Lecture 48: Hearing 2

Question 1

Describe the first step in the sound transmission sequence:

Answer 1

Sound enters auditory canal causing vibration of tympanic membrane

Question 2

Describe the second step in the sound transmission sequence:

Answer 2

Ossicles transfer and amplify vibrations to oval window to create compression wave in perilymph of scala vestibuli

Question 3

Describe the third step in the sound transmission sequence:

Answer 3

Pressure difference between scala vestibuli and scala tympani: since liquids are difficult to compress, vertical pressure wave transmits through Reissner’s membrane and endolymph (Organ of Corti)

Question 4

Describe the fourth step in the sound transmission sequence:

Answer 4

Downward bending of basilar membrane (repeatedly oscillates)
* Occurs in a tonographic manner
* Causes displacement of organ of Corti
* Pressure wave travels towards round window from point of basilar membrane deflection

Question 5

What effect do the oscillations of the basilar membrane have on the Organ of Corti?

Answer 5

Bowing of the organ of Corti at the point of oscillation

Question 6

What are the structures within the Organ of Corti?

Answer 6

• Hair cells - stereocilia extending from the apical surface and contacts tectorial membrane
• Tectorial membrane
• Surrounding fluid: endolymph - high in K+

Question 7

Describe the process of sound transduction:

Answer 7

Hair cells (-60mV) caused to move by the oscillations of basilar membrane
* Contain mechanically gated ion channels at the tip, which connect to neighbouring stereocilia (tip links)
* Movement of one stereocilia will pull on the other
* Causes opening of the ion channel

Question 8

What cause hair cell depolarisation?

Answer 8

Mechanical deformation towards kinocilium
* Endolymph (+80mv) has high K+
* Opening of kinocilium K+ channels leads to K+ influx in hair cells, causing depolarisation
* Leads to opening of voltage gated Ca2+ channels for vesicle fusion and release

Question 9

What happens after hair cell depolarisation?

Answer 9

• Release of neurotransmitter onto afferent axon
• Depolarisation of afferent axon
• Action potential initiation and propagation towards the brain

Question 10

Describe the first step in the auditory neural pathway:

Answer 10

Hair cells transduce sound into AP

Question 11

Describe the second step in the auditory neural pathway:

Answer 11

AP initiated in afferent fibres of Type 1 spiral ganglion neurons, which form the cochlear nerve (part of cranial nerve VIII)

Question 12

Describe the third step in the auditory neural pathway:

Answer 12

Axons enter brainstem and synapse onto neurons in dorsal and ventral cochlear nuclei

Question 13

Describe the fourth step in the auditory neural pathway:

Answer 13

Cochlear nuclei neurons project both contralaterally and ipsilaterally to the primary auditory complex via multiple synapses

Question 14

Describe the auditory neural pathway of sound from the cochlear nuclei

Answer 14

Cochlear nuclei → Superior olivary complex → inferior colliculus → Medial geniculate nucleus of thalamus → Primary auditory complex

Question 15

What is the role of the superior olive?

Answer 15

• Analyse input from both ears at same time
• Compares: important for spatial localisation of sound source (horizontal plane)

Question 16

What is the role of the inferior colliculus?

Answer 16

• Analyse input from one ear at a time
• Important for detecting frequency and intensity - signal analysis
• Coordinates reflexes - startle
• Sound source identification - vertical plane

Question 17

What is the role of the primary auditory cortex?

Answer 17

• Receives input from medial geniculate body of thalamus
• Basic sound processing: Frequency (tonotopic map), intensity, temporal patterns

Question 18

What is the role of the secondary auditory cortex?

Answer 18

• Receives processed input from primary auditory cortex
• Complex sound patterns, speech components, auditory recognition

Question 19

In terms of sound, what is frequency?

Answer 19

Hair cell activity on basilar membrane

Question 20

In terms of sound, what is intensity?

Answer 20

Number of APs in auditory nerve fibres

Question 21

In terms of sound, what is duration?

Answer 21

Duration of afferent discharge

Question 22

In terms of sound, what is direction?

Answer 22

Spectral notches, interaural time difference, interaural intensity difference

Question 23

What is deafness?

Answer 23

Increased threshold of hearing

Question 24

What is conductive hearing loss?

Answer 24

• Impaired sound transmission through outer or middle ear
• Ear wax, ear infection, ossicle damage
• Reduced tympanic membrane vibration

Question 25

What is surfer's ear?

Answer 25

Irritation of ear canal by cold water leads to bone growths to shield eardrum - Increased risk of water trapping and infections

Question 26

What is sensorineural hearing loss?

Answer 26

* Damage to cochlea or auditory nerve - Ageing, extreme loud noise exposure - Most common form of permanent deafness - Reduced sound signal transduction

Question 27

What causes hair cell damage?

Answer 27

Excessive amplitude of the pressure wave in cochlea damages stereocilia

Question 28

Which hair cells are most susceptible to damage?

Answer 28

Hair cells at the base of cochlea (oval window)

Question 29

What happens to damaged hair cells?

Answer 29

Leaky membrane leads to high frequency ringing in absence of the sound stimulus - Acute noise induced tinnitus

Question 30

What is the death of hair cells?

Answer 30

Permanent loss of hearing at the frequency range - Phantom sound: chronic tinnitus