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AUD fall 2019 electrophys I > Week 6: neuroscience and AEP > Flashcards

Week 6: neuroscience and AEP Flashcards

1
Q

definition of neuron

A

cells with plasma membranes, nucleus, cytoplasm, and intracellular organelles

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2
Q

three parts of a neuron

A
  • soma (cell body)
  • axon
  • dendrites
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3
Q

how are neurons classified

A

number of process

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4
Q

unipolar neurons

A

mainly in insects

  • unipolar bush cells-cerebellar cortex and cochlear nucleus
  • have one axon*
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5
Q

pseudounipolar

A

sensory neurons of CN V, VII, IX, X

*one axon with two branches

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6
Q

bipolar

A

specialized sensory neurons of CN I, II, VIII

*2 axons off 2 soma

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7
Q

multipolar

A

most neurons in the central nervous system

*one axon and several branches for dendrites

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8
Q

dendrites

A
  • tree like structures that receive info and deliver it to the soma
  • increases the surface area of the cell for synapses
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9
Q

axon

A
  • thinner than dendrites and some motor axons can be quite long
  • -takes info from the soma and delivers to other neurons
  • –longest axon in body=sciatic nerve
  • three regions:
    1) axon hillock
    2) axon proper
    3) synaptic bouton
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10
Q

axon hillock

A

transition between soma and axon

  • –rich with voltage-gated Na+ channels
  • –begins the signal (starts the transmission of signals)
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11
Q

axon proper

A
  • main extent of axon
  • fatty material called myelin surrounds many
  • –cell membrane layers upon each other
  • –myelin of CNS is formed by oligodendrocytes
  • –myelin of PNS is formed by schwann cells
  • **not all axons are entirely myelinated such as type II afferents of the VIII nerve
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12
Q

what are the three functions of the myelin sheath

A
  • help protect the axon
  • insulates the axon: prevents charges from leaking out of the nerves
  • facilitate the transmission of impulses along the nerve cells, fire quicker and fluently
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13
Q

nodes of ranvier

A
  • rich with voltage gated Na+ channels

* each node generates a new action potential which allows the signal to maintain its power to keep traveling

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14
Q

synaptic bouton

A
  • neurons communicate with one another through synapses
  • pre-synaptic-secretory vesicles which contain neurotransmitter and are released by exocytosis
  • –signal releases neurotransmitter which activated channels which allows sodium to take info to dendrite of next cell
  • synapses can occur at any region of the neuron
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15
Q

what are the three different types of synapses of neurons

A
  • axodendritic (bouton to dendrite of next soma)=excitatory
  • axosomatic (bouton to soma itself)=inhibitory which means it suppresses the function of that neuron
  • axoaxonic (axon to axon)= modulatory meaning changes function of the axon
  • –these different synapses aid in localization of sound
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16
Q

how is information transmitted by neurons? simple

A
  • neurons undergo rapid changed in electrical potential across the cell membrane
  • electrical charge is generated and maintained by the passage of ions (Na+, K+, Ca+2, and CL-)
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17
Q

modality gated channels

A

specific to sensory neurons open in response to mechanical forces and generate a receptor potential
—example is MET channels on stereocillia

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18
Q

ligand gated channels

A

open in response to a neurotransmitter (neuromodulators)

—neurotransmiter comes in and attaches to gate which causes it to open

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19
Q

voltage gated channel

A

open in response to changes in electrical potential across the membrane

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20
Q

resting membrane potential of neurons

A
  • RMP= -70mv
  • cell membrane acts as a capacitor that allows the separation and storage of electrical charge (hair cells and neurons have similarity in ability to hold charge
  • –creating an unequal distribution of electrical charge
  • —-two forces acting on each type of ions determine the distribution of it
    1) concentration gradient (higher concentration=lower concentration
    2) electrical gradient (opposites attract)
  • Na+ concentration and Cl- concentration are higher on the outside of the cell
  • K+ and organic ions are high on the inside of the cell
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21
Q

how is resting membrane potential of neurons maintained

A
  • negatively charged ions trapped inside the neuron (X-) gives the neuron a negative charge
  • active transport of Na+ out and K+ in to the cell helps to try and get the cell to go back to resting potential
  • baseline diffusion of K+ (and Cl-) through non-gated channels
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22
Q

protein pump in neurons

A
  • Na-K pump is electrogenic (3 Na+ out and 2 K+ in) protein
  • – this maintains electrical potential/ helps regain resting potential
  • —-kicks sodium outside of the neuron
  • end result is a separation of charges as long as there is energy (ATP; only happens of the cell is alive)
  • –ATP is the driver of cells
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23
Q

changes from resting membrane potential (excitation, inhibition,and modulation)

A
  • the neuron is depolarized when the membrane potential becomes less negative than the RMP (excitation)
  • the cell is hyperpolarized when the membrane potential becomes more negative than the RMP (inhibitory)
  • gradual and longer-lasting changes in membrane potential are referred to as modulation and are usually small changes in the membrane electrical potential
  • –synapses adjust the RMP to make it easier or harder for the cell to fire
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24
Q

local potential

A

initial change in membrane potential

  • –when allow + ions it, it will start changing the cell potential, if it doesnt hit the threshold to produce a signal, the cell will continue to fill
  • –when present threshold or above, there is a successful action potential and all gates open
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25
Q

action potentials

A
  • large changes in electrical potential consisting of a brief but large depolarization that can be repeatedly regenerated along the length of the axon
  • action potentials spread long distanced to transmit down the axon to pre-synaptic bouton to release a neurotransmitter
26
Q

local receptor potential

A

such as hair cells in the auditory system

  • is purely localized to the receptive surface of the sensory neuron as a result of voltage changes in the sensory receptor
  • –hair cells depolarization
27
Q

local synaptic potentials

A

generated in motor neurons (crucial to get info into the soma) and interneurons when stimulated by other neurons–a neurotransmitter is released–this chemical interacts with postsynaptic membrane–opening ligand-gated channels this changing the resting membrane potential in the post synaptic cell

28
Q

synaptic potentials basics

A
  • the greater the amount of neurotransmitter and the longer the time over which it is available, the larger the synaptic potential amplitude
  • the amplitude (strength) of potentials decreases with the distance traveled
  • to maintain action potential, it needs to be generated and regenerated
  • Ca+ takes the neurotransmitter and pushes them out through synaptic cleft, here the transmitter goes to the next neuron, attaches to ligand-gated channels and opens them allowing Na+ in which begins activation of the neuron (depolarization)
29
Q

action potential basics

A
  • essential for rapid movement of info over long distance (gets the potential to the end point)
  • action potentials are all or none
  • generation of an action potential involves the sudden influx of Na+ through voltage-gated Na+ channels
30
Q

threshold of action potenitals

A
  • the lowest stimulus intensity that produces an action potential
  • –usually 10-15mv change toward the positive is sufficient
  • –opening of voltage-gated Na+ channels allows Na+ to rush into the nerve cell
  • –Na+ channels close and voltage-gated K+ channels open with K+ leaving the cell (repolarization because cell has large + charge)
  • –an overshoot of K+ exodus causes the membrane to become more polarized then at rest-hyperpolarized
  • –back to RMP due to diffusion of ions and Na-K pump
31
Q

what are the two different types of refractory periods

A
  • absolute= wont fire no matter how strong the stimulus

* relative=will only fire to strong stimulus (stronger than the stimulus before)

32
Q

propagation of the action potential

A
  • changes in electrical potential passively spreads along the axon to adjacent regions of the membrane
  • –A is generated at every node of ranvier (Na+ rushes in and signal goes through)
  • when the potential of the adjacent area reaches thresholds, another AP is generated (again at nodes of ranvier)
  • –process is repeated the entire length of the axon
  • benefit of this is that the signal will not be allowed to leak back because area behind where it is firing is tin the refractory period
33
Q

volume conduction; collection of neurons (in other words why is it helpful to measure from many neurons at once instead of trying for just one)

A
  • important for the distance traveled through fluid, brain tissue, bone, and skin
  • is possible because many neurons firing at the same time will add up and make the response large enough to measure
  • temporal synchronization= many neurons firing together which adds up; loss of some energy occurs as signal travels through tissue, but still big enough to use
  • –in the auditory system, different conditions will affect the size and latency of the AP
34
Q

geometric orientation of the neurons

A
  • open field (auditory nerve) neatly organized laying next to each other
  • closed field (cortex) all neurons extended in all different directions
  • –open field shows nice big AP, closed field AP cancels each other out because they are all running in different directions
35
Q

recording location of AEPs

A
  • far field= trying to record from further from the source of the response
  • near field= trying to measure from where the response is generated
  • –near field gives a better spatial resolution (more concise response)
  • –near field also shows larger amplitude
36
Q

neuron firing rate of VIII nerve

A
  • one bit coding system so either on or off (AP or none)
  • absolute refractory period of neurons limits its “sampling rate”
  • spontaneous firing (types of neurons are divided by their amounts of spontaneous firing)
  • –there is always spontaneous firing happening, but the brain ignores it until the neurons is firing faster than the spontaneous rate
37
Q

what are the classifications of spontaneous firing rates

A
  • classified by rates up to 100/second
  • –high rate= more than 18 spikes/second (60-75%) lateral side of the hair cell
  • –medium firing rate= 0.5-18 spikes/second (15-30%) medial side of hair cell
  • –low firing rate= under 0.5 spikes/second (10-15%) medial side of hair cell
  • —-a lot harder to make low spont rate neurons fire than high spont rate neurons
38
Q

what is the importance of firing rate in terms of saturation

A
  • high and medium staurate at 20-30 dB SL (above threshold)
  • –stimulated with pretty soft sounds, but then plateau in firing rate about 20-30 dB
  • low spontaneous firing rate saturate at 60 dB SPL
  • –need more intensity to fire
  • –still pretty sharply tuned
39
Q

what is the frequency distribution of the VIII nerve

A

high in the periphery and low in the center

40
Q

how does the intensity affect firing of the VIII nerve

A

higher intensity means a larger # of fibers are recruited and the faster firing rate
*this helps the brain interpret the intensity

41
Q

phase locking

A
  • does not mean responding to each cycle but when responding, the neuron fires at or near signal negative peaks
  • at lower frequencies (1000 Hz or lower) can fire to every cycle
  • between 1000-4000 Hz not every single cycle (stochastic firing)
  • not as effective above 4000 Hz
42
Q

definition of AEP

A
  • auditory evoked potential (or responses) represent activity within the auditory system that is stimulated or evoked by sounds
  • –sounds range from very brief clocks or tones to longer duration and more complex sounds such as speech sounds
43
Q

what are the two different ways AEPs can be described

A
  • region of the auditory system where they are generated (ABR, ECochG)
  • temporal relation to other responses (early, late)
44
Q

relationship between AEPs and stimulus levels

A
  • larger responses with larger stimulus levels

* can be evoked with very loud to very soft sounds

45
Q

latency of AEPs

A
  • the period of time between the presentation of the stimulus and the appearance of the response (in msec)
  • –there is an inverse relationship between the stimulus intensity and response latency (latency decreases ans stimulus intensity increases)
46
Q

recording location of AEPs

A
  • can be recorded from the inner ear to the auditory cortex

- –in aud, ABR is the most common electrophys procedure

47
Q

what time frame to most AEPs fall into

A

after the presentation of sound, most AEPs occur in a continuous rapid sequence over a time period of about 300-400 msec

48
Q

importance of AEPs

A
  • contribute to the early detection and accurate diagnosis of auditory dysfunction
  • are feasible in pts who cant have behavioral eval
  • –babies and kids
  • –developmental delay
  • –sick
  • –malingerers
  • –intraoperative
  • revolutionized detection and diagnosis of HL in infants and young kids
  • even when behavioral testing is possible, gives more sensitivity to some problems like probs with auditory nerve and central auditory nervous system
  • provide more accurate info on site of lesion in the auditory system
49
Q

factors affecting measurement of AEPs

A
  • age (longer latency in babies which shortens and smaller amp as older)
  • gender (shorter latency in females because dimensions are smaller than males)
  • body temperature (higher temp means faster response and vice versa)
  • state of arousal (awake or asleep?)
  • muscular artifact
  • effect of drugs
  • –one must consider relationship between AEPs and pathology f the peripheral auditory system
50
Q

measuring AEPs

A
  • present acoustic stim using a transducer (earphone or bone oscillator)
  • response recorded with electrodes that make contact with the skin. wire plugs into preamplifier connected to evoked response system
  • electrodes are places at specific place on scalp depending on the test of interest
  • stimulus evoked neural activity goes through body tissue, fluid, bone, and skin to electrode
  • latency= post-stimulus time of peaks in the waveform (msec)
  • responses with shirt latencies are generated in the periphery
  • the longer the latency, the deeper in the brain the response is
51
Q

classification of AEPs based on generation

A
  • exogenous: based on the physical characteristics of the stimulus
  • endogenous: based on the events happening during recording–significance of the stimulus (P300 and mismatch negativity response)
  • –normally more complex signals for endogenous such as speech sounds
52
Q

electrocochleography (ECochG)

A

response components

  • within the 1st 5 msec
  • inner ear and the auditory nerve (distal part)
  • –AP= distal AN and is basically the same as wave I on the ABR
53
Q

ABR

A

response components

  • within the 1st 10 msec
  • wave 5 is from inferior colliculus
  • auditory pathways
  • clinical application= issues in pathway
54
Q

ASSR

A

auditory steady state response

  • –80 Hz ASSR= brainstem (not affected by state of arousal)
  • –40 Hz ASSR= cortex.thalamus (affected by arousal)
  • modulating signal and seeing if brain can identify modulations going on (AM and/or FM) meaning amplitude or frequency modulation
  • clinical application= estimate thresholds
55
Q

AMLR

A

auditory middle latency response

  • response components
  • –within first 50 msec
  • –from thalamus/cortex
  • –affected if pt falls asleep (response diminishes)
56
Q

LLR

A

late latency response

  • response components
  • –50-200 msec
  • –p= positive, n= negative, and # reflects response in msec
  • –from auditory cortex
  • –arousal state affects= asleep makes reduced response
57
Q

P300

A
  • endogenous response
  • brain tries to pick up change in stimulus delivered
  • odd ball paradigm= lots of similar stimuli but a few (called rare or odd) are different
  • from frontal/temporal lobes
  • to be able to see response/, pt must pay attention and identify different response
  • –attention is a must
58
Q

MMN

A

mismatch negativity

  • endogenous response
  • –very similar to P300, but pt is not asked to pay attention
  • –from frontal/temporal lobes
59
Q

why are higher level AEP responses harder to record

A

orientation or neurons; in nerve and pathway the cells are aligned and fire together all in phase, at cortical level is is closed field, so while there is a higher number of neurons firing, they fire in a complex pattern and cancel out the electric field generated
—getting further from the source of generation so not able to clearly pinpoint the response generation point

60
Q

signal averaging with AEPs

A
  • measured in microvolts–voltage measured between two electrodes
  • –response is amplified and noise is filtered out
  • the response is embedded within other brain activity and outside electrical noises
  • because the responses have very small voltage, two processes are essential for detecting a response
  • –amplifying the response
  • –averaging of the response
  • signal averaging is recognizing the pattern of auditory brain activity elicited by each stimulus
  • bkgd electrical activity is being added up and cancelled out
  • the remaining waveform is the AEP

AUD fall 2019 electrophys I (11 decks)

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