Electrophysiology Flashcards
Positives of electrophysiology
Great temporal resolution
Potential for great spatial resolution too
A diverse, flexible technique - wide and narrow field recording
Invasive and non-invasive
Biophysics are relatively well understood - direct measure of neural activity so should be able to isolate specific contributions
Neural events play out at a range of frequencies
Low frequency = less than 300Hz are sampled to obtain local field potentials (LFPs) = slow wave forms, dominated by synaptic activity, perpetuate over quite significant distances
High frequency = more than 300Hz sampled to obtain multiple-unit activity (MUAs) = extracellular APs, signals attenuate rapidly, most likely represent a weighted sum of extra cellular APs of all neurons in a certain range
What are electrophysiological signals?
Summation of current contributions from cellular process across a given area of neural tissue.
Neurons are embedded in an extra cellular medium (fluid surrounds cells and acts as a very strong conductor)
Current superimposed to extracellular medium to generate an extracellular potential
What membranes does electrophysiology measure signals from?
Any excitable membrane - spine, dendrite, soma, axon, axon terminal
Some are easier to access than others but deep brain electrophysiology can capture them all
Synaptic and spiking events are primary contributors but minor actions of glia can contribute
What sources contribute to the local field potential?
Spiking
Synaptic events primarily contribute to local field potential but also other potential finds that can affect the signal depending on the recording site
What is an electroencephalogram?
EEG = recordings from the scalp with measurement devices surrounding nearly the whole head. Measures the summed activity of post-synaptic currents
What is electrocorticography?
ECoG = when measuring from the surface of the brain
What is a magnetoencephalogram?
MEG = the magnetic moments produced by these electric currents. Allows great mapping as the measurement devices nearly surround the entire head
What determines contribution to the LFP?
LFPs represent mostly slow events so to contribute significantly need cooperative activity in neural populations - synchrony of current sources and a network arranged to maximise the likelihood of current sources being detectable
Lots of overlapping activity in an area maximised for detection
What contributes to the LFP? 2
Primary contribution typically is the summation of synaptic activity. Can also potentially get minor contributions from: spiking related activity (fast APs, spike afterhyperpolarisations, calcium spikes), intrinsic currents and resonances, gap junctions, glial cells. These potential contributions depend on the recording site
Dipole theory
A single extracellular event is usually too small to be measured and surface electrodes (EEGs) mostly detect summated LFPs which is most easily achieved for relatively slow events. Dipoles from multiple local neurons sum together.
Dipole generation is the current dominant theory of LFP
Dipoles
A region of positive charge (named the source) separated from a region of negative charge (the sink) by some distance. This spatial separation allows for ionic flow = open field (e.g. pyramidal cells) make a sizeable contribution to the extracellular field
Layer 5 pyramidal cells = most easily recorded
Cellular geometry and LFP contribution
Know it’s:
Synchrony or the current sources and
Architectural organisation of the network
But also:
Cortical folds can affect LFP - less dense current on concave side of gyri which can get complex ie the dentate gyrus
In vivo animal electrophysiology
Invasive recordings, deep brain or surface level
Can get simultaneous LFP and MUA
Requires chronic or terminal implantation for deep brain in animals and typically focus on neural populations
In vitro animal electrophysiology
Slice work or ex vivo
Neural tissue is preserved in artificial CSF to keep neural functionality
Can activate direct inputs into it and observe directly the output
Slice work = near perfect spatial resolution so get a good idea of where in the brain you are which is great for studying circuits and systems
Surface level LFP detection
EEG or ECoG - difficult to fully localise where the response is coming from
Primarily from synchronised synaptic activity in populations of cortical pyramidal (layer 5 pyramidal neurons)
A volume-conducted signal - as it reaches the edge of the volume it’s travelling through, capacitance becomes responsible for signal’s propagation - signal smearing = distortion of EEG signals by poorly conducting skull hampers identification of the underlying generators - can be partially corrected through current sources density estimations
