Electrophysiology in vivo

Claus Mathiesen

Department of Neuroscience and Pharmacology

Aim: Teach you the basics of in vivo electrophysiology

!Claus Mathiesen, M.Sc. Ph.D.

Electrophysiology (Ephys) A key feature when studying neuro-vascular and

metabolic coupling

Claus Mathiesen October 2012

Outline of my talk

Core of electrophysiology !EEG !Field potentials !Contributions from different cell types !Spike (action potential) activity !Pro and Cons with types of ephys recording



The core of Ephys

Ephys signal is measured in voltage (V), current (I), and resistance (R) or conductance (G=1/R) !These variables are related according to Ohm’s law:

V = I ◦ R or I = V ◦ G !Ephys signal has different frequencies !Frequency is measured as oscillation per second (Hz) !Each type of neuronal activity is located within areas in the frequency band running from 0 to 5000 Hz

Department of Neuroscience and Pharmacology Dias 3


The generator of the Ephys signal

Department of Neuroscience and Pharmacology (Dias 4)

Neurons are like a battery

Negative inside (-60 to -70 mV)

Generate action potentials via

voltage-gated ion-channels

Some have pacemaker activity


Excitatory-PostSynaptic-Potential (EPSP)

Presynaptic release of transmitter Transmitter-gated ion-channels Ion-flux Potential changes


EPSP

fEPSP

EPSP

fEPSP


EPSP

fEPSP

Intracellular potential changes as Synaptic events Spike activity Graded potentials

Extracellular potential changes as Evoked field excitatory-postsynaptic potentials (fEPSPs) Single unit (cell) activity (SUA) of spikes/action potentials Multi-unit activity (MUA) of spikes Non-spiking, graded potentials (EEG)

Commonly recorded Ephys signal in the in vivo brain?

SUA

MUA

EEG


From low frequencies to higher frequencies



EEG (ElectroEncephaloGraphy)

Richard Caton 1875: The electric currents of the brain. BMJ 2. 278. !Hans Berger 1929: Über das elektreenkephalogram des menschen. Arch. Psychhiatr. Nervenkr. 87, 527-570


In the beginning EEG was used as indicator for sleep stages (Slow wave, light, REM) together with recording of muscular tone (EMG) diseases like epilepsy and brain damage


EEG (ElectroEncephaloGraphy)


Brain activity and EEG

Possibly only a small proportion of nerve cells generate synchronous spikes in normal mental state !Cerebral rhythms picked up by EEG represent synchronous synaptic activity !EEG measures only a small fraction of the total brain activity due to

Dilution (distance-2 ≈ amplitude) Variability in conductivity Mixed orientation of active dendrites Lack of synchronous activity


Irregular activity leads to high frequency and low amplitude EEG

Synchronized activity leads to low frequency and high amplitude EEG


Subdivided EEG band

Delta <4 Hz (deep sleep)Theta 4-7 Hz (REM sleep, drowsy, meditation)

Alpha 8-13 Hz (eyes closed awake)

Beta 14-30 Hz (active awake, open eyes)

Gamma 30-80 Hz (memory)


Delta rhythms (<4 Hz EEG)

Marker for slow-wave sleep also called deep sleep. In slow-wave sleep the brain recovers


Theta rhythms (4-7 Hz EEG)

In rodents the theta rhythms (4-10 Hz) originate from hippocampus and is an indicator for

paradoxical sleep (rodents REM sleep) Exploration and sniffing !In humans the theta rhythms originate from cortex

and is an indicator for Drowsiness Meditation Light sleep states



Alpha- (8-13 Hz) and Beta-rhythms (14-30 Hz)

Open eyesQuiet awake closed eyes

BetaAlpha


Gamma rhythms (30-80 Hz EEG)

Represent spike timing of a large ensemble of neurons Dependent on GABA interneurons that synchronise the spiking of pyramidal cells !Synchronous neuronal activity is a tool for dealing with information with different modalities:

Perceptual binding Attention Working memory

!Can be observed at multiple spatial scales, from single-unit recordings to MEG and scalp EEG

Gamma


Gamma activity

(Adapted from Sumiyoshi et al., 2012, Neuroimage)


Bands for different Ephys signals





Evoked field potential 0.1-1000 Hz



What generates the evoked field potential?


Synchronic activation: •Transmembrane current flow •Extracellular current flow and the resistant properties of the extracellular media àvoltage changes in the field potential

0.5 mV


Shape of evoked field potentials as function of anatomy and location


Hippocam

pus or Cerebellum

Cerebral cortex


Interpretation of an evoked field potentials


Degree of excitation =

Number of open AMPA receptor

channels

Ca2+ dependent K+ current

+NMDA rec. antagonist


From field potentials to Current Source Density


Jessen et al 2014 & Mathiesen et al. 2011

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Current source densityCSD Map


Current Source Density (CSD)


Neuronal activity à Transmembrane current generating ensemble of current sources and sinks à Extracellular current flow à Potential differences (Field potentials) due Resistance in the extracellular media

The first spatial derivative of the Field potential is equal to Current Flow Density.

The Current Flow Density is a vector indicating the amplitude and direction of current flowing through a giving point in the extracellular medium.

The second spatial derivatives of the field potential is equal to the Current Source Density (CSD).

The Current Source Density correspond to the transmembrane current

Nicholson, Freemann 1975

Field potential

Current flow density

Current Source DensitySource

Sink

Spatial derivativeSpatial derivative


Contribution to Ephys from different cell type


GliaNeurons

Principal output Relay

Cortical pyramidal

Cerebellar pyramidal

Astrocyte Oligodendrocyte MicroglialInterneurons Stellate cell

K+ buffer Blood flow House keeper Calcium waves

Myelinate Phagocyte

Major Major Middle Minor (EEG)

(Minor) (Minor) (Minor)Contribution to the field potential




Evoked field potential (synaptic strenghts) 0.1-1000 Hz

Spikes /action potentials 300-3000 Hz peak 1000 Hz • Single unit activity (1-100 spikes/s) •Multi unit activity





Adrian & Moruzzi 1939: Impulses in the pyramidal tract. J. Physiol. 97, 153-‐199


Spiking in respons to synaptic input

Cascades: Transmitter release Transmitter-gated channels (spatial and temporal summation) Voltage-gated channels

Calcium spikes Sodium spikes Potassium re-polarize cell

Calcium mediated potassium current !Or sodium spikes as a consequence of pacemaker activity




5 ms 5 ms

Single unit spike activity (Purkinje Cell)

0 1000 2000 3000 4000 50000.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

Power

Frequency

Simple spike Complex spike

Kirsten Thomsen



Herrik et al. 2010

Example of single unit activity (SUA)

Event autocorrelation

InterSpikeInterval

Spike- waveform Regular

Irregular

Burst

BurstIrregularRegular


Methods in Multi-unit activity (MUA)

Single electrode !!!Stereotrode !!!Tetrode

Low resolution

•distance

Root-Mean-Square (RMS)

!Better resolution

•distance + location

!!Even better resolution

•Distance + 2D location



Pro and Cons with types of ephys recordingDepartment of Neuroscience and Pharmacology

Method Pro Cons

EEG Non-invasive, comparative to human studies, timing

Bad localisation (3-5cm), no info on cell types, or mode of actionEvoked field potentials Robust indicator of

synchronous synaptic activity

Invasive, only on evoked response, not well with non-aligned cellsCurrent source

density Robust indicator of transmembrane ion flux, better localisation

same as above

Single unit activity (SUA)

Info from one cell Only one cell

Multi unit activity (MUA)

Better overall estimation of spike activity

lack cell type information




Delta <4 Hz (deep sleep)Theta 4-7 Hz (REM sleep)

Alpha 8-13 Hz (light sleep, or quit awake)

Beta 14-30 Hz (active awake)

Evoked field potential (Synaptic strengths) 0.1-1000 Hz

Spikes /action potentials 300-3000 Hz peak 1000 Hz • Single unit activity (1-100 spikes/s) •Multi unit activity



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Electrophysiology in vivo