The dynamics of a neuronal culture of dissociated cortical neurons of neonatal rats

Neuronal networks of dissociated cortical neurons from neonatal rats were c ultured over a multielectrode dish with 64 active sites, which were used bo th for recording the electrical activity and for stimulation. After about 4 weeks of culture, a dense network of neurons had developed and their elect rical activity was studied. When a brief voltage pulse was applied to one e xtracellular electrode, a clear electrical response was evoked over almost the entire network. When a strong voltage pulse was used, the response was composed of an early phase, terminating within 25 ms, and a late phase whic h could last several hundreds of milliseconds. Action potentials evoked dur ing the early phase occurred with a precise timing with a small jitter and the electrical activity initiated by a localized stimulation diffused signi ficantly over the network. In contrast, the late phase was characterized by the occurrence of clusters of electrical activity with significant spatio- temporal fluctuations. The late phase was suppressed by adding small amount s of D(-)-2-amino-5-phosphonovaleric acid to the extracellular medium, or b y increasing the amount of extracellular Mg2+. The electrical activity of t he network was substantially increased by the addition of bicuculline to th e extracellular medium. The results presented here show that the neuronal n etwork may exist in two different dynamical states: one state in which the neuronal network behaves as a non-chaotic deterministic system and another state where the system exhibits large spatio-temporal fluctuations, charact eristic of stochastic or chaotic systems.