Calcium dynamics in the extracellular space of mammalian neural tissue.

In the brain, hundreds of intracellular processes are known to depend on calcium influx; hence any substantial fluctuation in external calcium ([Ca2+]o) is likely to engender important functional effects. Employing the known scales and parameters of mammalian neural tissue, we introduce and justify a computational approach to the hypothesis that large changes in local [Ca2+]o will be part of normal neural activity. Using this model, we show that the geometry of the extracellular space in combination with the rapid movement of calcium through ionic channels can cause large external calcium fluctuations, up to 100% depletion in many cases. The exact magnitude of a calcium fluctuation will depend on 1) the size of the consumption zone, 2) the local diffusion coefficient of calcium, and 3) the geometrical arrangement of the consuming elements. Once we have shown that using biologically relevant parameters leads to calcium changes, we focus on the signaling capacity of such concentration fluctuations. Given the sensitivity of neurotransmitter release to [Ca2+]o, the exact position and timing of neural activity will delimit the terminals that are able to release neurotransmitter. Our results indicate that mammalian neural tissue is engineered to generate significant changes in external calcium concentrations during normal activity. This design suggests that such changes play a role in neural information processing.