Hearing

Question 1

What is the function of the ossicles?

Answer 1

To match the impedence of the air to impedence of the fluid within the inner ear

Question 2

How is the reflection of sound which normal occurs when passing from air to fluid overcome by the structure of the ear?

Answer 2

Tympanic membrane has a pressure 200x greater than the inner ear, and is significantly larger than the oval window (20:1)
The lever action of the ossicles results in greater pressure on the stapes (1.3:1)

Question 3

What are the 3 chambers in the cochlear and the intervening membranes?

Answer 3

Scala vestibuli
Tectorial membrane
Scala media
Basilar membrane
Scala tympani

Question 4

What is the role of the pinna of the ear?

Answer 4

For localisation of sound in the vertical domain

Question 5

Describe the structure of the basilar membrane

Answer 5

Wider at the apex than the base

Stiffer base than apex

Question 6

What are the relative roles of the apex and base of the basilar membrane in hearing?

Answer 6

Apex responds to low frequencies

Base responds to high frequencies

Question 7

What are the auditory receptors and where are they located?

Answer 7

Hair cells

Sandwiched between basilar membrane and reticular lamina

Question 8

What are the 2 types of hair cells? What are their roles?

Answer 8

Inner (95%): project to brainstem nuclei
Outer (5%): efferent inputs from the superior olivary complex (for amplification of movements of the basilar membrane and therefore sound, particularly quiet sound, so that a smaller range of hair displacement is required for a large range of sound pressures)

Question 9

How is sound transduced into a neural signal by the hair cells?

Answer 9

Sound-induced vibration of the basilar membrane causes the tectorial membrane to exert a shearing force on the stereocilia of the hair cells
Deflection of the hair bundle towards the tallest stereocilium (the kinocilium) causes opening of mechanically gated K+ channels leading to depolarisation of the cell (opposite occurs for movement away from the kinocilium)
Depolarisation results in the opening of voltage-gated calcium channels and release of glutamate

Question 10

Does the hair cell experience larger depolarisations or hyperpolarisations?

Answer 10

Depolarisations

Question 11

Describe the cycling of K+ in the cochlear

Answer 11

Endolymph in the scala media is high in K+ to enable depolarisation of the hair cells following K+ channel opening
Once it has entered the cells, the K+ empties into the perilymph in the scala tympani via basolateral K+ channels
It is then recycled via the stria vascularis into the endolymph (so that the perilymph typically does not contain K+)

Question 12

What stimulates contraction of the outer hair cells?

Answer 12

Depolarisation of the inner hair cells (hyperpolarisation causes elongation)

Question 13

How do antibiotics cause hearing loss?

Answer 13

Via loss of outer hair cells

Question 14

How much do outer hair cells amplify basilar membrane movement by?

Answer 14

100x

Question 15

Describe the nuclei and pathways involved in the transmission of auditory informtion

Answer 15

Ear: sound vibration causes activation of hair cells and subsequent transmission of a neural signal via CNVIII and the spiral ganglion
Brainstem: several nuclei involved including cochlear nucleus (medulla) and superior olive, information carried by lateral lemniscus
Midbrain: inferior colliculus
Thalamus: medial geniculate nucleus
Temporal lobe: auditory cortex

Question 16

What are the main components of the superior olivary complex and what are their roles?

Answer 16

Lateral superior olive: localisation of sound by detecting intensity differences
Medial superior olive: localisation of sound by measuring time delay
Trapezoid body: medial nucleus also involved in intensity differences

Question 17

Which is more important for the localisation of low frequency sounds: time delay or interaural intensity difference?

Answer 17

Time delay

Question 18

Which is more important for the localisation of high frequency sounds: time delay or interaural intensity difference?

Answer 18

Interaural intensity difference

Question 19

What is columnar organisation in the auditory cortex?

Answer 19

Cells in same column are tuned to the same frequency

Question 20

Describe the pattern of input to the auditory cortex

Answer 20

Alternating regions of input from both ears (excitatory from one ear and inhibitor from the other)

Question 21

In what hemisphere are speech, environmental and music sounds processed?

Answer 21

Speech: left
Environment: both
Music: right

Question 22

What is sudden sensorineural hearing loss?

Answer 22

Sudden loss of hearing of at least 30db within 3 hours to 3 days

Question 23

List the 3 classifications of causes of sensorineural hearing loss, giving examples of each

Answer 23

Peripheral: meningitis, Guillain-Barre, acoustic neuroma, metastasis
Central: MS
Cochlear: infection (e.g. HSV), AI disease, traumatic, metabolic, vascular, ototoxicity