Wk2b - AN and Electrode Hurdles Flashcards
What is a neuron?
A specialized cell that generates an action potential in response to a stimulus
What is a nerve?
A bundle of nerve fibres (axons).
How is the tonotopic array preserved in the auditory nerve?
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.
In the resting state, the hair cell activation gates are ____ (open/closed) and a resting potential of ____ mV is maintained.
Closed; -70 mV
An action potential is generated at ____ mV in a hair cell
50 mV
The phase during which Na+ ions flood through the channels is called _______
Depolarization
What are the resting potentials of each compartment of the cochlea?
Scala tympani: 0 mV
Scala media: +80 mV
Scala vestibuli: +2-5 mV
Which structure is responsible for maintaining the positive potential of scala media in comparison to the other 2 compartments?
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)
What is the name of the neurotransmitter released into the synaptic space b/w the hair cell and dendrite?
Glutamate
What is spontaneous firing? How frequently is it found in auditory fibres?
The firing of an axon in the absence of an acoustic stimulus; auditory fibres may fire spontaneously 0-100 times per second
Higher velocities are generated by ____ (quieter/louder) sounds and they will generate _______ (Lower/higher) spike rates
louder; higher
True or False: Spike rate is not proportional to BM velocity under 200 Hz
False
Define refractory period
The period (about 1 ms) after a stimulus during which a single neuron cannot fire again
When strongly stimulated, most auditory neurons fire at rates up to ______ spikes per second
500
What is the neural threshold?
The min stimulus level causing a significant increase in discharge rate; spike rate will increase with level up until “saturation”
What does the spike rate encode?
The level of sound
What are the two ways amplitude is encoded?
Rate Level
Spread of Excitation
How does characteristic frequency relate to stimulus level?
The CF for a neuron is the freq at which the lowest stimulus level is required to achieve neural threshold
Lower spontaneous rate neurons have ____ (Lower/higher) thresholds
Higher
Amplitude can be more accurately encoded by combining different fibers with different thresholds. This increases the overall ________ _______
Dynamic range
How does the spread of excitation help encode amplitude?
More parts of the BM (more IHCs) will be moving/stimulated with louder sounds -> more stimulation of the AN
What are the two types of frequency coding?
- Place-coding: by knowing which fibers are firing, the auditory system knows which part of the BM was activated
- 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)
If a stimulus is <500 Hz, the neuron may fire once in each stimulus cycle. What is this called?
Entrainment
What is half-wave rectification?
When only one peak of the stimulus waveform causes depolarization and neuron firing (only depolarization will cause stimulus firing, not hyperpolarization)
What is Volley Theory?
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
Summary: What are the two ways to code:
Frequency
Level
Frequency: Place coding and temporal coding
Level: Firing rate and spread of excitation
Where does CI electrical stimulation need to reach?
Peripheral axons of auditory neurons and spiral ganglion
Name one problem with the location of the CI electrode?
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.
What is CI channel interaction?
The fact that electrical fields from individual electrodes overlap
What is another problem regarding the axons and hair cells of a deafened inner ear?
After the hair cells are lost (prolonged deafness), the axons begin to degenerate., and neurons are lost in the spiral ganglion
What are the 3 physiological obstacles to electrical hearing?
Anatomy - excitable structures are distanced and behind bone
Neural degeneration - spiral ganglion and peripheral processes
Central auditory system changes - cortical development and plasticity
What is a consideration regarding whether to have one CI or two?
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
What physiological and perceptual performance differences do we see between early and late implatation?
- 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
What happens if there is no input to the auditory cortex during the critical window?
Other areas of the cortex ‘take over’ and you won’t reach normal performance
What happens to the P1 latency of late implanted CIs compared to early implants?
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
When does CI stimulation provide the most benefit?
When provided prior/during sensitive periods in early development (as measured by PBK Word score)
True or False: CIs typically provide very good speech perception in quiet for pre-lingually implanted children and post-lingually deafened adults
True. Speech perception in noise and other challenging situations remain a challenge
