Ephaptic interactions in the mammalian olfactory system

Ephaptic coupling refers to interactions between neurons mediated by curren t flow through the extracellular space. Ephaptic interactions between axons are considered negligible, because of the relatively large extracellular s pace and the layers of myelin that separate most axons. By contrast, olfact ory nerve axons are unmyelinated and arranged in tightly packed bundles, fe atures that may enhance ephaptic coupling. We tested the hypothesis that ep haptic interactions occur in the mammalian olfactory nerve with the use of a computational approach. Numerical solutions of models of axon fascicles s how that significant ephaptic interactions occur for a range of physiologic ally relevant parameters. An action potential in a single axon can evoke ac tion potentials in all other axons in the fascicle. Ephaptic interactions c an also lead to synchronized firing of independently stimulated axons. Our findings suggest that ephaptic interactions may be significant determinants of the olfactory code and that such interactions may occur in other, simil arly organized axonal or dendritic bundles.