L08 - Hearing

Question 1

What are the ossicles and what is their role?

Answer 1

• Malleus, incus & stapes - found in the middle ear

- They amplify the vibration produced by the tympanic membrane

Question 2

Which muscles are involved in the movements of the ossicles and what is their role?

Answer 2

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 by reducing the movement of the ossicles

Question 3

List the 3 canals within the cochlea.

Which fluids fill these canals?

Answer 3

1 - Scala vestibuli

• Filled with perilymph

2 - Scala media

• Filled with endolymph

3 - Scala tympani

• Filled with perilymph

Question 4

What is the basilar membrane and what is its function?

Answer 4

• 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 5

How does the shape and flexibility of the basilar membrane change throughout the length of the cochlea and what is the relevance of this?

Answer 5

• 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 6

What is the tectorial membrane?

What is its function?

Answer 6

• 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 7

Describe the mechanism that causes hair cell depolarisation.

Answer 7

• 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 8

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

Answer 8

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

Question 9

What is the primary function of outer hair cells?

What is their secondary function?

Answer 9

• 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 10

What is tonotopy?

Answer 10

• 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 11

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

Answer 11

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

Question 12

What is phase locking?

Answer 12

• For low frequency sounds, each sound wave results in action potential – this is known as phase locking
• At high frequencies, the action potentials are not phase locked to the sound waves because the sensory neurones cannot be constantly stimulated to generate an action potential at such a high frequency due to the refractory period

Question 13

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

Answer 13

• Very low: phase locking
• Intermediate: phase locking & tonotopy
• High: tonotopy

Question 14

Which mechanisms translate sound intensity?

Answer 14

• The number of cells firing and the firing rate

- Determined by the movement of the basilar membrane

Question 15

Which phenomena contribute to sound localisation?

Answer 15

1 - Interaural delay

2 - Sound shadowing

Question 16

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

Answer 16

Integration of auditory & visual signals

Question 17

What are the functions of the eustachian tube?

Answer 17

• Equalisation of pressure both sides of the tympanic membrane
• Allows secretions that accumulate in tympanic cavity to empty into nasopharynx

Question 18

Why are children more prone to middle ear infections?

Answer 18

• Shorter, more horizontal Eustachian tubes - easier entry of bacteria/viruses to middle ear
• Narrower - more likely to become blocked

Question 19

What is presbycusis?

Answer 19

• Normal progressive age-related loss of hearing acuity or sensitivity
• Not preventable or reversible

Question 20

Which drugs can cause acquired sensorineural deafness?

Answer 20

• Aminoglycosides

- Methotrexate

Question 21

What causes tinnitus?

Answer 21

Amplification by outer hair cells -> motor proteins contract when there is no sound and move the tectorial membrane