Model systems and microscopic structure of the brain extracellular space as formulated in . a Schematic representation of the reconstruction of the brain extracellular space generated by electron microscopy. Sheets (the spaces between two adjacent cells) are in red, while tunnels (the spaces at the junctions of three or more cells) are in cyan. b Close-up of the electron microscopy reconstruction showing typical sizes of the 84 million tetrahedrons used in the simulation. c, d Electron microscopy reconstruction of the brain extracellular space by Kinney et al.  with a small tunnel volume fraction (c) and with a larger tunnel volume fraction (d).