Auditory

Question 1

First amphibians

Answer 1

They already had some sensory cells that were able to sense the movement of water around the body and a vestibular system.

When they migrated into land they learnt how to detect sounds. This lead to the appearance of the tympanic ear in mammals and birds.

Question 2

Sound is important for

Answer 2

Communication
Emotion
Navigation
Survival

Question 3

What features of sound need to be encoded.

Answer 3

Frequency in hertz-
Number of waves per second. We can hear a wide range. 10^3

Sound intensity loud in decibels-
A massive range can be heard 10^12. This is achieved by the firing rate of afferent nerve fibres.

Onset/latency- a rapid onset is important for localising different sounds. And creating a topographic map of auditory space.

Duration- the ear has to remain sensitive to sound for long periods without fatigue. It never rests. The sensory cell synapses are specialised for sustained ntm release.

Question 4

What is the function of the oscicles.

Answer 4

They amplify the signal so that the fluid in the cochlea will vibrate.

Question 5

How does sound enter the ear and get to the cochlea

Answer 5

Sound is shaped by the auricle (outside of ear).
It travels down the ear canal and causes a vibration on the tympanic membrane.

This will vibrate the tiny bones called the oscicles. (Malleus incus and stapes). The stapes is the smallest in the body.

This will vibrate the round window and cause vibration if the fluid in the cochlea.

Question 6

Labels

Answer 6

UTICLE and saccule

Question 7

Cochlea nerve

What is in the organ of corti

Answer 7

At the base of the cochlea

It enters through the Modiolus (core of the cochlea)

Sensory cells

Question 8

What are spiral ganglions and look at the picture of the cochlea.

Answer 8

Spiral ganglions come from the three chambers and carry info and join to the cochlea nerve.

Question 9

What do the three chambers of the cochlea contain.

Answer 9

Scala vestibuli and tympani contain perilymph. This is like normal extra cellular fluid with low K (5nM) and normal ca (1.3mM).
-70mV

Scala media contains endolymph which is similar to intra cellular fluid. With high k (150nM) and low ca.
80mV

Question 10

Stria vascularis

Answer 10

Pumps K into the scala media to give it the high K

Question 11

Describe the hair cells in the organ of corti.

Answer 11

A single row of IHC
Three rows of OHC.

16,000 hair cells in each cochlea and 4000 are the IHCs which are the sensory ones.

When they are damaged they cannot be replaced.

Question 12

Supporting cells in the cochlea

Answer 12

Dieters cells are below the outer hair cells.

Pillar cells are between the inner and outer hair cells.

Question 13

Neurons in the cochlea

Answer 13

Type 1 spiral ganglion neurons innervate the IHCs. Carrying info to the brain.

Type 2 spiral ganglion neurons innervate the OHCs. Thought to be involved with loud noises and nociception.

Lateral efferent synapse with the type 1 fibres
Medial efferents synapse to the OHCs.
They give efferent feedback from the brain that allows some control over the sounds we concentrate on and this prevents the fibres being overstimulated.

Question 14

Tonotopical organisation

Answer 14

The cochlea is tonotopically organised.

The cells at the base respond to high frequency sounds and the cells at the apex respond to low frequencies.

The cochlea relays the info to the cochlea nucleus in the brain stem.
The cochlea nucleus also has tonotopic organisation as all the apex cochlea fibres go to one side of the cochlea nucleus and the base cochlea fibres go to the other side.

Question 15

Animals different hearing ranges.

Answer 15

Humans are 20-20,000

Mice, dolphins and bats can hear high frequencies.

Birds cannot heat high so they cannot heat mice. It makes them harder to hunt.

Blue whales use very low frequency sounds that travel well in water.

Question 16

How is the tonotopicity established

Answer 16

By the basilar membrane

The movement of the fluid on the cochlea will cause a maximum stimulation of a particular region along the basilar membrane.

The membrane is narrow at the base for high frequencies and wide at the apex.
The base is stiff and the apex is floppy.

Low frequency sounds can travel further and vibrate the apex.

Question 17

Maximum deflection

Answer 17

The biggest wave caused by a sound on the membrane.

The hair cells in the region if the basilar membrane that has the maximum deflection will be stimulated.

Question 18

Tonotopicity preservation

Answer 18

It is preserved throughout the entire auditory pathway.

The cochlea and brain stem and cortex.

Question 19

The general hair cell

Structure and stimulation

Answer 19

A hair bundle on top of the cell which is stimulated by the movement of fluid in the cochlea if mammals. Or by water in fish. The hair bundle is also called stereocillia.

On top of each stereocillia there is a transducer channel which is mechanically gated. The stereocillia are joined by tip links.
Pulling the tip link causes activation of the channels.

They also have a K channel for removing K so the cells can repolarise
The have ca channels for ca entry to allow excocytosis of synaptic vesicles.

Question 20

When the hair cell is at rest

Answer 20

No sound.
There is resting tension on the tip links that opens some of the channels.
This allows some K into the hair cell and slight depolarisation to -55.

This activates some ca channels and produce the resting activity in the afferent fibres.

Question 21

Inside the hair cell is

Outside the hair cell is

Answer 21

Perilymph

Endolymph

Question 22

When the hair cell is stimulated

Answer 22

A sound occurs.
The stereocillia bend towards the taller ones.
This increases the tension in the tip links and open all the channels.
Lots of K enters and causes depolarisation.
It causes a maximum inward transducer current of -30.
Ca channels all open and lots of ntm released for high afferent firing.

then the sound pulls the stereocillia back in the opposite direction and this slackens the tip links and closes the transducer channels.
the cell will hyperpolarise and K will leave the cell

Question 23

why is the hearing system so effective

Answer 23

no Na is required

Question 24

how are the rows of stereocillia arranged on the IHC and where are the ion channels located

Answer 24

there is one tall row and two shorter rows.

the ion channels are located on the shorter cillia.

Question 25

what are synaptic ribbons

Answer 25

specialized electron dense bodies in the synaptic regions of the hair cell, they act as a store of synaptic vesicles so they can maintain high rates of vesicle exocytosis for long periods of time.

Question 26

what types of K channels do the IHCs have

Answer 26

they have a fast activating one, a slow activating one and one that is active at negative voltages.
this helps the IHCs respond and recover quickly.

Question 27

how does the frequency of the sound affect how the membrane potential changes

Answer 27

at low frequencies the receptor potential is able to follow the sound frequency well.
at high frequencies the cells respond with a sustained change in membrane potential for the duration of the stimulus, the fibres fire constantly and it is no longer pulses of activity.

Question 28

what do mammalian OHCs do and how are their hair bundles arranged

what are the innervated by and what do they do to the cell

Answer 28

they dont have a role in transmitting sensory info to the brain, they are involved with cochlea amplification.
their hair bundles are arranged in a W shape.

they are innervated by efferent fibres, they release ach which activates receptors linked to the K channels in the hair cell and this causes inhibition of the cell.
they have a small amount of afferent input but not as much as IHCs.

Question 29

what does prestin do and where is it found

Answer 29

in the OHC membrane there is prestin which is a motor protein, in response to chlorine it allows the cell to contract or elongate.
when the cell depolarises it causes a conformational change in the prestin that shortens the length of the cell.
when the cell hyperpolarises it elongates.
the cell contracts and elongates rapidly and provides the IHC with amplification because it makes their stereocilia move more.

Question 30

what is the innervation of the IHCs

Answer 30

each IHC is innervated by 10-30 type 1 afferents, it needs this much innervation because all the action potentials can summate together and better represent the derpolarisation of the IHC.
every fibre has a different threshold, low threshold fibres respond to quiet sounds and high threshold fibres respond to loud sounds.

Question 31

which are more depolarised at rest, OHCs or IHCs

Answer 31

OHCs so that they can respond faster.

Question 32

how many of the afferents in the cochlea are type 1

Answer 32

90-95%

Question 33

how are the OHCs innervated

Answer 33

type 2 afferents innervate 10-30 OHCs
they will only fire when all the OHCs they innervate are activated
they respond to dangerous loud sound and signal to the brain to turn the cells off using efferent fibres.

Question 34

what is the difference between the medial and lateral efferrents

Answer 34

the medial efferents are inhibitory and turn off the OHCs

the lateral efferents protect afferent fibres from over activity

Question 35

what is the path of the auditory system and what does each area do

Answer 35

from the cochlea the afferent fires go to the cochlea nucleus in the brainstem.
ventral cochlea nucleus fibres are responsible for sound localisation, dorsal cochlea nucleus fibres are responsible for sound recognition.
the superior olivary complex is responsible for sound localisation and is the first binaural site.
the info then goes to the inferior and superior coliculus where it is integrated with other sensory modalities.
then to the medial geniculate nucleus which is for memory and learning
then to the auditory cortex where the memory and decision making is done.