A semipermeable barrier surrounding the capillaries that supply the central nervous system. It largely prevents outside molecules (such as things we eat or drugs) from entering the nervous tissue and harming the tissue or altering neuronal function.

Ions and small and/or lipophilic molecules tend to cross more freely. It is structurally complex, composed of junctional complexes (such as tight junctions) between the capillary endothelial cells, as well as ancillary cells (mainly pericytes and astrocytes) surrounding the capillaries.