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Electrophysiology of neurons
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Some things to remember…
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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
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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
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Cells differ from one another – morphologically Scale bar = 100 micronsSegev, 1998
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Cells differ from one another – electrically
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There are lots of them Buzsaki, 2004 (≈ 5-10 million cells in a 3x3x3mm voxel)
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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
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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
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Classic example – miniature synaptic potentials Fatt & Katz, 1952
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Patch clamping after Neher & Sakmann, 1970s
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State of the art – in vivo patch clamp Bruno & Sakmann, 2006
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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!)
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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
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Classic example – visual cortex Hubel & Wiesel, 1960s
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State of the art – juxtacellular recording after Pinault et al., 1996
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State of the art – juxtacellular recording Ungless et al., 2004
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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…
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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
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Tetrodes Buzsaki, 2004
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Silicon probes Buzsaki, 2004
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Example – spike cross- correlograms Fujisawa et al., 2008
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Two-photon calcium imaging Ohki et al., 2006
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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
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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)
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LFP and cortical depth
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Current source density analysis Mitzdorf, 1985
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Relationship between LFPs and EEG: confusing! Mitzdorf, 1985
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Phase-locking between LFP oscillations and spike timing of different cells Klausberger et al., 2008
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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
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Relating neural activity to BOLD fMRI signals Red = BOLD fMRI timecourse Blue = LFP Green = single unit spiking Logothetis, 2001
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