Electrophysiology of neurons

Some things to remember…

Electrical properties of a (simplified) single cell

Differences in ion concentrations set up by Na+-K+ ATPase pump – high [K+], low [Na+, Cl-] inside cell; high [Na+, Cl-] , low [K+] outside cell

Voltage-gated ion channels alter permeability to Na+ and K+ during generation of action potential

Permeability of membrane to these ions determines the membrane potential

Ligand-gated ion chanels alter permeability to Na+,K+ , Cl- during generation of synaptic potentials

Electrical properties of a (simplified) single cell

ALL or NOTHING: binary, point process

SUMMED input from (tens of) thousands of synapses: continuous process

40ms

20pA

synaptic potentials

action potentials

Cells differ from one another – morphologically

Scale bar = 100 microns Segev, 1998

Cells differ from one another – electrically

There are lots of them

Buzsaki, 2004(≈ 5-10 million cells in a 3x3x3mm voxel)

What do we want to know about?To investigate… We use…

Membrane properties, properties of synapses

Intracellular recording: sharp microelectrodes or patch electrodes (but also calcium imaging)

Single cells, firing patterns in response to environmental stimuli, labelled cells

Extracellular recording

Multiple cells, firing patterns in response to each other, distribution of responses

Tetrode/silicon probe recording (but also population imaging)

Populations of cells acting in synchrony, synaptic input to a population

Local field potential recording

Intracellular recording

Aims to establish something about the properties of single cell, e.g. membrane properties or properties of synapse

Needs an electrode whose tip is smaller than the cell! (typically 50-500 nm)

a lot of mechanical stability

Classic example – miniature synaptic potentials

Fatt & Katz, 1952

Patch clamping

after Neher & Sakmann, 1970s

State of the art – in vivo patch clamp

Bruno & Sakmann, 2006

Intracellular recording: pros and cons

permits measurement of synapses/membrane properties

we can fill the cell with a dye (and reconstruct it afterwards)

difficult to obtain in vivo recordings (normally anaesthetised)

cell damage affects physiology

Sjostrom and Hausser, UCL(also state of the art!)

Extracellular recording

Aims to record firing patterns of a cell, typically with respect to environment/behaviour

Needs electrode that will remain stable during recording – less stringent than intracellular so in vivo recording more straightforward

May need spike sorting to differentiate cells recorded on same electrode

Classic example – visual cortex

Hubel & Wiesel, 1960s

State of the art – juxtacellular recording

after Pinault et al., 1996

State of the art – juxtacellular recording

Ungless et al., 2004

Extracellular recording – pros and cons

can use in awake, behaving animals

difficult to know which cell you’re recording (juxtacellular technique has low yield)

may bias sampling when listening for ‘noisy’ cells/cells with certain response property

spike variability assumed to be noise, when it might not be…

Multi-unit recording

Aims to record activity of populations of cells stimultaneously

Needs some clever maths and technology to pick out the individual voices in the chorus

Tetrodes

Buzsaki, 2004

Silicon probes

Buzsaki, 2004

Example – spike cross-correlograms

Fujisawa et al., 2008

Two-photon calcium imaging

Ohki et al., 2006

Multi-unit recording – pros and cons

can begin to ask sophisticated questions about populations carrying meaningful information (acting as ‘cell assemblies’)

can examine the interactions between cells and how these change during task

can never label cells (although can identify putative interneurons/excitatory cells)

limited by how well we can separate units from one another

Local field potential

Aims to record gross current flow in extracellular space

Reflects synaptic inputs into dendritic trees with particular orientations – so low frequency cf. action potentials (typically lowpass filter at 300Hz)

LFP and cortical depth

Current source density analysis

Mitzdorf, 1985

Relationship between LFPs and EEG: confusing!

Mitzdorf, 1985

Phase-locking between LFP oscillations and spike timing of different cells

Klausberger et al., 2008

What do we want to know about?To investigate… We use…

Membrane properties, properties of synapses

Intracellular recording: sharp microelectrodes or patch electrodes (but also calcium imaging)

Single cells, firing patterns in response to environmental stimuli, labelled cells

Extracellular recording

Multiple cells, firing patterns in response to each other, distribution of responses

Tetrode/silicon probe recording (but also population imaging)

Populations of cells acting in synchrony, synaptic input to a population

Local field potential recording

Relating neural activity to BOLD fMRI signals

Red = BOLD fMRI timecourseBlue = LFPGreen = single unit spiking Logothetis, 2001