Fig. 5

Possible routes of solute transport along arteries both in the subarachnoid space and in the parenchyma. Extramural influx may occur (dashed green line) via a “periarterial space"between a sheath composed of pia and artery wall [11] or (solid green line) via the subpial space [129, 130]. Whatever their route, the solutes must cross the pia (green double-headed arrows) at some stage leading into the parenchyma. Efflux of solutes may occur by reversal of the extramural route (green dashed line) [12, 62, 96, 138] in which case, they would reach intramural sites within the smooth muscle layer by diffusion through the wall (double-headed red arrows in the main figure, dashed red lines in the insert). Alternatively [8, 129, 130, 139, 242], efflux may occur via an intramural route (solid red lines in the main figure and insert), which requires movement of solutes over long distances via the basement membranes of the smooth muscle layer of the arterial wall (light grey in the insert). For this route to be dominant there must be some feature of the arterial walls that prevents escape of solutes from the smooth muscle layer to the extramural periarterial space. The thick black lines represent the glia limitans at the surface of the brain parenchyma and surrounding the arteries. Structures shown in the figure are modified from those shown in Fig. 6 of [130]