The Journal of Neurophysiology Vol. 83 No. 2 February 2000, pp. 828-835
Copyright ©2000 by the American Physiological Society

Intrinsic Dynamics in Neuronal Networks. II. Experiment

 ¹Department of Neurobiology, University of California at Los Angeles, Los Angeles, California 90095;  ²Laboratory of Neuropsychology, National Institute of Mental Health, National Institutes of Health; and  ³Laboratory of Developmental Neurobiology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892

Latham, P. E., B. J. Richmond, S. Nirenberg, and P. G. Nelson. Intrinsic Dynamics in Neuronal Networks. II. Experiment. J. Neurophysiol. 83: 828-835, 2000. Neurons in many regions of the mammalian CNS remain active in the absence of stimuli. This activity falls into two main patterns: steady firing at low rates and rhythmic bursting. How these firing patterns are maintained in the presence of powerful recurrent excitation, and how networks switch between them, is not well understood. In the previous paper, we addressed these issues theoretically; in this paper we address them experimentally. We found in both studies that a key parameter in controlling firing patterns is the fraction of endogenously active cells. The theoretical analysis indicated that steady firing rates are possible only when the fraction of endogenously active cells is above some threshold, that there is a transition to bursting when it falls below that threshold, and that networks becomes silent when the fraction drops to zero. Experimentally, we found that all steadily firing cultures contain endogenously active cells, and that reducing the fraction of such cells in steadily firing cultures causes a transition to bursting. The latter finding implies indirectly that the elimination of endogenously active cells would cause a permanent drop to zero firing rate. The experiments described here thus corroborate the theoretical analysis.