Computational properties of peri-dendritic calcium fluctuations.

Using a model of the extracellular space, we show how external calcium fluctuations, engendered during normal neural activity, can act as a rapid information-bearing signal in nervous systems. We demonstrate that action potentials propagating along a dendrite can induce large peri-dendritic calcium fluctuations, lowering significantly the external calcium available to overlying presynaptic terminals. The geometrical distribution of active calcium sinks critically influences the time and spatial extent of fluctuations in external calcium. In particular, clusters of coactive dendrites can prolong and amplify an external calcium fluctuation. This latter effect provides a natural substrate for a computational mechanism that locates specific volumes of neural tissue on rapid time scales. Such an interpretation suggests that the detailed structure of the extracellular space, in combination with the three-dimensional distribution of active calcium sinks, may play a role in neural information processing.