Auditory System

Question 1

What is sound?

Answer 1

The displacement of air particles following a sinusoidal pattern of compression and rarefacation

Question 2

What can humans hear?

Answer 2

20Hz-20KhZ

Ranges change during life

Optimal area of hearing is 40-50 decibels for humans normally

Question 3

Components of the auditory system

Answer 3

Outer ear- Pinna to eardrum
-air

Middle ear- eardrum to medial wall of the middle ear cavity -air

Inner ear- cochlear, vestibular system and vestibulocochlear nerve
-fluid

These are Central auditory pathways

Question 4

Outer pinna and ear canal

Answer 4

Pinna- cartillagenous structure

Formed from pharyngeal arches 1 & 2 ( 6x Hillocks of His)
Comes up in assessment

Forms between 10th and 18th week in utero
Directs soundwaves towards ear canal

Role of the pinna is to direct sound towards the inner subsystems
Folds of ear selectively filter sound out
High pitch sounds are detected more easily than Low pitch sounds due to evolution

Ear Canal made of 1/3 cartilage & 2/3 bone

Question 5

Outer ear- tympanic membrane

Answer 5

Top 1/3rd of eardrum is pars flaccida
2 layers- lacks a mesoderm
More likely to retract and become damaged

Bottom 2/3rds of eardrum is pars-tensa
This has 3 layers- mesoderm, endoderm, ectoderm

Question 6

The middle ear contains

Answer 6

Bones:
Malleus, Incus & Stapes
Malleus is attached to tympanic membrane
The head of malleus is attached to incus
Incus long process is attached to head of stapes

Muscles:
Tensor Tympani & Stapedius
Tensor tympani tenses the tympanic membrane
Stapedius attaches to the stapes

Tubes
Eustachian Tube

Question 7

Middle ears function

Answer 7

Slide 15

Is to house the the ossicular chain

Question 8

What is the role of the middle ear?

Answer 8

Acoustic impedance match between air and fluid- filled inner ear

When sound travels from air to fluid (into cochlea fluid) 97% of loss of energy due to impedance but the amplification then cancels this loss to cause acoustic impedance match

Question 9

Middle ear is the sound amplifier

Answer 9

The large eardrum concentrates its size down onto the foot plate of stapes which is very small

Amplifying the movement makes the airborne sound vibration louder

Because of the ratio of the tympanic membrane to stapes footplate( 14:1)

This means any force applied to the ear drum, which has air hitting it, is going to be concentrated down to a much smaller area, increasing the air pressure as force is same but area decreases and P=F/A

There’s a lever action of the ossicles- handle of maelleus is 1.3 times longer than process of incus
So movement of the tympanic membrane anchored to the malleus causes a moment force onto the incus, which then causes a little bit of force onto the stapes foot plate, which takes advantage of the fact that theres a big ear drum and small footplate, leads to gain (making things loud)

Total gain is 18.3:1 or 20-35 dB

Question 10

Transferring energy from outer ear to inner ear- oscillar chain

Answer 10

200 fold increase boost In pressure from tympanic membrane to inner ear

Question 11

What is the role of muscles in the middle ear?

Answer 11

Protection of the inner ear from acoustic trauma

Stiffens the ossciular chain

Stapedius stimulated acoustically (quicker of the reflexes)
Reflex arc: 3 or 4 neurones
25 ms in man

Tensor Tympani-voluntary and involuntary control
Stiffens to allow other sounds to be heard when chewing as otherwise nothin but chewing heard

Question 12

What is the role of the Eustachian tube?

Answer 12

Ventillation of the middle ear space
Drainage of secretions
Equalises air pressure

Often dysfunctional in children – causing hearing loss and middle ear infection

Question 13

The inner ear: vestibulocochlear apparatus

Answer 13

A set of fluid filled sacs, encased in bone

Cochlear- responsible for hearing

Labyrinth- responsible for balance

Innervation: Vestibulocochlear nerve

Question 14

The cochlea

Answer 14

2.5 turns fluid filled bony tube

2 openings- round window & oval window

3 compartments ( Scala Tympani, Scala Media & Scala Vestibuli)

2 Ionic fluids

Question 15

Scala vestibular

Answer 15

Label

Question 16

Cochlear fluids

Answer 16

Scala media contains endolymph
This contains high K+

Scala vestibuli/tympani contain perilymph
These are like Extra cellular fluid or CSF
Na+ rich

Gradients are maintained by Na+/K+ ATPase channels and NKCC1 CIC-K chlorine channels
Ion channel abnormalities lead to deafness- inner ear problems cause complete deafness

Question 17

The cochlea

Answer 17

If a fluid is compressed a wave can be generated as long as there’s somewhere for the wave to leave from

When the stapes moves near the oval window, the oval window moves in and out slightly which then sends a pressure wave coursing through the Scala vestibuli

The reason that this pressure wave moves at all is because the round window is elastic

When the pressure gets to the Scala tympani theres a tiny bit of movement upwards of the basilar membrane

The basilar membrane has the organ of corti sitting on it
Organ of corti has hair cells- inner and outer

The hair cell is attached to the tectorial membrane

Question 18

Basilar membrane

Answer 18

At base side the basilar membrane is narrow and stiff
High frequency sounds picked up here

At the apex the basilar membrane is wide and floppy
Low pitch sounds picked up here

This shows the basilar membranes tonotopy: different parts of the basilar membrane optimally responding to different frequencies

Question 19

Organ of corti

Answer 19

Movement in the basilar membrane causes movement in the organ of corti and specifically causing movement of the hair cells

This is where there is movement from waves to sound waves to fluid waves which then changed to electrical energy

Hair cells attached to auditory nerve fibres

Question 20

Displacement of the basilar membrane causes movement of specialized mechanical transducing cells

Answer 20

Hair cells
-Inner Hair Cells- Mechanical transduction
-Outer Hair Cells- fine tuning

Base of hair attached to basilar membrane
Stereocillia anchored to tectorial membrane

Shearing forces at the stereocillia

Question 21

From waves to electrical signals

Answer 21

The stereocillia of the inner hair cells move as a result of the shearing forces- this causes creation of electrical spark
Rapid response/depolarisation required

Mechanically gated (as the hair cells physically move to open up a k+ channel) K+ channels opened causing depolarization ( K+ rich endolymph)

Depolarization results in opening of voltage gated Calcium channels

Release of neurotransmitter- Glutamate (plus others)- afferent neurotransmitter in the auditory system, is thought to be the afferent transmitter between the cochlear inner hair cells and afferent neurone

Repolarization through K+ efflux ( into K+ poor perilymph)

Question 22

Tonotopy and role of outer hair cells

Answer 22

Each nerve responds maximally at a specific frequency.
Can respond to others but responds optionally to a specific frequency

But our ability to discriminate different frequencies is not fully explained by this theory

Outer Hair Cells can alter the stiffness of the basilar membrane to ensure maximal stimulation at one site and dampened response at another

Increased resolution

Question 23

How is sound information coded?

Answer 23

Sound analysed to encode in formation for neural transmission

FREQUENCY (PITCH) Encoded in nerves by location along the basilar membrane
INTENSITY (LOUDNESS) Encoded in nerves by numbers responding and by firing rate
SOUND TRANSDUCTION Inner Hair Cells (and OHCs)
AMPLIFICATION Outer Hair Cells

Question 24

Whole process

Answer 24

E. C. O. L. I

Auditory fibre – spiral ganglion.
Spiral Ganglion to vestibuloCochlear nerve ( VIII)
Then hits Central auditory pathway

Central auditory pathway is
8th cranial nerve
Cochlear nucleus- in the cochlear complex which is in junction between the pons and medulla
Olive- MSO
Lateral leminiscus
Inferior colliculus

Question 25

What does the Brian stem do

Answer 25

Localise sound

Question 26

Interaural Time differences: MSO- Medial Superior Olive

Answer 26

MSO neurones are coincident detectors: response only when exciters signals arrive simultaneously

Anatomical differences in connectivity allow each MSO neurone to be sensitive to sound source from particular location

Slide 45