Blood-Brain Barrier

The blood-brain barrier (BBB) is a selective boundary formed by the blood vessels of the brain that controls which substances pass from the blood into the nervous tissue. It protects neurons from toxins and pathogens and keeps the chemical environment of the brain stable.

Capillaries in the brain are lined by endothelial cells that are joined by tight junctions, which seal the gaps between cells and force molecules to pass through the cells themselves. Pericytes wrap the vessels, and the endfeet of astrocytes surround them. The cells' membranes allow small, fat-soluble molecules and gases such as oxygen to diffuse across the lipid bilayer.

Nutrients that the brain needs cross by specific transporters; for example, the GLUT1 transporter carries glucose. Efflux pumps such as P-glycoprotein push many drugs back into the blood. The result is that most large molecules and about 98 percent of small-molecule drugs do not reach the brain in useful amounts.

The barrier can be crossed or damaged by some pathogens, including West Nile virus and other arboviruses, and by parasites in cerebral malaria. Drug developers use carriers, focused ultrasound and engineered antibodies to get treatments across it, and some brain regions that sense blood chemistry lack the barrier.

• Cell Membrane — the boundary the barrier depends on
• West Nile Virus — a virus that can enter the nervous system
• Glucose — the fuel the barrier transports