Week 6: neuroscience and AEP Flashcards
definition of neuron
cells with plasma membranes, nucleus, cytoplasm, and intracellular organelles
three parts of a neuron
- soma (cell body)
- axon
- dendrites
how are neurons classified
number of process
unipolar neurons
mainly in insects
- unipolar bush cells-cerebellar cortex and cochlear nucleus
- have one axon*
pseudounipolar
sensory neurons of CN V, VII, IX, X
*one axon with two branches
bipolar
specialized sensory neurons of CN I, II, VIII
*2 axons off 2 soma
multipolar
most neurons in the central nervous system
*one axon and several branches for dendrites
dendrites
- tree like structures that receive info and deliver it to the soma
- increases the surface area of the cell for synapses
axon
- 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
axon hillock
transition between soma and axon
- –rich with voltage-gated Na+ channels
- –begins the signal (starts the transmission of signals)
axon proper
- 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
what are the three functions of the myelin sheath
- 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
nodes of ranvier
- rich with voltage gated Na+ channels
* each node generates a new action potential which allows the signal to maintain its power to keep traveling
synaptic bouton
- 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
what are the three different types of synapses of neurons
- 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
how is information transmitted by neurons? simple
- 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-)
modality gated channels
specific to sensory neurons open in response to mechanical forces and generate a receptor potential
—example is MET channels on stereocillia
ligand gated channels
open in response to a neurotransmitter (neuromodulators)
—neurotransmiter comes in and attaches to gate which causes it to open
voltage gated channel
open in response to changes in electrical potential across the membrane
resting membrane potential of neurons
- 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
how is resting membrane potential of neurons maintained
- 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
protein pump in neurons
- 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
changes from resting membrane potential (excitation, inhibition,and modulation)
- 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
local potential
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
action potentials
- 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
local receptor potential
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
local synaptic potentials
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
synaptic potentials basics
- 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)
action potential basics
- 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
threshold of action potenitals
- 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
what are the two different types of refractory periods
- absolute= wont fire no matter how strong the stimulus
* relative=will only fire to strong stimulus (stronger than the stimulus before)
propagation of the action potential
- 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
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)
- 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
geometric orientation of the neurons
- 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
recording location of AEPs
- 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
neuron firing rate of VIII nerve
- 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
what are the classifications of spontaneous firing rates
- 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
what is the importance of firing rate in terms of saturation
- 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
what is the frequency distribution of the VIII nerve
high in the periphery and low in the center
how does the intensity affect firing of the VIII nerve
higher intensity means a larger # of fibers are recruited and the faster firing rate
*this helps the brain interpret the intensity
phase locking
- 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
definition of AEP
- 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
what are the two different ways AEPs can be described
- region of the auditory system where they are generated (ABR, ECochG)
- temporal relation to other responses (early, late)
relationship between AEPs and stimulus levels
- larger responses with larger stimulus levels
* can be evoked with very loud to very soft sounds
latency of AEPs
- 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)
recording location of AEPs
- can be recorded from the inner ear to the auditory cortex
- –in aud, ABR is the most common electrophys procedure
what time frame to most AEPs fall into
after the presentation of sound, most AEPs occur in a continuous rapid sequence over a time period of about 300-400 msec
importance of AEPs
- 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
factors affecting measurement of AEPs
- 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
measuring AEPs
- 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
classification of AEPs based on generation
- 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
electrocochleography (ECochG)
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
ABR
response components
- within the 1st 10 msec
- wave 5 is from inferior colliculus
- auditory pathways
- clinical application= issues in pathway
ASSR
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
AMLR
auditory middle latency response
- response components
- –within first 50 msec
- –from thalamus/cortex
- –affected if pt falls asleep (response diminishes)
LLR
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
P300
- 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
MMN
mismatch negativity
- endogenous response
- –very similar to P300, but pt is not asked to pay attention
- –from frontal/temporal lobes
why are higher level AEP responses harder to record
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
signal averaging with AEPs
- 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
