Electrophysiology Flashcards
What are the advantages of electrophysiology?
- Excellent temporal resolution (potential for excellent spatial resolution also)
- Diverse, flexible techniques (wide and narrow field recording, invasive and non-invasive)
- Biophysics are relatively well understood (direct measure of neural activity)
What are electrophysiological signals?
Summation of current contributions from cellular processes across a given area of neural tissue
Neurons are embedded in an extracellular medium (primarily interstitial fluid and extracellular matrix, highly conductive - acts as a volume conductor for any excitable membranes and transmembrane current)
Current superimposes in extracellular medium to generate an extracellular potential - signals can get messy
What do local field potentials represent?
Represents slow waveforms - dominated by synaptic activity
What does multiple-unit activity represent?
Extracellular action potentials - signals attenuate rapidly so difficult to detect over distances
What are the following abbrievations: EEG, ECoG, MEG?
EEG = electroencephalogram when recorded from the scalp
ECoG = electrocorticography when at the surface of the brain
MEG = magnetoencephalogram - magnetic moments produced by these electric currents
How has the local field potential definition changed?
Both definitions by Buzsaki
2004 - The local field potential is the extracellular current flow that reflects the linearly summed postsynaptic potentials from local cell groups
2012 - Any excitable membrane - whether it is a spine, dendrite, soma, axon or axon terminal - and any type of transmembrane current contributes to the extracellular field. The field is the superposition of all ionic processes, from fast action potentials to the slowest fluctuations in glia
What factors determines contribution to the LFP?
It depends - multiple sources whose proportional contribution vary based on recording site and technique
LFPs mostly represent slow events reflecting cooperative activity in neural populations
Two important factors determine the extent to which current contributes to an extracellular field
1. Synchrony of the current sources - sources occurring at roughly the same timescale
2. Architectural organisation of the network - allows the slow signals to overlap such that you can actually detect them
What contributes to the LFP?
Primary contribution typically from summation of synaptic activity
Potential minor contributions from
- Spiking related activity (fast action potentials, spike afterhyperpolarisation, calcium spikes)
- Intrinsic currents and resonances
- Gap junctions
- Glial cells
== Contributions depend on recording site
Briefly explain synaptic activity
Neurotransmitter action leads to ions flowing into the cell
Depending on the type of neurotransmitter, this leads to IPSPs - extracellular source or EPSPs - extracellular sink
Neurons always strive for electroneutrality so opposing ion flux along the neuron occurs
This leads to an extracellular sink in IPSPs and a extracellular source in EPSPs
This leads to generation of a dipole
Explain sources and sinks
Sinks and sources refer to the sign of the local field potential measured with extracellular electrodes
Excitation involves positive charges entering cells, depolarising them
When positive charges move into a cell, there is less positive charge outside the cell where the electrode is, so it becomes more negative - this is called a sink because the electrode records a negative deflection
Inhibition causes an active source in the case of negative ionic current - the extracellular potential at the source is positive
What is a dipole?
A molecule with an area of negative charge and an area of positive charge that are separated
Why is synaptic activity important for extracellular current flow?
They are slow which allows for overlap and summation which boosts detection
What is dipole theory?
When an excitatory neurotransmitter is released, positive ions flow into the dendrites, leaving a net negative voltage in the extracellular space – this creates a dipole.
Dipoles from different neurons and different regions summate and conduct to the skull and give rise to the characteristic peaks and troughs of the ERP waveform
A single extracellular event is ordinarily too small to be measured
Surface electrodes in human electrophysiology e.g., EEG mostly detect summated LFPs
Dipoles from multiple local neurons sum together - measurable as a single dipole whose magnitude reflects the number of summated dipoles
Why is cellular geometry important for dipoles?
Open field = layer V pyramidal cells
Maximum ion flow = strong LFP contribution
Spatial separation between sink and source allowing for ionic flow
Closed field = thalamocortical cells
Limited ion flow = weak LFP contribution
Source and sinks overlap resulting in limited ion flow
Why is the structure of layer V pyramidal cells ideal for dipoles?
Show ideal architecture for superposition of active dipoles
- Parallel apical dendrites
- Open field
- Perpendicular afferent inputs
