Isolated neuronal networks still display spontaneous activity. This
activity is not uncorrelated noise but is structured into bursts of
concerted firing alternated with silent intervals. We analyzed the
dynamics of spontaneous bursts in two very different neuronal
networks: intact leech ganglia and dissociated cultures of rat
hippocampal neurons. The following features were common to the two
i) single neurons displayed three classes of spontaneous firing: Poisson, periodic and bursting.
ii) the power spectrum of the firing rate was 1/f at low frequencies, indicating the presence of long-term correlations.
iii) bursts size and duration had power law distributions with no characteristic burst scale. Power laws were followed by an exponential breakdown depending on the network size.
Furthermore, we could switch the two networks from this "critical" state to states dominated by very small or very large events with the same pharmaceutical modulations. We concluded that diverse neuronal networks display similar spontaneous activity features independently from their architecture. 
In the leech, we analyzed then the spontaneous activity of the ganglion when it was connected to the rest of the nervous system. Stimuli from neighboring ganglia and from the two "brains" increased the correlation among the neurons and drove the ganglion to a state similar to the one associated to decreased inhibition, in which bursts have a characteristic size. Finally, we studied how the spontaneous bursts determine the onset of the spontaneous movements in the semi-intact preparation. From the previous results it was possible to determine the relation between the threshold duration and firing rate necessary for a burst to trigger a movement.
 Mazzoni A, Broccard FD, Garcia-Perez E, Bonifazi P, Ruaro ME, et al. (2007) On the Dynamics of the Spontaneous Activity in Neuronal Networks. PLoS ONE 2(5): e439. doi:10.1371/journal.pone.0000439