NMDAR-Mediated Ca2+ Increase Shows Robust Information Transfer in Dendritic Spines

Abstract

A dendritic spine is a small structure on the dendrites of a neuron that processes input timing information from other neurons. Tens of thousands of spines are present on a neuron. Why are spines so small and many? We addressed this issue by using the stochastic simulation model of N-methyl-D-aspartate receptor (NMDAR)-mediated Ca²⁺ increase. NMDAR-mediated Ca²⁺ increase codes the input timing information between prespiking and postspiking. We examined how much the input timing information is encoded by Ca²⁺ increase against presynaptic fluctuation. We found that the input timing information encoded in the cell volume (10³μm³) largely decreases against the presynaptic fluctuation, whereas that in the spine volume (10^-1μm³) slightly decreases. Therefore, the input timing information encoded in the spine volume is more robust against presynaptic fluctuation than that in the cell volume. We further examined the mechanism of the robust information transfer in the spine volume. We demonstrated that the condition for the robustness is that the stochastic NMDAR-mediated Ca²⁺ increase (intrinsic noise) becomes much larger than the presynaptic fluctuation (extrinsic noise). When the presynaptic fluctuation is large, the condition is satisfied in the spine volume but not in the cell volume. Moreover, we compared the input timing information encoded in many small spines with that encoded in a single large spine. We found that the input timing information encoded in many small spines are larger than that in a single large spine when presynaptic fluctuation is large because of their robustness. Thus, robustness is a functional reason why dendritic spines are so small and many.