Hearing

Question 1

List the compartments of the ear.

Answer 1

1 - Outer ear (pinna).

2 - Middle ear.

3 - Inner ear.

Question 2

In which compartment of the ear are the ossicles found?

Answer 2

The middle ear.

Question 3

List the ossicle bones.

Answer 3

1 - Malleus.

2 - Incus.

3 - Stapes.

Question 4

What is the role of the ossicles?

Answer 4

To amplify the vibration produced by the tympanic membrane.

Question 5

List the muscles that are involved in the movements of the ossicles.

What is the role of these muscles?

Answer 5

1 - Tensor tympani.

2 - Stapedius.

• They contract by a reflex known as the attenuation reflex to prevent against damage against the inner ear that might occur as a result of loud sounds.

Question 6

In which compartment of the ear is the cochlea located?

Answer 6

The inner ear.

Question 7

List the 3 canals within the cochlea.

Which fluids fill these canals?

Answer 7

1 - Scala vestibuli

• Filled with perilymph.

2 - Scala media.

• Filled with endolymph.

3 - Scala tympani.

• Filled with perilymph.

Question 8

What is the basilar membrane?

Where is it located?

What is its function?

Answer 8

• A flexible membrane found in the scala media that contains the organs of Corti - the sensory organs of hearing.
• It moves with the endolymph as a result of movement of the ossicles.

Question 9

How does the shape and flexibility of the basilar membrane change throughout the length of the cochlea?

Why is this important?

Answer 9

• At the apex, it is wide and floppy.
• At the base, it is narrow and stiff.
• This means that the basilar membrane can respond to both low frequencies (at the apex) and high frequencies (at the base).

Question 10

What is the tectorial membrane?

What is its function?

Answer 10

• The membrane that sits above the organ of Corti.
• When the basilar membrane vibrates, the tectorial membrane shifts back and forth.
• This bends the underlying stereocilia that project from the hair cells of the organ of Corti.
• This induces a receptor potential in the hair cells.

Question 11

Describe the mechanism that causes hair cell depolarisation.

Answer 11

• When the stereocilia are moved by the tectorial membrane, mechanically gated K+ channels on their apical membrane are manipulated:
• When the stereocilia move in one direction, the K+ channels open, and when the stereocilia move in the opposite direction, the K+ channels close.
• K+ influx into the hair cell causes depolarisation, which leads to Ca2+ influx.
• The Ca2+ causes release of glutamate into the synaptic terminal.

Question 12

What is the difference between outer and inner hair cells with regards to the contribution of nerve fibres to the cochlear nerve?

Answer 12

The majority of fibres of the cochlear nerve originate from inner hair cells.

Question 13

What is the primary function of outer hair cells?

What is their secondary function?

Answer 13

• The primary function of outer hair cells is to amplify the sound produced by the movement of the tectorial membrane by contracting motor proteins on their plasma membrane which pull the tectorial membrane closer to the hair cell.
• The secondary function of outer hair cells is to generate action potentials in response to sound (like inner hair cells).

Question 14

How do different regions on the organ of Corti differ with regards to their response to sound?

How does this relate to their distribution?

Answer 14

• Different regions on the organ of Corti have their own characteristic frequency. This is the frequency to which they have the greatest response.
• Different regions of the organ of Corti have a characteristic frequency, such that the regions responding to low frequency sound will be situated at the apex of the basilar membrane and vice versa.
• This topographical organisation of the organs of Corti along the basilar membrane is called tonotopy, and is maintained throughout the ascending pathway.
• This is one method of frequency identification.

Question 15

List 4 areas of the brain to which the cochlear nerve travels after leaving the ear.

Answer 15

The cochlear nerve travels to:

1 - The auditory cortex of the temporal lobe.

2 - The superior and inferior colliculi.

3 - Medial geniculate nucleus.

4 - Brainstem neurones (outer hair cells only).

Question 16

To which side of the brain does a cochlea send its action potentials?

Answer 16

One cochlea sends its action potentials to both sides of the brain.

Question 17

What is phase locking?

Answer 17

• Phase locking is a mechanism that helps with frequency identification (the other being tonotopy).
• It involves matching frequency of action potentials with the frequency of the inbound sound wave.

Question 18

For very low, intermediate and high frequency sounds, give the method of frequency identification used by the auditory system.

Answer 18

• Very low frequencies use phase locking.
• Intermediate frequencies use both phase locking and tonotopy.
• High frequencies use tonotopy.

Question 19

Why can high frequency sounds not use phase locking?

Answer 19

High frequency sounds can’t use phase locking because there is a limit to the rate of generation of action potentials due to the refractory period.

Question 20

List the mechanisms that translate sound intensity.

Answer 20

1 - Number of cells firing (as a louder sound will result in a greater deflection of the basilar membrane).

2 - Firing rate (as long as the rate of action potential generation remains in phase with the sound wave).

Question 21

List the phenomena which contribute to sound localisation.

Answer 21

1 - Interaural delay.

2 - Interaural intensity difference.

Question 22

What is the function of the projections of the cochlear nerve to the medial geniculate nucleus and superior and inferior colliculi?

Answer 22

They are involved in the integration of auditory and visual signals.

Question 23

What is the eustachian tube?

List 2 functions of the eustachian tube.

Answer 23

• The connection between the nasopharynx and middle ear.

1 - It equalises the pressure between the middle ear and the environment.

2 - It drains the mucus that forms in the middle ear.

Question 24

Why are children more prone to middle ear infections?

Answer 24

Because children have straighter and shorter eustachian tubes, allowing for easier passage of infection.

Question 25

List 5 causes of conductive deafness.

Answer 25

1 - Obstruction of the auditory canal. 2 - Otosclerosis (sclerotic degeneration of the ossicles). 3 - Rupture of the tympanic membrane. 4 - Middle ear infections. 5 - Head trauma.

Question 26

List 4 causes of congenital sensorineural deafness.

Answer 26

1 - Congenital rubella syndrome. 2 - Human cytomegalovirus (HCMV). 3 - Toxoplasmosis. 4 - Hypoplastic auditory nerves.

Question 27

List 10 causes of acquired sensorineural deafness.

Answer 27

1 - Inflammation. 2 - DIabetes mellitus. 3 - Iodine deficiency. 4 - Hypothyroidism. 5 - Tumours. 6 - Meningitis. 7 - Viral infections. 8 - Trauma. 9 - Stroke. 10 - Drugs.

Question 28

Define presbycusis.

Answer 28

Normal progressive age-related loss of hearing acuity or sensitivity.

Question 29

List 2 drugs that can cause acquired sensorineural deafness.

Answer 29

1 - Aminoglycoside antibiotics (worsened by a rare mitochondrial mutation). 2 - Methotrexate.

Question 30

List 3 perinatal causes of acquired sensorineural deafness.

Answer 30

1 - Premature birth. 2 - Foetal alcohol syndrome. 3 - Syphilis.

Question 31

What causes tinnitus?

Answer 31

Tinnitus is caused by contraction of the motor proteins of outer hair cells in the absence of sound.