Fig. 3. The neurovascular unit.

The structure and function of the neurovascular unit allow bidirectional communication between the microvasculature and neurons, with astrocytes playing intermediary roles. Pial arteries in the subarachnoid space bathed in CSF become penetrating arteries upon diving into the brain parenchyma. The perivascular space around penetrating arteries is termed the Virchow-Robin space. As the penetrating arteries branch into arterioles and capillaries the CSF-containing Virchow-Robin spaces narrow and finally disappear. However, the perivascular space extends to arterioles and capillaries to venules where it is made up by the basal lamina's extracellular matrix that provides a continuity of the fluid space between arterioles and venules. Astrocytic vascular endfeet expressing aquaporin-4 (AQP4) surround the entire vasculature and form the boundary of the perivascular spaces.