Electrophysiology in vivo

Electrophysiology (Ephys)
A key feature when studying neuro-vascular and
Claus Mathiesen
Department of Neuroscience and Pharmacology
metabolic coupling
Aim:
Teach you the basics of in vivo electrophysiology
!
Claus Mathiesen, M.Sc. Ph.D.

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

Department of Neuroscience and Pharmacology Dias 3
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

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

Commonly recorded Ephys signal in the in vivo
EPSP
fEPSP
brain?
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)
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)
Quiet awake Open eyes
closed eyes
Alpha Beta

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
Delta <4 Hz (deep sleep)
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 àvoltage changes in the field
potential
0.5 mV

Shape of evoked field potentials as function of
anatomy and location
Hippocampus or
Cerebral

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
í
í
í
í

í
í
í
í
í
í
Depth [μm]


Time [s]
í
í
í
Jessen
et
al
2014

Mathiesen
et
al.
2011
Sink
Iso
Source
LFP
Depth profile

Time [s]
1st
2nd
Current source density
CSD 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
Field potential
Current flow density
Current Source Density
Nicholson, Freemann 1975
Source
Sink
Spatial
derivative Spatial
derivative

Contribution to Ephys from different cell type
Cerebellar
pyramidal
Contribution to the field potential
Neurons Glia
Cortical
pyramidal
Interneurons Astrocyte Oligodendrocyte Microglial
Stellate cell
Principal output Relay
K+ buffer
Blood flow
House keeper
Calcium waves
Myelinate Phagocyte
Major Major Middle
Minor (EEG)
(Minor) (Minor) (Minor)

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
Delta 4 Hz (deep sleep)
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

Single unit spike activity (Purkinje Cell)
5 ms 5 ms
0 1000 2000 3000 4000 5000
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
Power
Frequency
Simple spike Complex spike
Kirsten Thomsen

Example of single unit activity (SUA)
Spike- waveform Regular
InterSpikeInterval
Event autocorrelation
Herrik et al. 2010
Irregular
Burst
Regular Irregular Burst

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 recording
Method Pro Cons
EEG Non-invasive,
comparative to human
studies, timing
Bad localisation
(3-5cm), no info on
cell types, or mode of
Evoked field potentials Robust indicator of action
synchronous synaptic
activity
Invasive, only on
evoked response, not
well with non-aligned
Current source cells
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

THANK YOU FOR YOUR ATTENTION

Electrophysiology in vivo

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