Anatomy & Physiology of the Ear

Question 1

What is the measurement of amplitude?

Answer 1

Decibels (dB)

Question 2

What is the measurement of frequency?

Answer 2

Hertz (Hz)

Question 3

What is the frequency range of human hearing?

Answer 3

20Hz to 20,000Hz

Question 4

What does the EAM stand for?

Answer 4

External auditory meatus

Question 5

Where is the concha?

Answer 5

The bowl of the ear

Question 6

Where is the helix?

Answer 6

The line around the edge of the pinna.

Question 7

Name the 3 acoustic functions of the outer ear?

Answer 7

1. Collecting sounds
2. Sound localisation
3. Ear canal resonance

Question 8

How does the outer ear collect sound?

Answer 8

The pinna funnels the sound into our ear canal and boosts the sound by 5 dB.

Question 9

How does our outer ear help us decipher sound localisation? (where the sound has come from)

Answer 9

The pinna’s on our ears point forwards to collect more sound. This also helps us to determine if we had our eyes closed, if the sound had come from the front/behind/left/right.

Question 10

What is ear canal resonance?

Answer 10

This is an acoustic function our outer ears do because our concha/ear canal is a cavity that has its own resonance. These cavities resonate at 2,000-5,000Hz so the outer ear boosts these frequencies to the tympanic membrane (ear drum) by about 15-20dB.

Question 11

Why is our hearing most sensitive between 2,000-5,000Hz?

Answer 11

Because our ear canal generates resonances that have frequencies between 2-5,000Hz and the resonance boosts these frequencies to our ear drum by 15-20dB.

Question 12

What is the total resonant effect?

Answer 12

This is the total of the concha and ear canal’s resonance added together. It shows the total acoustic gain we get from the size/shape of our pinna’s and ear canals that boost the speech information frequencies.

Question 13

What is the attic region of the tympanic membrane called?

Answer 13

Pars flaccida

Question 14

What is the bottom part of the tympanic membrane called?

Answer 14

Pars tensa

Question 15

What shape is the tympanic membrane?

Answer 15

Conical (a cone shape)

Question 16

How is the tympanic membrane attached to the ear canal wall?

Answer 16

Via the fibrous annulus that runs around the edge.

Question 17

Name key components of the middle ear.

Answer 17

Tympanic membrane, malleus, incus, stapes, eustachian tube.

Question 18

What is the eustachian tube connected to?

Answer 18

The naso-pharynx.

Question 19

What are the auditory ossicles?

Answer 19

Malleus, incus and stapes. These form a chain which connects the tympanic membrane with the oval window of the cochlea.

Question 20

What is the function of the middle ear?

Answer 20

1. To transform acoustic energy in air at the tympanic membrane to acoustic energy in the fluids of the cochlea.
2. To increase the efficiency of sound transfer to the cochlea.

Question 21

What are the middle ear’s two main principles to increase the efficiency of sound energy transfer to the cochlea?

Answer 21

1. The tympanic membrane is 17 times larger than the stapes footplate in the oval window so because of the larger surface area difference ratio, the pressure at the oval window is much greater. This pressure is how sound energy is transferred to the cochlea.
2. The arm of the malleus is longer than the incus which creates a lever action. This action increases the pressure at the oval window by 1.3 times.

Question 22

What is the impact of the larger surface area ratio difference and lever action in the middle ear?

Answer 22

The combined factors of these two principles gives a person an increase of about 28 dB of sound entering the cochlea.

Question 23

What are the two main functions of the eustachian tube?

Answer 23

1. To maintain the pressure in the middle ear cavity so it is equal pressure to the ear canal (outside world). This is because the air in the middle ear is being diffused into its mucous lining but needs replenishing regularly. So every time we yawn or swallow, the eustachian tube opens and air rushes up it.
2. Fluid in the middle ear can cause hearing loss. So the 2nd function is to drain any middle ear fluids away.

Question 24

Why does the pressure need to be equal on both sides in the middle ear?

Answer 24

If the pressure is equal, this means the middle ear is in its relaxed position so when a soundwave comes along, the middle ear can vibrate backwards and forwards easily to deliver sound efficiently to the cochlea. However, if the pressure is not equal, then the ear drum may be bent inwards or stretched causing it to be tense. Therefore, it won’t vibrate as easily and you will experience a hearing loss.

Question 24

Why do children experience more middle ear infections compared to adults?

Answer 24

Anatomical differences in their eustachian tubes. For example, a child's ET is much more horizontal, shorter and although it's not wider, it's relatively wide compared to its length.

Question 25

What is the central conical core of the cochlea called?

Answer 25

The modiolus.

Question 25

What are the scala media and scala tympani separated by?

Answer 25

The basilar membrane.

Question 26

What is the cochlea duct called when it's in a cross section view?

Answer 26

The scala media.

Question 27

What separates the scala media and scala vestibuli?

Answer 27

The Reisner's membrane (sometimes called the vestibular membrane).

Question 28

What is the point at the top of the cochlea called where the scala vestibuli and scala tympani join together?

Answer 28

Helicotrema.

Question 29

What sits on the basilar membrane?

Answer 29

Our auditory receptor organ, called the organ of corti.

Question 30

What types of sensory hair cells does the organ of corti hold?

Answer 30

A singular row of inner hair cells (IHCs) and three rows of outer hair cells (OHCs).

Question 31

What sits above the inner and outer hair cells?

Answer 31

The tectorial membrane. This is quite a heavy membrane that gives the sensory hair cells something to push against. Remember the OHC stereocilia are embedded in the tectorial membrane, whereas the IHC stereocilia are not.

Question 32

How is sound transferred into the cochlea?

Answer 32

The stapes pushes against the oval window when it receives a soundwave. This causes fluid in the cochlea to vibrate at the same frequency as the sound and sets up a travelling wave in the basilar membrane. The round window moves back and forth to relieve the pressure when the stapes is pushing against the oval window.

Question 33

What is the travelling wave theory?

Answer 33

Soundwaves come into the inner ear via the stapes in the oval window. These soundwaves travel along the basilar membrane to form a travelling wave. Each section of the basilar membrane is finely tuned to respond to its specific frequency. The travelling wave then peaks at the corresponding frequency part of the basilar membrane and then dies away.

Question 34

What part of the basilar membrane responds to high frequency sounds?

Answer 34

The base of the BM because it is very narrow and stiff.

Question 35

What part of the basilar membrane responds to low frequency sounds?

Answer 35

The apex of the BM because is wider, broader and relatively loosely strung.

Question 36

What does the organ of corti do?

Answer 36

This can be referred to as the body's microphone because it takes soundwaves from the cochlea and converts them into neural code to send up to the brain.

Question 37

What type of neurons are inner hair cells innervated by?

Answer 37

Afferent neurons. These travel from the cochlea to the brain.

Question 38

What type of neurons are outer hair cells innervated by?

Answer 38

Efferent neurons. These neurons travel from the brain back to the cochlear (eff off).

Question 39

What do IHCs look like?

Answer 39

They are rounded in shape and have relatively straight rows of stereocilia attached to them.

Question 40

What do OHCs look like?

Answer 40

OHCs are cylinder in shape and their stereocilia are arranged in W or V shapes.

Question 41

How many nerve fibres do inner hair cells have?

Answer 41

10-20 nerve fibres per cell.

Question 42

What are the IHCs doing?

Answer 42

They are doing the hearing and then sending this information up to the brain via the auditory cortex.

Question 43

What are the OHCs doing?

Answer 43

These cells only have efferent connections from the brain back to the cochlea so they process the information via a feedback loop. This feedback loop enables our brain to tell our OHCs to fine tune its responses.

Question 44

What is the role of the IHC?

Answer 44

Their job is to fire and send signals along the auditory neurons, up the auditory nerve and into the cortex so that we can hear the sound.

Question 45

What triggers the IHC to fire and send off their signals?

Answer 45

When the basilar membrane moves up and down, the IHC's stereocilia are bent. When they're bent, the tip links open the ion channels and potassium + calcium ions flow into the cell (ionic exchange). This then depolarises the cell which causes the IHC to fire off its signals up to the brain.

Question 46

What is the 'active' role of the OHCs?

Answer 46

When a soundwave comes in and reaches the finely tuned frequency on the basilar membrane for that sound, the OHCs physically expand and contract so they pull the tectorial and basilar membrane together and apart which amplifies the sound. This motion sharpens the sound's frequency, allowing us to hear quieter sounds.

Question 47

What process are OHCs active participants of?

Answer 47

The mechano-electric transduction process.

Question 48

What do the OHCs do if they hear a loud sound?

Answer 48

Our OHCs stiffen when responding to loud sounds to suppress the level of stimulation. This enables us to cope when standing next to a jet engine.

Question 49

How do our OHCs increase our dynamic range of hearing?

Answer 49

The OHCs amplify quiet sounds by vigorously vibrating the tectorial and basilar membrane back and forth but they stiffen to suppress loud sounds. This means we can hear a range of frequencies from hearing a pin drop to standing next to a jet engine.

Question 50

What are action potentials?

Answer 50

When the basilar membrane vibrates, causing the IHCs to vigorously vibrate up and down which cause them to fire off signals up to the brain. The information they send up to the brain is called their action potentials which are carried to the brain in bundles of auditory nerve fibres.

Question 51

How is frequency information carried from the cochlea to the auditory cortex?

Answer 51

Some auditory nerve fibres carry low frequency sound, some carry high frequency sound. Each auditory nerve fibre are most sensitive to a certain frequency. This is called their characteristic frequency. Their characteristic frequency is dependent along the location on the basilar membrane of the hair cell to which it's attached,

Question 52

What are the 3 factors that impact intensity information being carried from the cochlea to the auditory cortex?

Answer 52

Intensity information is carried by the rate of firing, the number of fibres carrying the information and the type of nerve fibre carrying the signal.

Question 53

How do the 3 factors of intensity information transfer to the auditory cortex?

Answer 53

1. For louder sounds, firing is faster and more fibres carry the signal. 2. For quieter sounds, firing is slower and less fibres carry the signal. 3. Every IHC has 10-20 neurons firing off signals and some of these neurons are designed to carry low intensity and some carry high intensity sounds. 4. The neurons that are most sensitive have a high spontaneous firing rate and will pick up the quiet sounds. 5. The neurons that have a low spontaneous firing rate will carry high intensity sounds as they need a kick before they send off the signal.

Question 54

What do the various relay nuclei tell us about the particular sound it is processing?

Answer 54

A range of things such as the sounds intensity, whether the sound has come from the left or right ear and the pattern of the sound (as you go higher up the pathway).

Question 55

What pathways can we send information from the cochlea to the auditory cortex?

Answer 55

We have both an ipsilateral (same side) or contralateral (opposite side) pathway. The contralateral pathway is quicker and most efficient for receiving auditory information, when we hear sounds, both our left and right auditory cortex's are stimulated.