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seen. Tension forces hold tightly the neuron extensions between clusters. As is
also seen in the bottom of Fig. 12.2, in some cases the tension between cells may
lead to "holes" within the network connectivity.
The topology of the neuronal network had been the focus of many studies. It
has been mentioned that connectivity between neurons in culture takes the form
of a "small-world" network, consisting long-range connections that make short-
cuts between distant regions of the culture (Netoff et al . 2004). Highly clustered
networks, as the ones shown in Fig. 12.2, can take the form of a scale-free
network that includes hubs of dense connections. As was previously mentioned,
the scale-free parameter that defines small-world connectivity may be strongly
related to the density of the plated cells (Segev et al . 2003). The main causes for
a culture to collapse into a clustered network are the strong tension forces along
the axons, which grow as the number of cells increases, compared to the adhesive
forces to the surface. However, recent work by Breskin et al . (2006), in which
electrical stimulation was used in order to engage network bio-electrical activity,
had shown, by applying percolation theory, that the scale-free parameter found is
closer to a Gaussian distribution of connectivity.
12.4. The Formation of Synchronized Bursting Events
Synchronization is a fundamental phenomenon that is commonly observed in
network dynamics and takes a special form in cultured networks (Baruchi et al. ,
in press). In fact, the basic mode of network activity, appearing as early as in
day 10 in culture, is composed of synchronized eruptions of neuronal firing.
Traditionally, this activity mode is termed as a "synchronized bursting event"
(SBE) (Maeda et al . 1995), though other titles such as "neuronal avalanches"
(Beggs and Plenz 2004), "network spikes" (Eytan and Marom 2006), "network
bursts" (van Pelt et al . 2004) or "giant depolarizing potentials" (Menendez de la
Prida et al . 1998) are also used. Figure 12.3 shows network activity recorded
over 24 hours. The synchronized basic nature of network firing is clearly visible
as simultaneous events that involve the activation of most of the neurons. In
general, SBEs are a momentary outbreak of most of the neurons in the network,
usually lasting for several hundreds of milliseconds. As is seen in Fig. 12.3, in
between each event the network remains relatively silent for periods lasting
several seconds.
The occurrence of SBEs in the network activity is the major trademark of
cultured networks, and therefore requires a precise definition and a proper
measure to characterize it. A common definition for a SBE is a limited time
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