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activity and cell communication. The cultures are simple to handle and to design
in various formats, and by using a MEA setup, simultaneous parallel recordings
of many individual neurons, both under normal conditions and undergoing
chemical and electrical manipulations, are enabled.
In the studies presented here, cortical cells that are extracted from rat embryos
are dissociated and uniformly spread over a planar glass substrate combined with
a grid of 60 recording sites (electrodes). The cells are maintained for days and
weeks in a life-supporting environment with proper biochemical conditions,
ensuring functionality over long periods of time. During the time of incubation
the individual cells grow extensions (neurite growth) and establish connectivity
(synapse formation) between cells. After 10-20 days in vitro (DIV) the cell
assembly evolves into a mature planar neuronal network with functional synaptic
connections and a unique pattern of bioelectrical activity.
12.2. Recording the Network Activity
Using a set of amplifiers and appropriate software enables the recording and
saving of the electrical signals generated by the cells. Single action potentials
(spikes), which are the basic form of neuronal signal transmission, are
characterized by higher amplitudes in respect to noise and are detected using a
predefined threshold. The relatively large size of electrodes (30 µm in diameter,
as compared to the 10 µm diameter of neuronal cell body) allows the recording
of several cells from the same electrode. Therefore, further software tools are
applied in order to classify the recorded spikes from into clusters of similar
waveforms, leading to the association of each detected spike with a unique
neuron (a process known as spike-sorting, see Refs. Hulata
et al
., 2000, 2002 for
details). The resulting data is comprised of the relative timing of action-potential
occurrences (spike time-series) of all single neurons. With a common system of
60 electrodes per plate, it is possible to reliably monitor the simultaneous activity
of several tens of cells.
In Fig. 12.1 we show a microscope image of a network grown over a micro-
electrode array. The two-dimensional network lies over a glass plate placed
in the bottom of a well filled with a life-supporting liquid medium (Fig. 12.1A).
Figure 12.1B shows the entire recording area, covering 60 electrodes (distance
between neighboring electrodes is 500 µm). The area in view, of around 30 mm
2
,
covers a small section of the entire network, which is 2 cm in diameter and
includes 2 million cells. Figure 12.1C is a closer view showing the network
densely surrounding a single electrode.
