Extracellular recording
Neurons, cardiomyocytes or slices are grown on or laid over a grid of planar electrodes, and every electrode records at once. The readout is spikes and bursts across the network, repeatable on the same culture over weeks.
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Multi-electrode arrays
Neurons, cardiomyocytes or slices are grown on or laid over a grid of planar electrodes, and every electrode records at once. The readout is spikes and bursts across the network, repeatable on the same culture over weeks.
How it differs from the standard method
- Spike band, 300–3000 Hz, sampled at 20–25 kHz.
- Activity is spontaneous or evoked through one of the electrodes.
- High-density arrays add enough spatial detail to sort many units per electrode.
1Listening from outside the cell
When a neuron fires, current flows into it at one place and out at another, and that current passing through the resistive extracellular fluid creates a small voltage. An electrode a few tens of micrometres away picks it up without touching the cell: a brief negative trough of tens of microvolts for a spike, and slower, larger waves for the summed synaptic currents of thousands of cells.
Spikes
Action potentials of the few neurons nearest the electrode, tens of µV, under a millisecond long.
Field potentials
Summed synaptic and dendritic currents of many cells: the local field potential, or an evoked field EPSP in a slice.
Detection
Spikes are found by a threshold set at a multiple of the noise, so the noise level decides what counts as a spike.
2Threshold, noise and what counts as a spike
The recorded trace is noise with spikes on top, and a spike is whatever crosses a threshold. The noise is estimated from the median of the absolute filtered signal divided by 0.6745, which equals the standard deviation for Gaussian noise and is barely moved by the spikes themselves.
3From crossings to neurons
An electrode usually hears several neurons. Spike sorting groups the detected waveforms into units by their shape: amplitude, width, and on dense arrays the pattern across neighbouring electrodes.
4Evoked field potentials in a slice
In an acute hippocampal slice, stimulating the Schaffer collaterals evokes a fibre volley (the axons firing) followed by a field EPSP in the dendritic layer of CA1. The initial slope of the field EPSP measures synaptic strength.
Key terms
Practise these as flashcards →A dish or chip with tens to thousands of embedded electrodes that record from a culture or slice grown on it.
The low-frequency part of the extracellular signal, dominated by summed synaptic currents.
The voltage, usually −4 to −6 times the noise σ, that a deflection must cross to be counted as a spike.
Assigning detected spikes to individual neurons by waveform shape.
An inter-spike interval shorter than about 2 ms within a sorted unit: evidence of contamination.
A brief period when many electrodes across a culture fire together; a hallmark of a mature, connected network.
The extracellular reflection of synchronous excitatory synaptic currents; its initial slope measures synaptic strength.
The small, early deflection from the stimulated axons themselves, which reports how many fibres were recruited.
The summed action potentials of many neurons firing together after a large synaptic response.
When to use it
- Following the spontaneous activity of a neuronal network for minutes to weeks, for example as cultures mature or after a drug.
- Measuring synaptic strength and its plasticity (LTP, LTD) in brain slices without dialysing the cells.
- Screening compounds for effects on network firing across many wells at once.
Compared with alternatives
Prefer patch clamp for subthreshold detail, ion-channel currents or the identity of the recorded cell; it records one cell at a time, and the cell is dialysed.
Calcium imaging identifies which cells are active and where they are, at the cost of millisecond timing and individual spikes.
Sub-techniques
Choosing one here switches the whole page, including the protocol and the lab, to it.
Neurons, cardiomyocytes or slices are grown on or laid over a grid of planar electrodes, and every electrode records at once. The readout is spikes and bursts across the network, repeatable on the same culture over weeks.
- Spike band, 300–3000 Hz, sampled at 20–25 kHz.
- Activity is spontaneous or evoked through one of the electrodes.
- High-density arrays add enough spatial detail to sort many units per electrode.