9

This is similar to intracellular solutions

It has a high concentration of potassium ions (150mM)

and has a low amount of calcium ions (20mM)

Where is it found?
Found in the scala media.

Question 10

What is the membrane potential (endocochlear potential) of the scala media? And what is this caused by?

Answer 10

The membrane potential is +80mv

This is due to the high concentration of potassium 150mM in the endolymph of the scala media.

Question 11

What are the two hair cells in the cochlear?

Answer 11

Theses are the inner and outer hair cells

The inner hair cells are sensory organs in the cochlear

The outer hairs cells are the sound amplifiers in the cochlear

Question 12

Where is the organ of corti found? And the nerves that innervate it?

Answer 12

Bottom of scala MEDIA and innervated by neurones on the basilar membrane

Question 13

What determines animals hearing frequency ranges?

Answer 13

This is determined by the size of the animal

The need to communicate over long distances

The need to hunt and for survival

Question 14

How do bats and dolphins have echolocation?

Answer 14

Their hair cells in the base respond to very high frequency sounds.

Question 15

What is preserved along the auditory pathway?

Answer 15

The tonotopic organisation

Question 16

How does the hair cell detect sound?

Answer 16

Sound causes a compression wave to travel from the base of the hair cell (bottom) to the apex of the hair cell (Top)

This causes MAXIMAL movement in the hair cell which corresponds to the frequency of sound

This movement made at the hair cells is determined by the width and the stiffness of the basilar membrane

Question 17

Structure of the hair cell?

Answer 17

At the base they are narrower and stiffer. At the top they are wider and more floppy.

Question 18

What channels do hair cells have? And what do they let in?

Answer 18

These hair cells have mechanoelectrical transducer channels at top of the hair cell

and voltage gated calcium 2+ channels which allow calcium to move in (at bottom of the hair cell

There are also voltage gated potassium channels.

Question 19

What is around hair cells? What is the membrane potential of the hair cell?

Answer 19

Apical surface of hair cells is surrounded by endolymph

The body of the cell is surrounded by perilymph

The membrane potential is 80mV

Question 20

What is the steps in excitation of hair cells?

Answer 20

There is sound stimulation

This causes the tip links (top of the hair cell) to be pulled

This opens the transducer channels

This leads to a large transducer current that depolarises the cell

Question 21

What happens when the hair cell becomes depolarised?

Answer 21

Depolarisation causes calcium channels to open at the bottom of the hair cell

This causes an action potential in nerve fibres . Thus sound signals are detected and sent to the brain

The depolarisation also activates potassium channels at the bottom of the cells. This helps the hair cell repolarise.

Question 22

What happens in the inhibitory phase of sound detection

Answer 22

This is after the sound has caused movement. The hair cell resets itself.

The hair bundles move in the opposite direction to which it usually moves.

Question 23

What channels are open longer in a hair cell?

Answer 23

The potassium channels are open longer than the calcium channels to allow for hyperpolarisation.

Question 24

What do inner hair cells do? What do they form? How do the differ from general hair cells?

Answer 24

They are primary receptors in the cochlear

They form hair bundles, and they have synaptic ribbons inside of the cell

In comparison to general hair cell inner hair cells have a greater number of different potassium ion channels

They have different properties also such as a voltage action range and different current sizes

Question 25

What happens when hair cells deflect in inner hair cells?

Remember the inner hair cells have a resting transducer current

Answer 25

In response to a sound the hair bundles deflect (move)

The inner hair cell then depolarises which increases nerve cell activity

The sound is then fully absorbed and the hair bundles are pulled back, hyper-polarising the cell

This reduces the firing below the spontaneous gradient.

Question 26

Why does the membrane potential of inner hair cells oscillate between being depolarised then hyperpolarised? What does this cause?

Answer 26

this is because sound causes movement of the hair cell so they become depolarised
But when this movement stops the sound is absorbed which stops movement causing hyperpolarisation.

This causes sound to be detected depending on the frequency of oscillations

Question 27

What happens when inner hair cells detect low frequency sounds? What happens as the frequency increases? What happens to action potentials at low and high frequencies?

Answer 27

At low frequencies membrane potential of inner hair cells changes - so hyperpolarises and depolarises - in relation to sound frequency

This occurs up to two or three KHz

However oscillations begin to saturate as you increase sound frequency

At a low frequency the nerve fibres show PULSES of action potentials that MATCH sound frequency

At high frequency there is SUSTAINED action potential rate

Question 28

Outer hair cells role? Innervations from the brain?

Answer 28

The role is to amplify signals in the cochlear

Note they dont send information to the brain!!!

Instead they are innervated by efferent fibres from the brain. These carry instructions from the brain to the outer hair cell. These turn the outer hair cells off.

Question 29

What is the shape of outer hair cells?

Answer 29

Their shape is different to inner hair cells.

These have a more v shape

The outer hair cells also have prestin in their inner membrane. Which inner hair cells dont have

These allow the hair cells to contract and elongate.

Question 30

How does transduction occur in the outer hair cell? And how does this differ to inner hair cells?

Answer 30

Same with inner hair cells as in there is a transducer current, which is pulled open, causing depolarisation, when there is sound stimulation. How long depolarisation lasts matches sound frequency

However the resting potential is MORE depolarised in outer hair cells and the resting transducer current is bigger in outer hair cells than inner hair cells.

Depolarisation however causes the outer hair cell to shorten.

Whereas hyper polarisation causes it to elongate.

Question 31

How do outer hair cells cause signal amplification?

Answer 31

The movement of outer hair cells causes the hair cell bundles to move up and down

This acts as a positive stimulation to increase the movement of the basilar membrane

This increases stimulation of inner hair cells which does

In other words the displacement of the basilar membrane is above threshold.

Question 32

What are the inner hair cells innervated by?

Answer 32

Type 1 and afferent fibres

Question 33

Type 1 afferent fibres for inner hair cells?

Answer 33

These represent most of the nerve fibres that enter the cochlear

Type 1 innervate the inner hair cells (they CONTACT them - key word)

Each single inner hair cell has 10 - 30 different fibres

Each fibre connects to a single inner hair cell

Question 34

Type 2 afferent fibres? Where do they enter? What do they innervate? Whats their function

Answer 34

These innervate the high frequency outer hair cells!! (5%) of cochlear nerve innervation

They enter the organ of corti - remember this is in the scala media

The type 2 afferent fibres are branched. They CONTACT (key word) up to 30 outer hair cells. They also synapse onto the cochlear nucleus

Their function may be to detect OVERSTIMULATION or pain in the cochlear

These fibres only become active when all of the outer hair cells contact them - thus to high stimulation.

Question 35

Key point of outer hair cells? And inner? And organ of corti? And cochlear structure?

Answer 35

They enhance the mechanical stimulation of inner hair cells.

They improve to tuning of the cochlear.

Inner hair cells:

Inner hair cell activation releases neurotransmitters onto the neurones which they work on

Organ of corti.

The ORGAN of corti is activated with the displacement of basal membrane inner hair cells. This causes MET channels to open causing the cells to depolarise

Cochlear structure?

• it is a spiral. The base is the outer region of the cochlear and the apex is the inner
• the base is stimulated by high frequency sound and the apex is stimulated by low frequency sound