Multichannel Systems MEA-System Manual - Manual (Page 14)

3.3 Signal Types
3.3.1 Single Unit Activities
Usually several cells are plated onto a MEA. The waveform of a single unit spike depends on
the signal source, the geometry of the extracellular space, and the distance of the signal source
to the electrode. The property of a waveform derived from a single neuron is reproducible over
time and therefore specific for that neuron. That is, the differences of waveforms from separate
signal sources can be used to distinguish the activities and to sort spikes into single unit spikes.
Thus, you can acquire single unit data from multiple cells in parallel by recording from a single
electrode.
You have to discriminate between independent activities and network responses. Responses
of cells on a MEA triggered by a chemical, electrical, or light stimulus can be either statistically
independent or show a specific pattern. The latter is quite interesting for studying the role of
cells and different tissues in a pathway. MEA recording allows such studies under controlled
experimental settings and is much easier and less labor intensive than an in vivo experiment.
3.3.2 Local Field Potentials
If the dendrite soma axes of the active cells are aligned, the waveforms from multiple units on a
MEA overlay and from a compound potential, or local field potential (LFP). The higher the activity,
that is, the spike rate, the higher is the amplitude of the LFP. A modulation of the stimulus results
in a higher frequency of action potentials that will result in a graded multiunit response.
LFPs often show a high signal-to-noise ratio, which is very beneficial for the analysis.
If dendrites are arranged in a nonparallel or radial fashion forming a closed field, the waveforms
may cancel each other out, when the neurons fire in synchrony.
3.4 Recording and Stimulation
The MEA sensor is placed directly into the small sized MEA amplifier or MEA2100 headstage.
When the amplifier is closed, the contact pins in the lid of the amplifier are pressed onto the
MEA contact pads. The very close location of the amplifier to the MEA sensor is very favorable
concerning a high signal-to-noise ratio.
If you use the MEA amplifier with blanking circuit, MEA_Select software control allows to select
any electrode on a MEA for stimulation and recording. In MEA2100 headstages the blanking
circuit feature is always provided. A user defined, typically 500 μs long, blanking signal switches
off stimulating electrodes during stimulation and thus removes stimulus artifacts.
With the MEA_Select software, it is easy to change the electrode selection during the experiment,
for example, to use stimulation electrodes for recording and vice versa. It is also possible to use
the same electrode for recording shortly after stimulation provided that you use a dedicated
biphasic pulse protocol that compensates for the slight DC offset that a stimulation electrode
always shows after stimulation. When using MEA2100-Systems the MEA_Select software is already
included in the data acquisition program MC_Rack, Cardio2D or LTP-Director, you do not have to
change the software program for blanking. Please read the MEA2100-System manual.