Lecture 2 Electrophysiology Flashcards
What are the 3 main strengths of electrophysiology?
excellent temporal resolution (at ms levels can capture data quickly)
diverse/flexible techniques (wide and narrow field recording with invasive and non-invasive options)
well-understood biophysics (allows the inference of neural activity from electrophysiological recordings- unlike BOLD fMRI)
define electrophysiological signals
summation of current contributions from cellular processes across a given area of neural tissue
what are neurons embedded in?
extracellular medium (made up of extra cellular matrix and interstitial fluid)
what is the property of extracellular medium which makes it useful for electrophysiological recordings?
high conductance for excitable membranes/transmembrane currents
why do you need to use a more deeply placed electrode in the brain to measure high frequency signals?
high frequency signals are attenuated by the lipid bilayer/ ECM so they cannot travel as far as low frequency, hence, unlike low-frequency, cannot be recorded at surface level as clearly.
what does LFP stand for?
local field potentials
where are LFPs measured from- inside or outside cells?
outside cells (extracellular environment)
where does an electroencephalogram take measurements from?
the scalp
where does electrocorticography take measurements from?
the surface of the brain
Name the 2 important factors which determine the extent to which current contributes to extracellular field:
1) Synchrony of current sources (where signals are able to overlap)
2) Architectural organisation of neurons
What is the primary contribution to LFPs?
summation of synaptic activity
other than the summation of synaptic activity, what other factors contribute to LFPs?
spiking related activity- fast action potentials, after-hyperpolarisation and calcium spiking
intrinsic current and resonances
gap junctions
glial cells
describe how dipoles are formed in synaptic activity recorded in the form of LFPs
while recording LFPs, you’re recording dipoles- summated PSP at a given time
an event causes excitatory neurotransmitter action at a dendritic level and ions flow into the cells, this forms an area of negative charge outside the dendrites (positive inside) known as an extracellular sink. Neurons strive for electroneutrality so this negative EC charge is balanced by a positive one at the soma end of the neuron- also occurring through ion flux.
this happens the same but with opposite charges for inhibitory neurotransmitters
what is faster, synaptic events or action potentials?
action potentials
why is it important that synaptic events are slow in order to be recorded in LFPs?
so that the signals can overlap as many are happening in the same kind of time, a single extracellular event is too small to be measured so need several at once
(this is not really seen with APs)
why is neuronal organisation important in forming a dipole?
they need to be all orientated the same way, otherwise the dipoles will cancel each mother out, giving no overall charge signals
what is the dominant theory of LFP?
dipole theory
describe the relationship between cellular geometry and dipoles
the geometry of the neurons affects whether they an form robust dipoles.
For example layer V pyramidal cells have long thick apical dendrites, which generate a strong dipole, giving rise to an open field as there’s a considerable spatial separation between the extracellular sink and source.
On the other hand, cells such as thalamocortical cells emanate dendrites in all directions so are spherically symmetrical which doesn’t cause a strong dipole, hence forms a closed-field.
how do cortical folds affect LFP?
it makes LFP signals more complex as opposite dipoles/ changing cell orientation means dipoles may cancel each other out e.g. dendate gyrus.
how do fast action potentials contribute to LFP?
when they spike, they give off a strong signal, however as it is high frequency it may not be recorded as an LFP, unless deeper electrodes are being used- so not really effecting EEG
how do afterhyperpolarisations contribute to LFP?
they occur after fast spiking and can last as long as synaptic events, so may be seen in the form of temporally coordinated neuronal bursts
how does calcium spiking contribute to LFP?
it’s not fully understood in vivo
they have a large amplitude and long duration so can contribute
describe in vivo animal electrophysiology
invasive recordings taken from living animals
often simultaneously recording LFP and MUA
can be done deep or superficial levels
describe in vitro animal electrophysiology
neural tissue put in artificial CSF
it has near perfect spatial resolution
it’s great for studying circuits and systems
has high levels of environmental control
why are surface level LFP recordings, such as EEG not recording direct electrophysiological signals?
it’s volume conducted as there’s a build up of capacitance in meninges/skull/scalp etc.
what is spatial smearing?
where there is a distortion of the EEG signal caused by meninges
what are the 5 EEG frequency bands?
delta, theta, alpha, beta and gamma
