Fig. 1.

Thin-film model of the myelinated axon. a Cross-sectional image of a myelinated axon in the mouse brain obtained using transmission electron microscopy (TEM). b A schematic cross-sectional view of the myelinated axon, with an interrogating light beam (purple arrows) focused at the center of the axon. c Physiologic thin-film model for the myelinated axon with corresponding refractive indices and thicknesses indicated. Upward arrows (light blue) indicate Fresnel reflections at the interfaces. Not drawn to scale. m, membrane; c, cytosol; e, extracellular space; p, pericellular space