Intrinsic dynamics in neuronal networks. II. Experiment

Neurons in many regions of the mammalian CNS remain active in the absence o f stimuli. This activity falls into two main patterns: steady firing at low rates and rhythmic bursting. How these firing patterns are maintained in t he presence of powerful recurrent excitation, and how networks switch betwe en them, is not well understood. In the previous paper, we addressed these issues theoretically; in this paper we address them experimentally. We foun d in both studies that a key parameter in controlling firing patterns is th e fraction of endogenously active cells. The theoretical analysis indicated that steady firing rates are possible only when the fraction of endogenous ly active cells is above some threshold, that there is a transition to burs ting when it falls below that threshold, and that networks becomes silent w hen the fraction drops to zero. Experimentally, we found that all steadily firing cultures contain endogenously active cells, and that reducing the fr action of such cells in steadily firing cultures causes a transition to bur sting. The latter finding implies indirectly that the elimination of endoge nously active cells would cause a permanent drop to zero firing rate. The e xperiments described here thus corroborate the theoretical analysis.