Electrophysiology Flashcards by Amelia Ram

What does EEG stand for?

Electroencephalography

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What is the EEG and how does it work?

Electrophysiological method used to record the electrical activity of the brain. Non-invasive. Electrodes placed on scalp.
EEG measures voltage fluctuations resulting from ionic current within the neurons.
Clinically EEG refers to recording the brain’s spontaneous activity over a period of time

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What is an evoked potential?

The brain activity is time-locked to a stimulus. While background activity may make it difficult to determine the brain response, the experiment can be repeated to cancel out the background, highlighting the salient response (SIGNAL AVERAGING)

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What does ECoG stand for?

Electrocorticography

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How does ECoG work?

This is a type of electrophysiological monitoring that uses electrodes placed directly on the exposed surface of the brain to record electrical activity from the cerebral cortex. The spatial resolution of ECoG is much better than the EEG since the signals do not have to conduct through the skull and thus attenuate.

Using depth electrodes, the local field potentials (LFP) can be detected. These are transient electrical signals generated in nervous and other tissues by the summed and synchronous electrical activity of the individual cells (e.g. neurons) in that tissue. LFP are “extracellular” signals Spikes in nearby neurons cause local extracellular current flow, which can be detected as transient voltage change.

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What is intracellular monitoring?

Measurement of voltage differences across the cell membrane

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What Law does external stimulation of the nerve obey?

Ohm’s Law

Only when the stimulus results in a membrane potential greater than threshold in an AP generated

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Give three examples of graded potentials

Sub-threshold changes in the membrane potential due to intracellular current injection
Excitatory Post-Synaptic Potentials and Inhibitory Post-Synaptic Potentials
Receptor potentials from sensory transduction

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Define threshold

a value below which, the response is indistinguishable from background noise

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Define saturation

The maximum response

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Five differences between AP and graded potentials?

Large (~100mV) vs small (<=1mV)
Fast (1ms) vs Slow (10ms)
All or nothing vs. graded
Cannot summate vs can summate
Active process vs. passive process

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How do individuals graded potentials meet threshold in the CNS?

SUMMATION

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What are the two types of summation?

Spatial:
Different pre-synaptic neurons with synapses on different spatial LOCATION on the post-synaptic neuron can be active simultaneously, in which case the individual PSP’s can summate

Temporal:
The time course of the AP is slower than the PSP due to the CAPACITANCE of the neuron. If two pre-synaptic AP’s are fired in rapid succession, the PSP due to the 2nd AP can sum with that due to the first.

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What are the two ELECTRICAL properties of the neuronal membrane that determine the summation?

Space/ length constant (lambda)

2. Time constant (Tau)

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What is the space constant?

It is the length it takes the graded potential to degrade to 37% of its original

Increased with diameter and membrane resistance

Long lambda means that a longer distance is needed to degrade the signal –> Increases likelihood of summation

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What is the time constant?

Tau = resistance * capacitance

The length of time it takes for the signal to degrade

Decreased tau due to decreased resistance and capacitance. C is a constant. Resistance can be altered by number of ion channels

To measure coincidences, you want a short tau to safely say that new stimuli cause the AP

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What is the function of transmembrane transporters?

They (i) maintain concentration gradients over the long term and (ii) they are NOT directly involved in neuronal signalling. They allow for the conditions required for the generation of an AP

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What are four features of a channel?

They allow the selective movement of ions across the membrane.
It is a passive process.
The directions depends on the concentration and electrical potential gradient
Ion flow constitutes an election current

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Units of charge

Coulombs

Units of current

Amps

Units of voltage

Volts

Units of resistance

Ohms

Units of conductance

Siemens (1/R)

Units for capacitance

Farads

True or False: The total charge on either side of the membrane is the same

TRUE | There is a fractional imbalance on the membrane itself

What is the equilibrium potential?

The voltage at which the generated electrical force opposes the movement of ions. Illustrated with the Nernst equation Equilibrium Potential= (Gas constant * absolute temp in kelvin)/ (valence of ion * Faraday constant) ln (X out/ X in) SIMPLIFIED TO: Ex= 58/z log (X out/ X in)

TRUE OR FALSE: Ex is proportion to charge of ion

FALSE | It is proportional to log (X out/ X in)

Ions are flowing OUT of the cell. Is the Ex positive or negative? E.g. ?

NEGATIVE | Potassium

Ions are flowing INTO the cell. Is the EX positive or negative? E.g.

POSITIVE | Sodium

What is the electrochemical driving force?

This is the potential that drives the ion flow across the membrane. Driving force= Vm - Eion

TRUE OR FALSE: The ionic flow tries to change Vm closer to the Ex?

TRUE | Vm is largely affected by conductance though.Practise proof

Recall the experiment where an IPSP was applied to various Vm. Why was there a shift in polarity?

When ion channels open, and their conductance thus increases, the potential will move towards the Eion. Recall that Eion moves towards its Ex. Post-Ex, E ion moves in the opposite direction The experiment highlights the fact that for a single ion channel, the reversal potential is that voltage where the driving force is 0. For a single ion, it is the same as the ion’s equilibrium potential.

What is an IPSP?

A Eion whose Ex is lower than the threshold for AP generation

Explain hyperkalemia?

In hyperkalaemia, the extracellular potassium levels are elevated. Thus log [K+]out/[K+]in decreases. Equilibrium potential of potassium thus becomes less negative . It becomes easier for an action potential to be generated.

What two findings were established in the beginning of the 1950's with respect to AP's?

1. The action potential involved a transient reversal of Vm to a positive value 2. This was dependent on the presence of extracellular sodium Hodgkin (1951)

Where does the Vm value usually lie?

Closer to the Eion of the ion of the greater conductance

What causes the changes in membrane permeability?

Membrane permeability depends on Vm itself. A change in the membrane potential in the positive direction triggers molecular changes in the membrane that increase the conductance of sodium, through the opening of VgNa channels. This resulting increase in sodium current moves the membrane potential futher in the direction of ENa, resulting in further increases to sodium conductance.

How does one track membrane potential changes over time?

Voltage gated clamp Key equation: Q=CV and I = dQ/dt We know that the membrane current is the sum of the capacitor current and the ion flow current Calculate current through capacitor using equation 2

What equation is used to track voltage changes over time?

(I m - I ion)/ Cm = dVm/dt

What additional factor does the Hodgkin Huxley model take into account?

The ionic currents= IK, INa and leakage current

Hodgkin Huxley model calculation?

``` Q= It C= QV ``` Work out ionic current= I = g (driving force) Work our change in Vm: V= t (ionic currents)/ capacitance Calculate new Vm Repeat with new Vm The conductance of ions at a particular Vm must be known.

What two factors play a major role in the shape of membrane current/ time graphs

1. Conductance at a particular Vm | 2. Driving force (Vm-Vion)

How can one rationalise that driving force is a factor?

No current at time=0

How can one rationalise that conductance is a factor?

The graphs plateau i.e. the maximum number of channels are open

What blocks potassium channels?

Tetra-ethyl ammonium | Recall potassium is a positive current

What blocks sodium channels?

Tetradotoxin | Recall sodium is a negative current

How were Hodgkin and Huxley able to account for the voltage dependent inactivation?

Hyperpolarisation of membrane to remove all inactivation Compared gNa with and without prior hyperpolarisation With hyperpolarisation, it would be more difficult to react the threshold to cause inactivation of the sodium gates