Wk2b - AN and Electrode Hurdles

Question 1

What is a neuron?

Answer 1

A specialized cell that generates an action potential in response to a stimulus

Question 2

What is a nerve?

Answer 2

A bundle of nerve fibres (axons).

Question 3

How is the tonotopic array preserved in the auditory nerve?

Answer 3

Nerve fibres from the apex carrying low frequency information are found in the center of the nerve. High frequency fibres are found on the outside.

Question 4

In the resting state, the hair cell activation gates are ____ (open/closed) and a resting potential of ____ mV is maintained.

Answer 4

Closed; -70 mV

Question 5

An action potential is generated at ____ mV in a hair cell

Answer 5

50 mV

Question 6

The phase during which Na+ ions flood through the channels is called _______

Answer 6

Depolarization

Question 7

What are the resting potentials of each compartment of the cochlea?

Answer 7

Scala tympani: 0 mV
Scala media: +80 mV
Scala vestibuli: +2-5 mV

Question 8

Which structure is responsible for maintaining the positive potential of scala media in comparison to the other 2 compartments?

Answer 8

The stria vascularis; this leads to a difference of 150 mV across the tops of the hair cells (the highest voltage difference in the human body)

Question 9

What is the name of the neurotransmitter released into the synaptic space b/w the hair cell and dendrite?

Answer 9

Glutamate

Question 10

What is spontaneous firing? How frequently is it found in auditory fibres?

Answer 10

The firing of an axon in the absence of an acoustic stimulus; auditory fibres may fire spontaneously 0-100 times per second

Question 11

Higher velocities are generated by ____ (quieter/louder) sounds and they will generate _______ (Lower/higher) spike rates

Answer 11

louder; higher

Question 12

True or False: Spike rate is not proportional to BM velocity under 200 Hz

Answer 12

False

Question 13

Define refractory period

Answer 13

The period (about 1 ms) after a stimulus during which a single neuron cannot fire again

Question 14

When strongly stimulated, most auditory neurons fire at rates up to ______ spikes per second

Answer 14

500

Question 15

What is the neural threshold?

Answer 15

The min stimulus level causing a significant increase in discharge rate; spike rate will increase with level up until “saturation”

Question 16

What does the spike rate encode?

Answer 16

The level of sound

Question 17

What are the two ways amplitude is encoded?

Answer 17

Rate Level

Spread of Excitation

Question 18

How does characteristic frequency relate to stimulus level?

Answer 18

The CF for a neuron is the freq at which the lowest stimulus level is required to achieve neural threshold

Question 19

Lower spontaneous rate neurons have ____ (Lower/higher) thresholds

Answer 19

Higher

Question 20

Amplitude can be more accurately encoded by combining different fibers with different thresholds. This increases the overall ________ _______

Answer 20

Dynamic range

Question 21

How does the spread of excitation help encode amplitude?

Answer 21

More parts of the BM (more IHCs) will be moving/stimulated with louder sounds -> more stimulation of the AN

Question 22

What are the two types of frequency coding?

Answer 22

1. Place-coding: by knowing which fibers are firing, the auditory system knows which part of the BM was activated
2. Temporal coding: low and intermediate frequencies (<500 Hz) result in neurons firing in a phase-locked manner (cannot do at high frequencies d/t refractory period)

Question 23

If a stimulus is <500 Hz, the neuron may fire once in each stimulus cycle. What is this called?

Answer 23

Entrainment

Question 24

What is half-wave rectification?

Answer 24

When only one peak of the stimulus waveform causes depolarization and neuron firing (only depolarization will cause stimulus firing, not hyperpolarization)

Question 25

What is Volley Theory?

Answer 25

Combines temporal coding across many fibers (even if some cycles are skipped, when the response is observed across many fibers, the cycles are all filled)
- utilizes phase locking in auditory neurons up to 5 kHz

Question 26

Summary: What are the two ways to code:
Frequency
Level

Answer 26

Frequency: Place coding and temporal coding
Level: Firing rate and spread of excitation

Question 27

Where does CI electrical stimulation need to reach?

Answer 27

Peripheral axons of auditory neurons and spiral ganglion

Question 28

Name one problem with the location of the CI electrode?

Answer 28

It is surrounded by fluid, which prevents it from being able to effectively target one neuronal area, and it is separated from the neurons by bone.

Question 29

What is CI channel interaction?

Answer 29

The fact that electrical fields from individual electrodes overlap

Question 30

What is another problem regarding the axons and hair cells of a deafened inner ear?

Answer 30

After the hair cells are lost (prolonged deafness), the axons begin to degenerate., and neurons are lost in the spiral ganglion

Question 31

What are the 3 physiological obstacles to electrical hearing?

Answer 31

Anatomy - excitable structures are distanced and behind bone
Neural degeneration - spiral ganglion and peripheral processes
Central auditory system changes - cortical development and plasticity

Question 32

What is a consideration regarding whether to have one CI or two?

Answer 32

Normal hearing uses two ears best for noise management and localization.
- persons with bilateral CIs are better at sound localization than those with unilateral CIs

Question 33

What physiological and perceptual performance differences do we see between early and late implatation?

Answer 33

• cortical areas of the brain are similar for early-implant users and normal listeners
• cortical reorganization can occur in late-implanted individuals, which might make it difficult to get good results with a CI later on

Question 34

What happens if there is no input to the auditory cortex during the critical window?

Answer 34

Other areas of the cortex ‘take over’ and you won’t reach normal performance

Question 35

What happens to the P1 latency of late implanted CIs compared to early implants?

Answer 35

Early implanted CI’s will eventually enter the normal range (by 6-8 months).
For those implanted late, the P1 latency will decrease, but never to the normal range, suggesting some difficulty

Question 36

When does CI stimulation provide the most benefit?

Answer 36

When provided prior/during sensitive periods in early development (as measured by PBK Word score)

Question 37

True or False: CIs typically provide very good speech perception in quiet for pre-lingually implanted children and post-lingually deafened adults

Answer 37

True. Speech perception in noise and other challenging situations remain a challenge