APA Dictionary of Psychology

a semipermeable barrier formed by cells lining the blood capillaries that supply the brain and other parts of the central nervous system. It prevents large molecules, including many drugs, from passing from the blood to the fluid surrounding brain cells and to the cerebrospinal fluid, and thus it protects the brain from potentially harmful substances. Ions and small molecules, such as water, oxygen, carbon dioxide, and alcohol, can cross relatively freely. Entry is also possible for lipid-soluble compounds, such as anesthetics, which diffuse through plasma membranes.

Several anatomical features contribute to the barrier. Cells lining the capillary walls are joined together by tight junctions, which block the passage of molecules through the intercellular spaces found in capillaries elsewhere. Also, the brain capillaries lack pores, called fenestrations, which normally promote the passage of fluid and solutes. Furthermore, the brain capillaries are tightly enveloped in a sheath formed by star-shaped glial cells, called astrocytes. The barrier formed by these features helps maintain a constant environment in which the brain can function, but it also means that many potentially useful drugs cannot enter the brain from the bloodstream. See also area postrema.