The Auditory System

Question 1

What are 2 critical features of sound that our ears need to recognise?

Answer 1

Frequency

Amplitude

Question 2

What happens to air molecules for sound to be produced?

Answer 2

Compression and rarefaction of air molecules

Question 3

What is frequency and how do we interpret it?

Answer 3

Number of complete oscillations that happen in a given time.
High frequency sound = high pitch.
Low frequency sound = high pitch.
Humans can tell between 20 and 20,000Hz.

Question 4

What is amplitude and how to we interpret it?

Answer 4

Amplitude is how big or small the increase and decrease in pressure is.
It is measured on a log scale.
We can hear from 0DB upwards and then over 100DB can damage the sensory apparatus.

Question 5

What parts of the ear are air filled and what parts are fluid filled?

Answer 5

External and middle ear are air filled.

The inner ear is fluid filled/

Question 6

What is the external ear (pinna) made out of?

Answer 6

Fibrocartilage

Question 7

Explain the shape and composition of the ear drum

Answer 7

The ear canal is an S shaped curve and takes us to the ear drum.
It is approx 2.5cm long and takes us to the tympanic membrane.
Outer 2/3 of the canal is cartilaginous and the inner 1/3 lies within the temporal bone.

Has sensation to it by the trigeminal mainly but some vagus innervation.

Question 8

Why does the doctor need to straighten out the pinna when looking down the ear?

Answer 8

The canal is S shaped

Question 9

What happens in a middle ear infection?

Answer 9

The area can become filled with fluid. Fluid will need to be drained out by a grommet.

Question 10

Explain how a vibration goes from the outer ear to the middle ear

Answer 10

External ear collects a sound and particles of air will travel down the auditory canal and hit the tympanic membrane.

Hitting of the tympanic membrane causes it to vibrate and then the vibrations and sent down 3 bones in turn, the malleus, incus and the stapes.

The stapes is located at the oval window (entrance to the cochlea).

Question 11

Why is the area of the tympanic membrane greater than the oval window (entrance to the cochleae)?

Answer 11

So that the ear can concentrate the sound vibrations.
This concentrates this pressure wave onto the cochlea and allows the fluid molecules to vibrate.
The middle ear needs to be air filled so the bones can vibrate properly so fluid in the middle ear is dangerous with going deaf.

Question 12

What is the inner ear composed out of?

Answer 12

The cochlea (the bony labyrinth) and the cochlear duct (the membranous labyrinth).

Question 13

What are the 3 parts of the cochlea duct called?

What are they filled with?

Answer 13

The scala vestiboli, the scala media and the scala tympani.

The scala vestibuli and the scala tympani is filled with perilymph and inside the scala media is endolymph.

Question 14

How do the vibrations travel through the cochlea?

Answer 14

Have the vibration to the oval window causes the fluid molecules in the scala vestibuli to vibrate. This pressure wave travels through the vestibuli, though the media and then out through the scala tympani to the round window to dissipate the energy.

Question 15

Where inside the cochlear are the hair cells found?

How are the hair cells arranged?

Answer 15

Between the scala media and the scala tympani, there is a basilar membrane splitting the two.

On the basilar membrane, sits the organ of Corti and the hair cells are here.

These cells are arranged in rows.
1 row of inner hair cells and 3 rows of outer hair cells.

Question 16

How do inner hair cells transduce sound waves?

Answer 16

The sterocilia protrude into the scala media and into the tectorial membrane in the scala media.

The tectorial membrane is very stiff and attaches to the bone so doesn’t move.

When the basilar membrane is vibrating, this causes the hair cells to vibrate backwards and forwards, causes the sterocilia to bend back and froth as their tips are being restricted by the tectorial membrane.

Basilar membrane vibrations shown causing the sterocilia to bend backwards and forwards.
When in the axis of polarity, they depolarise and vice versa. The potential of the hair cells therefore goes up and down.
This vibration can be transducer into an electrical signal.

Question 17

What is the difference in the basilar membrane at the base compared to the apex?

Answer 17

At the base (next to oval window), it is narrow and stiff.

At the apex (unrolled end of the cochlear), it is wide and flexible.

Question 18

How does a sound wave of a) high and b)low frequency, move down the basilar membrane?

Answer 18

The sound wave will want to go down the path of least resistance.

Least resistance of the membrane is down to the apex bu this means that the whole column has to vibrate.

At low frequencies, it is not difficult to get all these vibrations.
At high frequencies, the becomes hard and takes a lot of energy.

Lower frequencies = travel to apex
High frequencies = go through the stiff basilar membrane passing through from the scala vestibule to the scala tympani

Question 19

What is a place code?

Answer 19

position of the basilar membrane tells us the frequency of sound

Question 20

What is the role of outer hair cells?

Answer 20

Outer hair cells have a motor protein called Prestin which allows the cell to change length in response to a voltage change.

At a low voltage (hyperpolarised), the cells are length and at high voltage (depolarsed), the cells contract.

This change in cell length amplifies the vibrations of the basilar membrane.

Question 21

Why are outer hair cells useful for hearing tests?

Answer 21

As the outer hair cells are vibrating themselves, they will generate their own sound pressure waves which are called otoacoustic emissions.

These emissions will travel back out through the middle ear to make the tympanic membrane vibrate.
These emissions can be tested to show vibrations can go to the inner ear and that the outer hair cells are working, tests to check babies aren’t deaf.

Question 22

How are inner hair cells innervated in order to send information to the brain?

Answer 22

Each inner hair cells is innervated by 10-20 primary afferent fibres. These fibres are bipolar neurons, peripheral axons that innervate the hair cell and the central axon which heads into the brain stem.

Each of the inner hair cells respond to sound waves by making the basilar membrane vibrate up and down which bends the sterocilia.

Every time the sterocilia are bent, this causes a depolarisation which causes voltage gated calcium channels open and a release of NT glutamate is released. This causes the afferent fibres to fire an action potential.

Question 23

How do the hair cells react to different amplitudes of sound?

Answer 23

Loud sounds create big pressure waves and this causes the sterocilia to be bent more in each direction, this causes more NT to be released and therefore a higher firing rate of action potentials in the primary afferent fibres.

The AP firing rate codes for the amplitude of the sound (loudness).

Question 24

Where are the cell bodies of the spinal nerves found?

Answer 24

In the spiral ganglion found in the cochlear tree trunk.

Question 25

How is the vestibule-cochlear nerve formed?

Answer 25

The central axon of the spiral nerve neurons bundle up together to form the cochlear nerve and then this leaves the inner ear and joins with the vestibular nerve to make the 8th cranial nerve to enter the brain stem via the internal auditory canal.

Question 26

Where does the 8th cranial nerve enter the brain stem?

Answer 26

In the Pons at the cerebellopontine angle.

Question 27

What nucleus does the 8th cranial nerve travel to in the brain stem?

Answer 27

The cochlear nuclei.

Tonotopy is maintained in the nucleus.
Low frequency sounds largely synapse in the ventral lateral sides of the nuclei.
High frequency sounds tend to synapse in the dorsal parts.
All the other frequencies line in between the nuclei.

Question 28

Why is it important that we have bilateral representation of the ears up to the brain?

Answer 28

From the cochlear nuclei, the neurons project contralaterally via the superior olivary complex and ascend up to the midbrain and to the inferior colliculus.

There is also an ipsalteral projection as well.
This means there is a representation bilaterally from the ears to the higher reaches of the auditory system which is important for sound organisation.
Also have a back up pathway to each ear and it allows for spatial localisation of sound.

Question 29

What other area of the brain does the tonotopic frequencies go to?

Answer 29

To the speech sensory cortex

Question 30

What nucleus controls auditory reflexes (moving head towards sound) ?

Answer 30

The inferior colliculus

Question 31

What are the 3 main types of deafness?

Answer 31

1) Conductive = blockage of propel with outer eat such as trauma, ear infections, earwax
2) Sensorineural = problems with the inner ear cochlear e.g. the ossicles (3 bones) fuse together
3) Central = damage higher up in the brain in the auditory pathway

Question 32

What is tinnitus caused by?

Answer 32

Spontaneous activity in hair cells, spiral ganglion cells or neurons of the CNS.
It indicates damage to hair cells.